Dissolved Oxygen Processes 35 Neap-Springtidal Effects on Dissolvedoxygen and River-Bay Interactions in the Lower York River Leonard W

DissolvedOxygen in the ChesapeakeBay I'xi~

Proceedingsof a Seminar on Hypoxicand Related Processes in ChesapeakeBay

Editedby Gail B. Mackiernan

Sponsoredby Maryland and Virginia SeaGrant Co1Ieges

;, A Maryland Sea Grant Publication = College Park, Maryland J'. $1 publication Number UM SG-TS-57-03

Copiesof thispublication are available from:

Maryland Sea Grant C ollege 1224 H.3. Patterson Hall University of Maryland College Park, Maryland 20742

This publication is made possible in part by a grant from the National Oceanic and Atmospheric Administration, Depart- ment of Commerce,through the National SeaGrant College Program, grant number NA86AA-D-SG-006 to the Univer- sity of Maryland Sea Grant College and grant number NA86AA-D-SC-042 to the Virginia Sea Grant College Program. The University of Maryland is an equal opportunity employ er. vii Acknow ledgements

Introduction

5 Recent Trends in Hypoxia Hypoxiain Virginia'sEstuaries: An Assessmentof Historical Data Leonard W. Haas and Bruce W. Hill 12 ChesapeakeBay Mainsternand Tributary Monitoring Program Richard Batiuk l9 Hypoxia in ChesapeakeBay: Resultsfrom the Maryland Of fice of EnvironmentalPrograms' Water Quality Monitoring, 1980 - l986 Rober t E. M agnien

22 Discussion

35 Dissolved Oxygen Processes 35 Neap-SpringTidal Effects on DissolvedOxygen and River-Bay Interactions in the Lower York River Leonard W. Haas

38 Intrusion of Low Dissolved Oxygen Water into the Choptank River Lawrence P. Sanford

02 Discussion PhosphorusCycling and Nutrient Limitation in the Patuxent River Christopher F. D'Elia !v / Cpntents

49 gutrjent Cyclingin ChesapeakeBay fhp+as R Fisher and Robert D. Doyle

SeasonalOxygen Depletion and Phytoplankton Prpduc- tjpn in ChesapeakeBay: PreliminaryResults of 198~- g6 Field Studies Thomas C. Malone Sourcesof BiochemicalOxygen Demand in the Chesa- peakeBay: The Role of MacrophyteDetritus and Decpmppsition Processes jpseph C. Ziernan, Stephen A. Macko and Aaron L. Mills

Discussion

y~ ChesapeakeBay Dissolved Oxygen Dynamics: Roles of phytoplank ton and Micr oheterotrophs Robert B. 3onas

Sl Bacterial Carbon Pools and Fluxes in Chesapeake Bay Plankton Hugh Ducklow and Emily Peele

86 implications of Microzooplankton Grazing on Carbon Flux and Anoxia in Chesapeake Bay Kevin G. Seilner, David C. Brownlee and Lawrence W. Harding, 3r.

9l Physical and Biological ProcessesRegulating Anoxia in Chesapeake Bay: Zooplankton Dynamics Michael R. Roman i00 Cpntributipn of Sulfur Cycling to Anoxia in Chesa- peake Bay 3on H. Tuttle, Eric E. Roden and Charles L. Divan l03 Relative Rolesof Benthic VersusPelagic Oxygen- ConsumingProcesses in Establishingand Maintaining Anoxiain ChesapeakeBay W- Michael Kernp, Peter Sarnpouand Walter R. Boynton Contents j v

107 Discussion

117 Biological Effects of Hypoxia

117 Depicting Functional Changesin the Chesapeake Ecosystem Robert E. Uianowicz

.' 125 Biological Monitoring of SelectedOyster Bars in the Lower Choptank 3ohn F. Christmas and Stephen 3. 3ordan

129 Discussion

.' 139 Influence of Low Oxygen Tensionson Larvae and Post SettlementStages of the OysterCrassostrea ~vir inica Roger Mann, Brian Meehan, 3ulia S. Ranier Victor S, Kennedy, Roger I,E. Newell and William F. Van Heukelem 140 Effects of I ow DissolvedOxygen in the Chesapeake Bay on Density, Distribution and Recruitment of an Important Benthic Fish Species Denise L. Breitburg 147 Bay Anchovy Ecology in Mid-ChesapeakeBay Edward D. Houde, Edward 3. Chesney, 3r. and Timothy A. Newberger

149 Discu ssion

157 Maryland Stock Assessment Studies Hariey Speir 159 Quantifyingthe Severityof HypoxicEffects Howard T. Boswell, G.P. Patil and 3oel S. O' Connor

171 Discussion

175 List of Participants afwfAuthors ACKNOVLEDGEMENTS Theeditor wishes to thankall the seminar participants for theirenthusiasm and perseverance in the face of adver- sity theworst winter storm in fiveyears!, and the authors for their cooperationduring subsequent preparation of this document.I am also grateful to David Smith of Virginia Sea Grantand Games Thomas of NOAA'sEstuarine Programs Of- fice for their helpwith preparationand logistics for the seminar. The supportof the EstuarineProgram Office, NOAAis especiallyappreciated. My colleagues at Maryland SeaGrant whohave been essential to the productionof the seminarproceedings include Merrill Leffler and3ack Greer for assistancein transcribingand editing; Sandy Harpe for designand graphics; and Lisa Griffin for wordprocessing, l thank them for their patienceand supportin the past months.

Supportfor the HypoxiaSeminar and this publication were providedthrough NOAA'sNational Sea Grant College Program, grant number NA86AA-D-SG-006 to the Univer- sity of MarylandSea Grant Programand grant number NA86AA-D-SG-042to the Virginia Sea Grant College Program. INTRODUCTION

In recent years, seasonal oxygen depletion in Chesa- peake Bay has been identified as a major indicator of reducedenvironmental quality and as a potential stresson the Bay'sliving resources.Consequently, one of the goalsof the on-goingfederal/state ChesapeakeBay restorationpro- gram is the reduction of the extent and duration of these hypoxic events.

However, such management strategies need a sound scientific basis for their development. The processeswhich lead to the establishment and maintenance of low dissolved oxygen, the relative importance of natural e.g., climatic! versus anthropogenic forces, and the nature and extent of impacts on living organisms have not been well-understood. In order to fill in these information gaps, the National Oceanicand AtmosphericAdministration NOAA!, through the Maryland and Virginia Sea Grant Programs,has been fundinga major effort to study the processesand impacts associatedwith hypoxiaand anoxia. This study program, initiatedin September1985, has already begun to shedlight on many of these problems. To begin the processof synthesizingthe knowledge gained,Maryland and Virginia SeaGrant jointly sponsoreda Seminaron Hypoxiaand RelatedProcesses in Chesapeake Bay, whic washeld 3anuary21 and 22 1987,in College Park, Maryland. e meettngfocused on selectedresearch related to low dissolvedoxygen, estuarine processes, and the biologicalimpacts of hypoxia. The primaryobjectives of the meeting were: To providefor the exchangeof ideas,research findings, data, and other informationamong investigators participatingin the SeaGrant Low DissolvedOxygen Programand related researchprojects, NOAA'sStock Assessmentprogram, and the federal/stateChesapeake Bay Monitoring Program. 2 / Introduction

d!scussthese findings, their interpretation, and To archimplications among seminar participants. To encouragefuture integrationand exchangeamong participating programs and investi ga tors. Theseminar was more than a presentationof individual projectresults; the intent was for investigatorsto discuss th;«esearch programs,their findings to date, and the relationshipof their researchto other work. Theseminar wasorganized to facilitatethis information exchange, and stimulate discussionamong participants. The latter includeprinci pal investigators of pertine nt SeaG rant projects researchersworking on relatedprojects with stateor otheragency support, NOAA stock assessmentP.l.s, repre- sentatives from the federal/state Chesapeake Bay monitoringprogram, and other invited attendees. The energetic and information-rich discussions which resulted were an irnpor- tant part of the meeting and are summarized in this docum ent.

This seminar represents the first step in evaluation and dissemination of information gained in the important researchprogram. A more in-depth presentation and synthesis of research results will occur at the end of the current research program. Recnit Trends in Hypoxia HYPOXIA IN VIRGINIA'S ESTUARIES: AN ASSESSMENT OF HISTORICAL DATA

Leonard W. Haas and Bruce W. HIH Virginia Institute of Marine 5cience College of William and Mary

As an initial step in elucidating the dynamicsof hypoxia in Virginia's saline waters, a grant was awarded to VIMS to collect and organize historical dissolved oxygen data from the Chesapeake Bay and its subestuaries. The specific objectives of the one year study were: l, To create a computer-based, data.management system for existing dissolved oxygen data in Virginia's saline waters.

2. To document, through the analysis of the historical data, the temporal and spatial characteristics of hypoxia in Virginia's saline waters.

3. To analyze the data set with a view toward defining which environmental processes natural and anthropogenic! are critical in regulating the hypoxiaprocess.

To use the results of this study to guide future research on h ypoxia i n Vir gi nia

The compilation and organization of the data have been completed and the analysis of the data is currently underway. The results provided in this report should be con- sideredpreliminary and are subject to revision. 6 / D>ssolvedOxygen

-BasedData Management Descriptionof the Computer- System sefor this project was created and managed

S stern DBMS!! with' h aFortran-like or programming language andcan interface wit 'th sta tatistical is ' packages such as SPSS and BMDP. The databased b is hierarchiali in structure,i,e., one record owns many other er recordsin a top-down approach !, Th f' t recordtype is referred to ast h eha eader r Figure1!, d Thet irs ' various reco stationinformation, i.e., river record and containsd d timevario cruise,river, latitude andlongi- ID,ud date, de, car co ' e,tes im, andtenths total depth,Secchi disc visibility,wind direction and speed, and air temperature.

Dept th recorrecordsare organized into the followingrecord types:

Record Type 2 - Temperature Record Type 3 - Salinity Record Type 4 - Bacteria Record Type 5 - Nutrients Record Type 6 - Organics Record Type 7 Algaeand Chlorophyll Record Type 3 - Dissolved Oxygen and pH RecordType 9- SuspendedSolids and Turbidity Record Type l0 - Conductivity Record Type l l - Inorganics Record Type l2 - Sample Time Record Type l3 - Current Direction and Speed Record Type 2l - Table Look-up

Each depth record observation contains the following '"fo mation: river lD, date, year, time, cardcode, depth, method code,value and sequencenumber. Every style of gear and methodof analysisis assignedan alpha-numeric code a"~ may be found in record type 2l, The rivers and the Bay were divided into 5 nautical mile segments to facilitate spatial analysis of data. RecentTrends in Hypoxia/ 7

Theexisting historical hydrographic data base at VIMS, was stored on mag a Prime-supported/DBMScalled Power. Becausethe data- basewas so large it wasdivided into two-yearintervals, exceptfor theyears 1902-1962 and J,9/1-1983 and programs were written to extract and modify tbe data in the power format so that they could be loadedinto a SIRschema. The schemadefined record size, variablenames, formats, value ranges,missing values, labels, sort IDsand the relationship betweenthe header record and depth records. Severalother sourcesincluding Old DominionUniversity, Hampton Roads Sanitation District and VIMS provided additional dissolved oxygen data.

The data were checkedand corrected for duplicated observations; tbe latitude and longitude were checked againstthe Hydro segmentsfile andassigned an appropriate segment name river ID!. All the information was sorted andsplit into the variousrecords and each record type was added one at a time to the database. The database was then unloadedin a sequential format and written to magnetic tape,

Various retrieval programs were written to extract the necessary header information, temperature, salinity and dissolved oxygen DO! data, making use of the sort ID vari- ables defined in the schema. Tbe data were sorted by river ID, date, time, carcode an integer assignedto each station! and sequence number. A criterion was established to de- clare dissolved oxygen values of 0,0 rng/l or less to be low DO values. Salinity, temperature, DO, river ID, depth and date were written to 5pSS systems file for later analysis.

Preliminary Results

Oxygen levels less than 0 rng/1 occur throughout tbe Say proper and its subestuaries almost exclusively at water 'ernperatures above ca. 20C. At this latitude water tern- >eraturesabove 20C correspond roughly to a time span from ~ay through September. Despite the apparent threshold ffect of temperature on low dissolved oxygen, oxygen oncentrations appear not to be inversely correlated with .mperatures above 20C. 8 /Dissolved Oxygen inthe Chesapeake Inboth the York and Rappahannock, oxygenlevels I than4 mg/1occur primarily inthe lower 20km and 40 km therivers, respectively. These sections ofboth river~ ar characterizedbywater depths of up to 20rn in the channel,which isconsiderably deePerthan the > IPm d upriver.Theclose association oflow dissolved oxygen with thesedeep holes atthe river mouths isfurther substantiated byan inverse relationship between dissolved oxygen and waterdepth in the lower sections ofthe rivers. The3ames River differs substantially from the York andRappahannock inthat oxygen levels less than 4 rng/Iare rarelyobserved inthe lower river despite the presence ofa deephole ca. 20 m! at the mouth. In the 3ames River, low oxygenvalues are obser ved predominantly in the upper river,roughly between Hopewell andRichmond. During the 23year interval covered bythe data set, the relative fre- quencyof low dissolved oxygen observations in the upper 3amesRiver hasdecreased. Duringthe ten-year span 1971-1980, thedata indicate a gradationof hypoxic stress in theseries Rappahannock ~ York > 3ames.During this period,oxygen concentrations below4 mg/1in theRappahannock River were approximately equallydistributed above and below 2 mg/l.In theYork River,low dissolved oxygen concentrations were predominately between2 and4 rng/1. ln the 3amesriver oxygen concentrationsbelow 4 mg/1were predominately within the rangeof 3-4mg/1. This gradient of hypoxicstress is more clearlyapparent if the dataset is restrictedto the lower sectionof eachof the three rivers, since oxygenvalues less than4 mg/1were rarely observed in the lower3arnes River, Therelative frequency of oxygenvalues between 0 and 2 mg/lin theRappahannock River has increased consistently duringthe 23years that dataare available. Similartrends were not apparent in the York or 3arnes rivers. Theapparent lack of hypoxiain the lower 3amesRiver, comparedto the lower York and Rappahannockrivers, is perhaps surprising given their gross morphometric similarities, i.e., all three rivers have deep holes at the Recent Trends in Hypoxia / 9 mouth, and the fact that the 3arnesRiver receiveshigher loads of BOD and COD from anthropogenic sources than either of the other two rivers. Kuo and Neilson unpublished manuscipt! suggest that the environmental influence most consistent with the observed gradient of hypoxia in these three systems is the relative magnitude of the longitudinal salinity gradient in the rivers. They point out that The longitudinal salinity gradient, and hence the magnitude of the gravitational circulation, is strongest in the 3ames River and weakest in the Rappahannock River. Consequently, oxygen depletion of upriver-flowing bottom water, primarily via benthic oxygen demand, is least in the 3ames River short residence time! and greatest in the Rappahannock River long residence time!, giving rise to the relative levels of hypoxia observed in these rivers. Another potentially important factor may be the oxygen concentration of the bottom water entering at the mouth of each river E.P. Ruzecki, personal communication!. Because of its greater distance from the mouth of the Chesapeake Bay, deep water entering the Rappahannock River may be expected to have a lower oxygen concentration, owing to its longer transit time from the mouth of the Bay, than deep water entering the mouth of the 3ames River. These proposed differences in oxygen content of the bottom source water to each river could be expected to contribute to relative levels of hypoxia like those observed.

Both of these explanations for the observed gradient of hypoxia among the 3ames, York and Rappahannockrivers emphasize the important influence of physical rather than anthropogenicprocesses in regulating oxygendynamics in these river systems. At present, with phosphate bans, eutrophication, and low dissolved oxygen levels as primary indicators of water quality, it is important to remember that natural physical/hydrographical processesas well as anthropogenicinfluences may be importantwhen attempting to elucidate the dynamics of a hypoxia process. It is also important to remember that different processesmay act to varyingdegrees in different parts of the Bag no single modelwill applyto all regions.Many important parts of the puzzleremain to be quantified e.g., ratesof benthicoxygen demand and sources and rates of carbon supply to the sedi- lp / DissolvedOxygen in the Chesapeake

>ents!. The analysis of historical data can help to deter- »ne which processes regulating oxygen concentations are importantin a givenregion of the Bay, and which processes needto be more carefully! quantified to adequately elucidate the cause and possible amelioration! of hypoxia. OJ r5 J2 nf r0 V-~ 0 E

bO nf Urd U E

U LR CHESAPEAKE BAY tNAINSTEM AND TRIBUTARY NOH1TORING PROGRAM

Richard Batiuk EPA Chesapeake Bay Program

The Chesapeake Bay Monitoring Program, initiated in 3une l980, is a coordinated network that samples for a variety af water quality, sediment quality and biological resource parameters. The station network covers the entire ChesapeakeBay, from the Conowingo Darn above the Sus- quehannaFlats down to the Virginia Capes Figure l!. The goals of the program are to characterize present conditions, detect long-term trends in water quality for documenting the responseof the system to remedial actions, and assist in establishing the relationships between water quality and living resources.

Maryiand and Virginia Maiastem

Nineteen water quality parameters are currently measured at 28 mainstem stations in Virginia ancf22 mainstern stations in Maryland. Eight additional parameters are calculated using these measured values Table l!. Sampling frequencyis twice monthly from March through October and once monthly from November through February a total of 20 cruisesper year. Cruisesare coordinated between Mary- landand Virginia with samplingefforts mergingat an over- lap station off the mouth of the Potomac River. The entire 50 station network is normally covered in three days.

Temperature, salinity, dissolved oxygen and pH are profiledat all mainstemstations, Readingsfor thesepara- meters are taken at a maximum of 2 m intervals through the water column. Additional measurements at l rn intervals are taken aroundthe pycnocline,if present. Recent Trends in Hypoxia / l3

Grab or pumpedwater column samplesare collected for analysis just below the surface and l m above the bottom. ln addition, at all 22 Maryland mainstem stations and an axial set of 9 Virginia mainstem stations where stratification normally occurs, two additional samplesare taken above and below the pycnocline.

The rnainstern stations were situated to represent each of the ChesapeakeBay segmentsidentified during the research phase of the Chesapeake Bay Program. Additional factors guiding station selection included critical areas such as important fisheries habitats, areasexperiencing severe anthropogenicimpacts, areas experiencing prolonged periods of hypoxic and anoxic conditions, and locations of historical water quality stations.

Table I. %'ater quality parameters measured or calculated» at the ChesapeakeBay mainsternmonitoring program stations

Temperature A mmonia Dissolved oxygen Total organic car bon Salinity Particulate organic carbon" pH Dissolved organic carbon Secchi depth Total phosphorus Total Kjeldahl N Particulate P» Dissolved Kjeldahl N Total Dissolved P" Total nitrogen" Dissolved organic P Particulate N» 0rthophosphorus Total dissolved N» Total suspended solids Dissolved organic N" S ili con Dissolved inorganic N» C hlorophy 11a Nitrite Pheophytin Nitrite+ ni t rate

MaryhmdTributary Monitoring Maryland has a total of 55 tributary stations, 36 of which are sampledat the same frequency as the mainstem stations. The remainingl9 stations are sampledmonthly. The stationsthat are sampled20 times per year includeall 1«dOxygen in the Chesapeake le / Dissol« ntgiver and Easter~ Shore estuary stations, the the patuxen> >

The same parameters are measured at the Virg nia tributary stations as the mainstem stations except as noted below: Recent Trends in Hypoxia / l5

Specific Conductanceand Salinity every two meters! Total Kjeldahl Nitrogen unfiltered only! Total Organic Carbon DOC not measured! Total SuspendedSolids selected stations only! Fecal Coliforms upper DamesRiver! Districtof ColumbiaTributary Monitoring

The District of Columbia monitors 27 stations on the Potomac River, 28 stations on the Anacostia River and 22 stations on several smail tributaries. The samplingfre- quency is the same as for the mainstemprogram. The District's program is part of the Coordinated A,nacostia River Monitoringand the PotomacRegional Monitoring P rograms.

MarylandBiological Monitoring Maryland collects benthic samplesten times each year at 70 stations in the mainstem Bay and tributaries, The benthicstations were situated to coincidewith the long term Maryland Power Plant Siting Programbenthic monitoring stationsand enhance the temporaland spatial coverage of the e xisting program.

phytoplankton is sampled twice monthly at l6 stations in the mainsternand tributaries. Zooplanktoncollections are made monthly throughout the year at the same l6 stati ons.

VirginiaBiological Monitoring Virginia collects quarterly benthic samplesat a subset of 5 mainstemstations and ll tributary stations. The l6 benthic stations were located in the major salinity-sedimentary regions of the tidal waters within the major Virginia tributaries and mainstem. Phytoplanktonsamples are collected 20 times p«year at 7 rnainstemstations and monthlyat 6 tributary stations. Depth integrated zooplanktonsamples are collectedonce a month at all 13 of these plankton stations. d Oxygenin the Chesapeake 16 / Dissolve

f~bia BiologicalQoottoring District 4f + ln the Potomact macand pnacostia rivers and smaller ajoin- D strict of Columb;a inton tributaries,and phytop an t e samplesata subsetof stat monthly basis. ~~Vftsnd 5ediment Monitoring

Mar ylan d ollected co sedimentsamples ' 19 at the22 mains e io s andin 19 tarystations. Further sampling isbeing delayed for several years cue to lowow temporalvariation found under the current samplingf requency. Virg|niaSediment Monitoring y>rginiacollected sediment samples once per year at 3 mainstemstations since 1984and in l 985 at 27 tributary s ations. Further samplingis being delayed for several yearsbecause of low temporalvariation with currentsamp ling frequency. !.ceg-Termihaoitoring Program There are many other monitoring programs not described within this broad overview of the Chesapeake Bay MonitoringProgram. We selectedthe programsdescribed here becauseof their significance in monitoring the occurrence of hypoxia and anoxia in the Bay's mainstem and tributaries.

The fixed station network outlined in Figure 1 is only a part of an extensive set of other monitoring and research activities occuring on dif ferent spatial or temporal scales. The core programs describedhere are the framework of the more extensive Bay-wide monitoring program. Related short term scientific researchand monitoring activities can be linked to eachother throughthe longer, constant record of water quality andbiological resource information being developedby the coordinatedChesapeake Bay Monitoring Program. Recent Trends in Hypoxia / l7

Thereis a growingawareness of the valueof long-term ecologicalmonitoring programs, but is thereenough support to sustain and maintain this network of stations to answer the questionsmanagers and researchers are asking? The basis of that support must begin within the research communityitself. A key questionthat participantsin the MonitoringProgram must address in comingyears in the definitionof "longterm" in thecontext of managingthe ChesapeakeBay. Althoughwe needto commitourselves ta a set of stationsand an approachto monitoringwhich will continue with minimal alteration, there is a needfor minor refinementsof the currentmonitoring program. We must sustainthe coordinateddata collectionfor manyyears to comewith theflexibility to answerother data needs as they arise.

As we beginto understandthe temporalcharacteristics of DOand the mechanismsimportant in establishinglow DO, the water quality andbiological resource data collected by the monitoringprogram can further clarify these initial findings. However,with the currentbiweekly sampling regimeduring the critical months,we can'tmeasure many of theshort-term events often associated with seiching of thepycnocline. The monitoring program, as it is currently structured,was not set up to examineevents occuring on a temporalscale of lessthan a month. Collectively,we are beginningto examinethe use of long-terminstrument deploymenton mooredplatforms to improvethe temporal and lateral characterizationof the mainstem. The use of satelliteimagery to increasethe program'sdetection of shortterm events and to enhancetrend analysis is also being actively explored.

The Monitoring Program becomesweakest when we leavethe planktonand benthic links in the foodchain and proceed to what is often most important to us as users: finfishand shellfish. Special habitat monitoring programs havebeen set up to providethat linkage. How we can bettermonitor habitats and biota to detectthe biological effects of hypoxiais a questionswe hopethe joint NOAA/SeaGrant program will beginto answer. 77DI DM 76010M 75DDOM 7BDIOII >BD <5II >BPO5II

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FIgurc 1. Maryland,virginia and D.c. mainstem, tribotary andfall line waterquality monitoringstations. HYPOXIAIN CHESAPEAKEBAY: RESULTSFROM THE MARYLANDOFFICE OF ENVIRONMENTALPROGRAMS' CATERQUALITY MONITORING, 1984 - 1986

Robert E. Magnien Office of EnvironmentalPrograms Mar yland Department of Heal th

TheMaryland Office of EnvironmentalPrograms initiateda multidisciplinary,Long-term water quality monitoring programin 3une,1984 that encompassedMaryland's tidal ttibutariesand mainstem of theChesapeake Bay. Several largescale, seasonal features concerning deepwater hypoxia were evident from the monitoring data as well as obser- vationsindicating that significantchanges in oxygencon- centrationswere occurring on timescales of hoursto days. Despitethe fact that the relativelyrapid changes in Chesa- peakeBay hypoxia were not completelyresolvable by this monitoringprogram which samples twice-monthly, strong inferencescould be drawn about the associated processes. Developmentof hypoxia was rapid in thespring of each year as dissolvedoxygen levels generallydeclined to less than1 rnid'1by 3une in theupper deep trough region. The upperdeep trough exhibits the mostsevere hypoxic/anoxic conditionswith onset and dissipation of theproblem occurring in this areafirst andlast, respectively,relative to lowerrnainstem areas south of thePatuxent River. During the highflow summerof 1984,hypoxic waters of Lessthan 1 mg/1extended well into Virginiamainstem waters while in 1985and 1986the affectedzone was more restricted to Marylandwaters. Strong mixing usually occurred in Sep- tember, dissipatinghypoxic conditions. In the October throughNovember period of 1980- 1986, dissolved oxygen concentrationsdeclined once again by oneto severalparts per millionf romearlier levels bef ore increasingagain duringthe winter. These autumn dissolved oxygen declines 20 / Dissolved Oxygen in the Chesapeake resulted in transient levels as low as 2 rng/1 in bottom waters. Dissolved oxygen concentrations in deep-trough mainstem bottom waters were quite variable although generally below2 mg/1from 3une through early September.Physical processesapparently dominated the short-term changesin dissolvedoxygen concentrations. The degree of stratification in the deeptrough region, as measuredby the change in densityfrom surfaceto bottom,was particularly variable from cruise to cruise through the course of each year, althoughgenerally higher in springand summer. Springand summer of 1984 exhibited the highest sustained density stratification observed since 1983. The relatively rapid changesoccurring in the physicalstructure of the water column are indicative of changesof freshwater input and vertical mixing. The monitoring program was also able to document some important processesoccurring in the lateral or cross- Bay direction. A major reaerationevent was observedin the upper deep-troughduring a cruise on july 10 - ll, 1980. Combininginformation on dissolvedoxygen, salinity, density and nutrient concentrationsin the vertical, lateral and longitudinaldimensions with local wind records,it was possible to evaluatehypotheses concerning the sourceof the dissolved oxygento sub-pycnoclinewaters in all three dimensions. This evaluation led to the conclusion that north to northeasterly winds causedthe pycnocline to tilt upward on the EasternShore bringing sub-pycnocline waters suffi- ciently close to the surface to become reaerated. A sub- sequentshift in winds to a southwesterlydirection brought the newly reoxygenatedwaters -0 mg/1! into the rnid- channel as the pycnocline tilt reversed. The lowest dissolved oxygen concentrations in sub-pycnocline waters were advected to the western shore. Thus, reaeration occurred without destratification. Additional evidence for the importance of lateral processescame from dissolvedoxygen measurementsin embayments and lower tributary areas. These regions experienced occasional intrusions of high salinity, low Recent Trends in Hypoxia / 2I

dissolvedoxygen waters from the mainstem. These events are almostcertainly the resultof wind-driventilting of the pycnocline that permits sub-pycnocline waters f rom the mainstemto intrude over the sills that are typically found at rnainstem-tributaryboundaries. !n onecase, an intrusion wasobserved as far upstreamas Cambridge on the Choptank River.

An importantseasonal feature in the onsetof hypoxia was a large pool of algal biomassthat developedfrom Decemberto Mayof eachyear in bottomwaters of thedeep trough region. This pool reached typical concentrationsof over 30 og/l of active chlorophylla throughextensive reaches of the mainstem along with similar concentrations of pheophytina. Resultsfrom the phytoplanktonand processes components of the monitoring pragrarn shed some additionallight on the accumulationof highalgal biomass. Diatomswere an importantpart of the phytoplanktoncam- munityduring winter and spring. Sedimenttrap data indicatedthat this sameperiod was characterized by higher proportionsof primary productionreaching bottom waters whencompared to summermonths. Thesesettling algal cells were probablyable to survivefor extendedperiods belowthe euphotic zone in winterand early spring because of law temperaturesand high oxygen levels. ln May,this algalpool rapidly diminished coincident with the greatest decreasesin bottomwater dissolvedoxygen. Thus, it is likely that the decompositionof late winterand spring phytoplankton growth that has settled to battom waters and sediments,coupled with spring increasesin stratification and temperatures,is a major contributor to the establish- mentof hypoxiain the deeptrough region, Similarly,phy- toplanktonblooms in October,with diatomsagain an important part of the community,may be an importantcause af the transientdecreases in oxygenlevels observed in the fall. geweJJ: We've heard a lat of presentations which refer to mg/l oxygen, while some have presented data which refer to "corrected"mg/l oxygen. Pm nat sure what is meantby that. Sincetotal mg/l of oxygendoes change with tempera ture, surely a better way than absolute concentration is percent saturation because DO differences between dif ferent river systems could be just due to temperature differences. What does the audience feel about this? Magnien:Corrected mg/l meansit's correctedfor salinity and temperature. Some of the field instruments da ret correct automaticallyfor these. Certainly the temperature patternstend to reinforce low dissolvedoxygen patterns that we see. Haas: ln the York River, the longitudinaltemperature gradientfrom the mouth to Westpaint is less than 1 C. Vertically,in the summer,there is nat that much difference.1 don'tthink the temperature gradient is a veryhigh magnitude.I don'tknow how much effect that wouldhave an DO probably very little. Mountfard:1 have a concern with using percent saturation in this contextbecause we' ve donesome exercises in the pastand when you start gettinga lat of photosynthetic activity,the oxygenvalues are sometimeswell aver 100% saturation.You start gettingexcursians that are relatedto biolagicalactivity in the watercolumn that are hard« interpretand wauld end up confusing thepicture more than clarifying it. Ncwell:Da you see same DO values more than fully sa«r- atedd? Mountfard: Correct we might see 1 15%ormore. RecentTrencls inHypoxia / 2~

indicateewell:whetherl would thethink water that w

CM refusedto usean oxygenmeter below about 0.5 mg/l becausehe flat out did not believe the data, Magnien:There are argumeots both ways:one could say measuringDO in situ hasan advantagebecause you' re not riskingcontamination of the samples.With the Winkler technique,you have to bringa sampleup and we found whey youuse a waterbottle or pump any method-- it is going to risk contamination. I think that's essentially the problem

Tuttle: There was a study done in the Black Sea in i9gp evaluatingthese things. lf you use a Niskin bottle, the loss of hydrogensulfide is quite minimal and if you haven'tgpt hydrogensul fi de, oxygen doesn't change. So there are oceanographicconfirmations of samplingtechniques.

3ordan: For the large scale processes and even for ammoni um and phosphate flux, does it really matter whether we know if it is zero?

Tuttle; lt certainly does to the animals.

3onas: l think we're going to see later on today, with regard to microheterotrophs, there is a very substantial difference between 0.5 mg/l of oxygen in the water column and zero oxygen. Zero oxygen means a lot to trophic dynamics, particularly in regard to rnicroheterotrophs, and in the type> ot metabolic activities that are going to go on. Probably mare redox potential phenomenathan oxygen per se, but it will make substantial differences.

We can make a big case about a little bit of oxygen and zero oxygen crabs may not go there in any event, but as far a> dynamics driving the system, it will be important. There- fore, measuringand confirming this anoxia is very important.

Tuttle: We've seensome interesting data here concerni~S oxygen dynamics. Going back to the 1950s, there is @ enormousCBI data set. Could we not have figured out t"< same dynamics from the l 950's information instead? R ecentTrends in Hypoxia / 25

question:How many cruises were in a givenyear in the 1950s? Magnien:My impressionis that, in a givenyear, you' re probabiylimited to 2 or 3 cruises,so the resolutionwooldn't be too good. Tuttle: Myother question is thatafter hearingall this,is thehypoxia situation getting worse or is it gettingbetter? t=urthermore,haven't the physicalconditions been the same for at least the last half a century? Magnien:Certainly we' re addressingthe historicalperspec- tive in the next coupleof years. EPA did it, Taft did it a numberof yearsago. Wewant to revisit that,use some moredata, put togethera pictureof the historicalrecord as bestwe can. Wewant to do it carefully becauseof all the problemsthat havebeen raised here. Certainly,factoring outphysical factors would be a criticalelement. Maybe the patternwill be so strongit will overwhelmthe physical dynamics,though that remainsto be seen. Haas: !n that regardwe' ve seena lot aboutthe different flowconditions in 1980and 1985 and the effecton vertical stratification.In theYork River at VIMSwe do plankton monitoring,which is in thevicinity of the deepbole in the lowerriver where we have oxygen problems. During April andMay, 1980, the salinitieswere 10-12 ppt and we had consistentlyover 50p g/1of chlorophyllin the watercolumn inthat vicinity. In 1985, at thesame period, with salinity at 18-20ppt, there was no spring bloom. Unfortunately, I don' t havethe oxygen data to saythat those differences in pro- ductionreally resulted in differences in oxygen inthe deep river.Is somebody going to saythat for theBay? When you havethese big differences in flowconditions which resulted in differentamounts of stratification, did they in factresult indifferent production conditions forthe spring bloom or is theBay so big that the spring bloom in a highflow year is furtherdown Bay but still impacting the main part of the Bay?Are the differences in 1980 and 1985 DO due to a greaterdegree of stratificationfrom the highflow in 1980?This seems a critical question in trying to sort this out. >6 I Disso lyed« Qxygenin the Chesapeake

Laff y we don'thave primary product i on data f rom Sellner: j9g4 our part of the programdidn't start until ear ly l9 I 9/4 so we just havethe high summerproduction PLugust0 period. ge have!august I984 on but we missedthe critical 'ngbloom Q fI 984 pI'is h er: . p commenton the "spring bloom" we' ve beenhear- Productivity increases from winter through summerand drops off again,There is no part icularlywell- definedspring bloom like wesee in a lakeor ocean.Highest productivitiesoccur in warmestmonths. The slide that Rob M>gnienshowed illustrated that. If you're talkingabout settlementof fecal materials,maybe a spring pulse of that secondaryproductivity in surface water is feeding a maximum summer oxygen demand.

Haas; Rates of production will be highest in the summer whenlight and temperatureare the highest. What I think Sob is getting at, is that the spring bloom may not be a "bloom" of production, but an accumulation of biomass becausewe' re talking about large diatoms that aren't grazed murh by zooplankton; that's probably the reason settling out is so great. High productionin summer is mostly small flagellates that are grazed in the microbial food web and they probably don't settle out as much.

But there is certainly a tremendous peak of phytoplankton biomass in the lower York River in springtime. This may not be associatedwith a higher rate of production per ~se maybe just an absence of grazing pressure which results in greater accumulation of biomass. Certainly, the highest sustained levels of biomassthroughout the year occurring in tl~ lower Bay and the tributaries of the lower Bay are in spring, ~here chlorophyll levels are five times higher than theyare duringthe restof the year, I'i»«: Thatmay be theresult of shifting patternsof where the rhlorophy!lmaximum is. It maybe pusheddownstream andthen move back upstream in responseto flow. Haas: That's a possibility, But evenin summertime,the highest accumulations of biomassare still in the frontal Recent Trends in Hypoxia/ 27

regions set up between the rivers and Bay. I think there is a pulse of biomass available for settling out in spring that is not available in the summer, Fisher: I just wanted to raise the issue of productivity versus material settling out.

Haas: Right, that's a good point.

Comment: Some decomposition work I did on freshly settled particulatesshowed dramatically higher rates of decomposition in May. The only thing that was associated as far as I could tell wasnot necessarilya bloombut very htghper- centagesof organic matter in the suspended material which waspresumably settling out. And the oxygenconsumption rates were dramatically higher than any other time of year.

Magnien: One other point I wanted to make is that most of this material accumulatingin bottom waters duringlate winter and springis living, whichindicates it's probably fairly labile. It may be importantthat in the springthe carbonmaking it downis "ready to process"and that it's a bit morerefractory at other timesof the year. 3onas: All of the Bay literature with regard to organic demandon oxygen centers on particulates. Last year during the 1985season, we were able to run engineering-type biochemicaloxygen demands on both unfiltered and filtered water samples,filtered with GF/F glass fiber filters. There actually is a shift over the seasonfrom the dominant labile poolbeing particulate earlier on, to beingwhat we' ll call "dissolved"for the rnornentand which passes a GF/F filter. Sothere is a shift, andthe full oxygendemand may be shiftingover to thedissolved portion. That's simply being missedwhen you look at particulatesettling rates. Comment: I saw that, too. Mountford:Bob, does that meanmaybe we shoulddo some- thinglike BODsin monitoring? 25/ DissolvedQxyge»n the Chesapeake

ldnt argueagainst that. It's a»mpl««hnique gonas'd in I wouengineering. The problem, like the oxygenone, is youare very careful the variancein the tech- thati 'll unless l youave ayou with oxygen values all overthe place. It'sone of thosethings that requires hand manipulation and t are becausewe' re lookingat sometimesonly 1.0 mg/Itotal demand ona particularsample and that means a dayimubation in the dark and there are a lotof logistical problems.Whether or not the big monitoring effo't can h Ale that, well, my argumentis that there is no Eood substituteor alternativeunless you wantto go to theextent of measuringtotal carbohydrate or amino acids.Maybe amlytically,if onewere tooled up to do that, thatwould be cheaperand easier in the longrun. DucklowIf youwant the simplesttechnique to addressthat question,just count bacteria. It is precise,very easy and you canput themaway and do them later. And it correlateswith all that stuff. Mountford: Thequestion that Larry asked is seminal to the whole thing. He was talking about whether production in l980 and I98f were measurably different. Rob Magnien may secondthis or argue with it, but scanning the surface chlorophyll data I see no such pattern between the two years; but in thinkingback on the figure that Rob showedof bottom chlorophyll, I wonder if you could do a mass comparison over those two years and get a feeling for whether the subpycnoclinal chlorophylls were different from one year to another, It couldbe worth looking at.

Sellner: We do have that data; it is in our report. Getting back to this whole question and the point that Tom brought up if you look at standing crop, not production, there are two distinct peaks; one that Rob had as his bottom chlorophyll that is principally the prorocentrum recirculation phenomenonthat Mary Tyler and Howard 5eliger talked about some time ago, and also diatorns. If you look at depth- integrated chlorophyll, they are higher in the winter than in summ««me 5o in fact the standing crops are higher at that time, and if you look at speciescomposition of the phytoplankton, the bottom chlorophyll is principally the large RecentTrends in Hypoxia/ p9

shelftype diatom assemblages.Then as you shift from summerto the late fall peakthat Robrefers to, thereis anotherdiatom pulse that is notshelf-related, but is truly estuarine.But there are two distinct standing crop peaks: onein late winter-earlyspring is principallydia toms andprorocentrum-like cells, and then the October peak is diatoms. Malone:I don'tthink you can say a lotabout productivity basedon chlorophyll. The interannualvariations in surnrner productionare pretty clear. The productivity in the summer of I954 was much higher than in the summerof I955. Both seasonaland interannual variations in production in the watercolumn and oxygen demand compared quite well. I'd like to get backto the original monitoring!data. I am justoverwhelmed by the richness of thatdata. The question I reallyhave is, do youthink the sizeof thisprogram will affect howlong you will be able to run it? Andif so,are thereany plans to objectivelyscale down the program so that it will provideyou with consistentinformation, but over a longer period of time? Batiuk: I wouldanswer that with a question:How many years of data do we need to take a look at the current structureof the mainstemand tributary program,and then to go aboutreevaluating to say we can dropthis stationand keepthese others? We've beengoing back and forth in termsof getting an agreement,and talking aboutwhat the fedswill put in andwhat states will takein overthe long run.We would like to keepit asit is a lot of peoplesay it is a minimum but youhave to berealistic that you may notbe able to sustainsomething this large.But we would wantto keepit for a goodcouple of yearsor moreso we can makesome careful scientific judgments on keeping different stations.That is, keepit asit is aslong as we can, and then whenwe are more comfortable, pull back a bitand perhaps supplementwith remotesensing, e.g., buoyson lateral stations,or try andsupplement it in someother ways. I wouldnot like usto seeus chop it upnext year, for exarn- Ple. We'll try to developa scientificbasis to makeman- agementdecisions on the program. I wouldlike to seeit go lo or 20years, but wehave to be realistic. 30/ DissolvedOxygen in theChesapeake

Wedon't have enough data to saywhat stations are or a« notuseful now. That's the kind of giveand take we need fromthe scientific community, from people that are usin< thedata. We may need to emphasizesome areas, and there maybe areaswe can pull back. 3ordan:This monitoring program may seem expensive -- but comparedtoresources thatare u/timately going to be pot intorestoration of the Bay, it isa dropin thebucket. thefinancial considerations maynot be so critical. Malone;l don'tsee that you can maintain a monitoring programofthis site for a decadeortwo. A long-termtime seriesis a lot moreimportant in the long run than high resolutionspatial coverage, for example.There are a lotof reasonsfor not being,able to maintainit forever. Energy, for one. Magnien:When you get into the details, you see that there is sometimesan economyof scale. You say, "v,e'Hdo half as manystations," but in reality whenyou get the boat out thereand you haveto coverthe samedistance, what's the incrementalcost of taking anotherDO profile and taking samples? ln certainaspects, w'e may have too much detail, and in other aspectsthere may be parameterswe' re missing. l think we probablyhave pretty reasonablecoverage. This doesn't mean we can't continue to evaluate and maybe scale back certain aspects or expand others. l would also agree with Steve'spoint l don't think we're over-resolving the systemso that we knoweverything that is going on all the time. Manyaspects of the programare not as intensiveas the chemical suite of stations that represents sort of a "backbone." But when you look at plankton, there are only l6 stations and only one station where vertical flux measurements are taken. Some aspects of the program we've scaled back initially because of cost. We're hoping that the "backbone" will be a way of interpolating some« the reduced subset of measurements, along with research projects, of course, so we can get more of a corn- plete picture. Again, costs may seem large in termsof Recent Trends in Hypoxia / 3l

researchgrants, but look at the resourcesbeing expended pn the pay in Pennsylvania, Maryland and Virginia. We're ~aking progress and it seems like a worthy investment tp ensure that we' re on track and continue on track. summers: At least for the present. Since I' ve been at OEp fpr three years, monitoring really has taken off. We have this big program, and we still have people from counties with more localized concerns coming in saying"this is top broad;we can't do anythingwith it," and sothey' re actually fundingadditional monitoring on top of this at a higherlevel pf detail. So it is still growing, it hasn't gotten scaled back, D1SY VCAOxyggI1 Processes CHEAP-SPRINGTIDAL EFFECTS ON DISSOLVED OXYGEN ~Q RIVER-BAY INTERACTIONS IN THE LOWER YORK RIVER

Leonard W. Haas virginia Institute of Marine Science College of William and Mary

it is now well established that the lower York River undergoes a predictable and persistent oscillation between vertical density stratification and homogeneity in close association, respectively, with neap and spring tides Haas, l977; Hayward et al., l986!. Wind and mean sea level play a minor role in regulating this phenomenon and freshwater river flow appears to have no effect on this process Hay- ward et al., l 986!. The portion of the York River which undergoes this oscillation is also subject to chronic seasonal hypoxia stress, which is usually first apparent in May, most severe in August and dissipates in September ordan, l979!. ln the one instance in which the effect of the fortnightly tides on the vertical distribution of dissolved oxygen was examined, it was observed that destratification resulted in an increase in bottom water oxygenfrom about 0.5 mg/I to about 0 rng/1 in a maximum of five days, The rapid subsequent restratification of the lower river was coincident with a decreasein bottom water oxygen to less than I mg/I Webband D'Elia, 1980; O'Elia et al., 1981!.

ln addition to a direct effect on the vertical distribution of dissolved oxygen in the lower York River, the process of a fortnightly, tidally-driven, density gradient oscillation also illustrates the close hydrographic coupling between the lower York and the ChesapeakeBay. The onset of spring tide destratification is triggered by an influx of le» sa4ne water from the Say into the lower York. This w««originates upbay and is advected into the river as a result of a tidal phase shift in which flood tide in the river geoin theChesapeake 36/ DissolvedOxyg dent with the endof ebbtide in the Bay. mouth is coinci des thi»nf lux « less salinewater to the Durin springti need andcauses a reversal in the longitu- owerriver is e> djent + in t"e lo ver river, which diminishes gravitationaldinal salinity gr~culatio»nd allover's spring tidal mixing educevertical de»it y stratification Hayward processest l 1982!to re The < restratification of the lower river follow- tidesresults from the rapidadvection of high lngli spring 't terti esfrom the ChesaPeake Bayalong the bottom of salinitythelower waterYork rRiver Ruzecki and Evans, l986!. Thus, both det t'I'cationand restratification in the lower York R dearestra regulated t i ica ionby hydrographic Processes occurring betwe th«iverand the Bay~ not bY processes occurring u river. Th»significance of these observations related tp poxiaProcesses in the Chesapeake pay widelyrecognized seasonal cycle of hypoxi shortterm alterations in dissolvedoxygen do occur p p esent,it is not clearto whatextent this densit o il- lationand possible effects on dissolved oxygen distributions occurin the lower Rappahannockand 3amesrivers or the lowerChesapeake Bay, but one should be cognizant of the possbilltyof short term variation in deepwater dissolved oxygenconcen trations. Furthermore,these observations illustratethe close hydrographicassociation of lower river- ChesapeakeBay processes.In Virginia,chronic hypoxia in the York and Rappahannockrivers is restricted largely to the lowerportions of eachof these rivers, andthe possibility that hypoxia is affected by ChesapeakeBay processes or conditionsshould not be ignored.

References

O'EliasC. F.i K- I.. Webb and R. L. Wetzel. 1981. The varying hydrodynamicsand water quality in an estuary pp >'97-605- In; B. 3. Neilson and L. E. Cronin eds.!. international Symposiumon Nutrient Enrichment in Estuaries. Humana press,Inc., C.lifton, N, 3. l977. The effect of the spring-neaptidal cycle on the vertical salinit structure of the 3ames York RaPP+hannockRiver, Virginia, U.S.A. Estuar. Coast Mar. S I > g8~O96 DissolvedOxygen Processes / 37

Hayward,D. M.~ C. S. Welch and L. W. Haas. 1982 York River destratification: An estuary-subestuaryinter- action. Science 216:1413-1414.

Hayward,DL. W. Haas, 3. D. Boon,III, K. L. Webband K. D. Friedland. 1986. Empirical models of strati fication Variation in the York River estuary, Virginia, U.S.A pp. 346-367. In: H. j. Bowman, C. M. Yentsch and W. T. Peterson eds.!. Lecture Notes on Coastai Estuarine Studies, Vol. 17, Tidal Mixing and Plankton Dynamics. Springer-Verlag, Berlin, jordan, R. A- 1974, Lower York River dissolved oxygen study May-October 1974. Virginia Institute of Marine Science, Gloucester Point, VA 23p62

Ruzecki, E. P. and D. A. Evans. 1986. Temporal and spatial sequencing o f de stratification in a coastal plain estuary. pp. 368-389. In: H. j. Bowman, C. M. Yentsch and W. T. Peterson eds.!. Lecture Notes on Coastal and Estuarine Studies, Vol. l.7, Tidal Mixing and Plankton Dynamics. Springer-Verlag, Berlin.

Webb, K. L. and C. F. D'Elia. 1980. Nutrient and oxygen redistribution during a spring-neap tidal cycle in a temperate estuary. Science 207:983-985. mmUSIONOF I OWDHSOZVFD OXYGEN %ATER INTO THP CHOPTANKRIVER

La~~ P. 5anfard HornPoint Environmental Laboratories, CEES University of Maryland

Oneaspect of the apparentincrease in the persistence and extent of low dissolvedoxygen DO! in the deep meso- halineChesapeake Bay over the last 30-00years has been an apparentincrease in low DO in adjoiningtributary estuaries. The causeof low DO in the deeper waters of these tributariesis not fully understood.One hypothesisis tbat it results from local processes, i.e., the same processes re- sponsiblefor low DOin the rnainstemlead to the formation of pocketsof low DO in tributaries. Alternatively, Seliger et al. 985!, havesuggested that DO effectively acts as a traCerfor the deepwater af the rnainSte, and is adveCted directly into tributariesunder the proper conditions.

The Choptank River estuary is an example of a corn- merciallyproductive tributary that may be impactedby episodesof low DO. Previousmeasurements of hypoxic or anoxic conditions in the Choptank, however, have been too spatially limited and too infrequent to adequately establish the extent, duration or source of the low DO. To better addressthese questions,the Maryland Department o< Natural Resources and Horn Point Environmental Laborato- ries carried out a joint researchstudy during surnm« l936~ incorporating both a water quality monitoring effort throughoutthe lowerChoptank at roughlyweekly inter«ls and a program of mooredobservations in the Chop«n" entrancechannel. This presentationis a description « the preliminary results of that investigation.

The study's primary hypothesis is that intrusions of lo+ DO from the lower layer of the Chesapeake Bay «oj' DissolvedOxygen processes / >9 duringepisodic, wind driven surgesof lower layer water into the Choptank.Steady advection of low DO by the density drivenestuarine circulation is not as likely. This hypothesis is basedon the previouswork of Wardand Twilley986!, +ho only measured low DO on the bayward side of the broad shallow sill at the mouth of the Choptank; on the work of Boicourt et al. personal communication!, who showedthe lower Choptank to be strongly wind forced; and simple considerations of the topography of the lower Choptank. Lower layer water from the mainstemof the Bay only reach the Choptank through the old entrance channel, bounded on the Northwest side by Sharps!sland Bar and on the Southeast by the neck district of Dorchester County. The channel depth varies between lQ and 2Gm, and the channel is about 15 km long. The sill at the mouth of the Choptank is at the head of the channel, and at 8 rn depth is shallower than the mean depth of the summertime pycnocline in the mainstem. Surges of lower layer water up the entrance channel might be forced by some combination of local wind driving and the remotely forced cross-Bay pycnocline tilting events reported by Malone et al. 986!. Tidal mixing is presumably much stronger on the sill than it is in the entrance channel; thus, low DO is more likely to persist longer and affect a greater portion of the river during a surge than it is during steady, slow advection.

Moored observations were made at three stations: one at the outer end of the Choptank entrance channel, one at the inner end just bayward of the sill, and one on top of the sill south of Broad Creek. Current speed and direction, temperature and conductivity were measured at aH stations. In addition, an attempt was made to obtain long time series of near bottom DO at the inner two stations, u»ng new Endeco pulsed DO sensors. The water quality monitoring program covered the entire lower Choptank from the Cambridge bridge to the outer end of the entrance channel with 26 stations.

preliminary results indicate that our primary hypothe»s is correct, with the caveat that 1986 was the lowest river flow year in the last seven for the Choptank drainage ba"n T. F'isher, personal communication!, so that the density 40 / Dissolved Oxygenin the Chesapeake

driven circulation was weak. Results of the monitorinl program show that DO is highly variable near the mouth o the river; near bottom DO on the Bayward side of the sil changed by as much as 6 mgj'I over a one-week interval The two time seriesof DO obtained from the mooring on th~ sill, though limited in extent, also show a great deal o variability, with near-bottom DO levels changing by almos' 6 mgII'1 in 6 h. During apparent intrusion events, characterized by hypoxic water upthe entrance channel and slightly over the sill, DO was highly spatially conservative wit> salt, indicating advection from the Bay. This conservative behavior of DO with salt also is apparent in the time serie. of near bottom DO from the sill mooring. Advected hypoxia was apparently confined to the immediate vicinity of the mouth of the river, however, and to depths greater thar about f m. In contrast to the behavior of hypoxia near the mouth of the river, an area of near bottom hypoxiain the deep water near Castle Haven was fairly constant, and possibly can be attributed to local processes.

A tentative model for advective surges of low DO near the mouth of the river is of 1-2 day intrusion events followed by 3-0 day mixing and reaeration periods. The surges are clearly related to the wind, but depend in a complex manner on wind direction and on the pattern of wind variability. Further analysis of existing data and more observations during l987 should help to answer remaining questionsand provide a complementaryview during a higher river flow year,

References

Malone, T.C., W.M. Kemp, H.W. Ducklow, W.R. Boynton, 3.H. Tuttle and R.B. 3onas. 1986. Lateral variation in the production and fate of phytoplankton in a partially stratified estuary.Mar. Ecol. Prog. Ser. 32:149-160. Seliger, H.H., 3.A. Boggs and W.H. Biggs. 1985. Catas- trophic anoxia in the Chesapeake Bay in 1984. Science 228: 70-73. Dissolved Oxygen Processes/ Vl

%'ard,L.G. andR. Twiliey. 1986. Seasonaldistributions of suspendedparticulate material and dissolved nutrients in a coastalplain estuary.Estuaries, 9 !:156-168,

Dissolved Oxygen Processes / 43

Mountford: It seems as you move up the Bay maybe tidal amplitude is enough to drive the spring and neap mech- anisms Chris D'Elia looked at it in the Patuxent and found it kind of ephemeral.What do you think about the Potomac, where USGSdid a lot of work. Does it happenthere?

Haas: I have some information from USGS I'm not sure if it's actual data or from a model-- but it seemsto suggest that certain parts of the Potomacundergo this oscillation. The York River seems to be the one river where it is well documentedin time and space. The james, Rappahannock, Potomac and maybe the Patuxent give indications that sometimesit might be an influence.It is somethingyou have to keep in mind in your samplingprogram and look out for from a practical point of view whenyou' re studyingthese kinds of vertical processes. Boicourt: I can backthis up, althoughI will disagreewith that particular model becauseit was run on the hydraulic modelat Mattapeake,which is not an appropriateway of lookingat vertical mixing;however, the neap/springvaria- tions we see in the Bay proper and the Potomac are reduced f or the reasons that Larry Haas has talked about and reducedover what we seein the lower Bay tributaries Mountford: Is this, then, a nutrient pumpthat hassome sort of frequency of moving material up into surface layers? Doesit affect phytoplanktonstanding crop?

Boicourt: Probably not in the Bay proper. Haas: Whilenutrient concentrationslook very high in the deepwater, if you look at the cross-sectionalarea, it's a relatively small volumeof deepwater comparedwith the cross-sectional area above the pycnocline and so there is a tremendous dilution effect when you mix the bottom water up. Mixing appearsconservative; in other words you have just as much material per unit area through the water columnbefore and after mixing,so there is a largedilution effect. So the impact of the nutrients may not be that great on the surface water. Probablyfarther up the tribu- tariesyou don't have such high concentrations in the deep 0Q/ Dissolved Oxygen in the Chesapeake water as yoU do downstream; the dilution effect would be less as you go up river.

Kemp: Prn not sure if we should leave this issue with that final statement, because I think there are indications that there is some impact. It's pretty fuzzy at this stage, and maybe Tom should comment on this but, after tilting events for examplein [984 whenwe had a pretty gooddata set!, there is an indication of increased chlorophyll in the flanks -- in shallow stations to the side of the channel. But you have to use a time lag, and I don't know what the appropriate time lag is. Clearly, the concentrations of nutrients in the shoal water are substantial following such events-

Malone: One of the puzzling things we ran into is that while increases in chlorophyll appear to be related to nutrient supply, nutrients are rarely exhausted and usually occur at concentrations that probably do not limit growth rate.

Haas: The whole time that that }982 data set was taken for salinity at the Coast Guard pier, we also took chlorophyll and cell count data with the express purpose of linking Up phytoplank ton dynamics with physical dynamics. I was hoping the relationship would be so dramatic it would hit me over the head. But it takes a lot of work to get the signal out and that's why it isn't published yet. There may be an influence and there may not be but it is certainly not dramatic. PHOSPHORUSCYCLING AND NUTRIENTLIMITATION IN THE PATUXENT RIVER

ChristopherF. D'Elia ChesapeakeBiological Laboratories, CEE.S Univer sity of Maryland

No singleissue in the ChesapeakeBay region has focused the attention of the public, federal and state management agencies and elected officials more than excessive nutrient enrichment and its consequences.Sea Grant's presentinterest in the studyof hypoxiarelates directly to this issue,although in the eyesof thepublic especially, the conceptuallink betweennutrient enrichmentand hypoxia is unclear.The present abstract considers this relationship andthe relevance of someongoing research and monitoring efforts.

Interest in the nutrient enrichmentproblem has typically centeredon the freshwaterportions of the tributaries. For example,the Potomacclean-up effort has attempted, with a disputed degreeof success,to reduce algalbiomass and the presenceof noxiouscyanobacteria in theregion just south of theWashington metropolitan area. As concernhas shifted to theproblem of hypoxiain the lower reachesof westernshore tributaries andin the main- sternof the bay,so too has interest shifted to understanding possibleanthropogenic factors that cause it. In theeyes of somemanagers, the transportof allocthonousoxygen-de- rnandingmaterial to the salineportions of the estuaryis at fault. Tothem, the obvious solution is to reducedischarges withhigh BOD and to establishnutrient reduction strategies aimedat controllingthe growth of algaein freshwaterareas of thetributaries. The nutrient reduction strategy of choice of freshwaterareas has traditionally been based on phos- phorusremoval at sewagetreatment plants. 46/ DissolvedOxygen in the Chesapeake

Manyin thescientific community have argued instead thatwhile such steps may be necessary, they are not sufficient for dealingwith the degradationof salineareas: onlyautochthonous primary production in theseareas is capableof producingsufficient organic matter to be re- sponsibleforoxygen consumption in the deep waters of the estuary.Accordingly, steps must be takento understand nutrient limitation and dynamicsin these areas. The PatuxentRiver is a convenientmodel subsystem to studyenrichment effects for a varietyof reasons: I! it is well-studiedhistorically and is believedto be exhibiting signsof moresevere hypoxia inthe last several decades; ! its sizemakes it relativelyeasy to sample;! it is located in thefast-growing area between Baltimore and Washington, andthus is receivingmore point-source inputs; ! goad estimatesof pointand non-point source nutrient inputsare available;and ! it is particularlyaccessible for studyby scientistsfrom both ChesapeakeBiological Laboratory CBL! and the BenedictEstuarine Resear ch Laboratory BERL!,which are both situatedon the estuary. A jointstudy by BERL and CBL scientists, supported in partby SeaGrant, has shown that nitrogen is thekey ele- ment that stimulatessurnrnertime autochthonous prima.ry productivityina sectionofthe Patuxent just downstream of the turbiditymaximum. If, in fact, summertimeproduc- tivity is coupledclosely with oxygen-demandingprocesses that contributeto hypoxiain the lower Patuxent,then a nitrogen-basednutrient strategy must be adoptedto irn- provewater quality. However, several important questions remain unanswered: 1. Canphosphorus be made limiting'? Many contend that althoughP is not nowlimiting, a phosphorusstrategy will reduceP levels enoughto causeit to be. This shouldlargely depend on the degreeto whichP is retained in the sedimentsof the estuary and is cycled within the water column. Historical evidencefor many temperateestuaries suggests that sedimentreserves of P elevate water column concentrations in the summer; this processappears to bestimulated by hypoxia. Thus, Dissolved Oxygen Processes/ 07

feedbackmay exist in whichthe settlingof particulate matter and associated BOD results in P enrichment of the water column. If a P strategy can limit productivity adequately to interrupt the feedback,it shouldbe effective. Prediction of this will require ! under- standingof the relationship betweenprimary productivity, sedimentationand benthic oxygendemand and nutrient regeneration, and ! appropriatemathematical models and nutrient budgets see below!.

2. Are the kinetic equations used in models correct? Water quality models may provide projectionsof future autochthonousproductivity and oxygen concentrations in the estuary. However, kinetic coefficients now used have largely been derived from work with marine and f reshwaterphy toplank ton. Do estuarinespecies behave similarly? My Sea Grant project is aimed at determining not only phosphateuptake and growth co-effic- ients for estuarine phytoplankton, under different conditions of nutrient limitation, but also to establish the rates at which P cycles in the water column. This informationshould be usefulfor modeling. 3. Are presentcapabilities for developingnutrient budgets for the Chesapeakeand its tributaries adequate?lf so, what can we learn from nutrient budgetsthat has bearing on management issues? There is a wealth of data available on nutrient standingstacks and trans- formations; however, much of the data is unsuitable for ad quate budgetary work. Inadequateprecision or inappropriatemethodologies have been employedthat makemass balance estimates impossible. For example, in saline regionsof the Chesapeake,the precisionused in manysampling programs has resulted in reportsof "not detectable"for phosphateand ammoniafor large periodsof the year. Althoughthe standingstocks of these nutrients may be low, the volumesof water are so large that the mass volume x concentration! of N or p involved is enormous. I hypothesizethat when adequatebudgets become available,we will find that the nitrogenbudget is a rather lved Oxygenin the ChesaPeake 48 / Disso "

"open"one e and> the phosphorusbudget is rather "closed." By that much of the N entering the estuary is Qyden i tri fication and is not retai ned in the eco- re moved p pn the other hand,is conservedin the ecosystem system. ent reservoirs. Unlike many ecosysternsin which in sedimentiyity is controlledby availablephosphorus's regula- praductivit' af Aitragenfixation, in the ChesapeakeN fixation is tionjvig[ af andni denitrification playsa far greater role. The ~anage~entramification is abvious: ni.trogen con trol may be necessar y. NUTRIENTCYCLING IN CHESAPEAKEBAY

ThomasR. Fisherand Robert D. Doyle HornPoint Environmental Laboratories, CEES Universi ty of Mar yland

Concernover anoxia in ChesapeakeBay waters inevit- ablyconcerns nutrient supply. The current paradigm linking nutrientswith anoxia is thatan enhanced supply of nutrients stimulatesalgal growth and sedimentation of organics into deeperwaters where respiration consumes a limited supply of oxygen.In orderto understand theorigins and control its occurrence,it is necessaryto quantifythe total supplyof nutr ients.

Theavailability of nutrientsto phytoplanktonin surface watersis thesum of boththe new inputs and recycled N and P. Inputsresult from rain and advective additions from the surrounding watershed, and nutrientsare recycledas a resultof sedimentdiagenesis and heterotrophic activity in the watercolumn Figure 1!. Thetotal supply of N andP is the sumof theseprocesses, and the hypothesis to be tested in thisresearch is that recyclingin the watercolumn is quantitatively the most importantprocess of thoselisted above.

In order to test this hypothesis,direct measurements of watercolumn recycling of N andP wereconducted with 15 N-NH~and 33 P-PO<.Both particulate and nutrient pools were removedin time seriesand analyzed for poolsize pM! and isotopiccontent. Isotopedilution of the ammoniumand phosphatepools was used to computerates of regeneration, and uptakewas calculated from isotopicincorporation into tbeparticulate pools. These measurements weremade using samples of surface and bottom water obtainedfrom the central partof theChesapeake in May and August 5p/ Dissolved Oxygenin the Chesapeake

An exampleof the dataobtained in this studyis shown in the attached Figure 2, The 15 N in the ammoniumpool wasslowly dilutedand appeared in the particulateN pool duringthe six hourincubation. In contrast,the soluble reactivephosphate pool SRP!was rapidly diluted note Log scaleon y axis and minute scaleon x axis!, and label ap- pearedquickly in theparticulate P pool. Uptake U! and regeneration R! were computed from the rates of changein thepools and are indicated in eachpanel in nmol/I/hr These data indicate that the ammonium and SRP pools were turningover on time scalesof hoursand minutes, respectively, andthat P wascycling faster than N in relationto Redfield stoichiome try. There were significant differences between the rates obtained at the two times of year and in surface and bottom waters. !n general,turnover times were longerand rates of uptakeand regenerationwere lower in bottomwaters than in surface waters. Nitrogen cycled more rapidly in summer than in winter, but phosphateexhibited the opposite. The ammonium pool turned over in about an hour in August, whereasthe SRPpool had a turnover time of about 40 h Table 1!. Thesedata are consistent with the concept that phosphorusis more limiting under spring, high runoff conditions, and nitrogen is more limiting under summercondi- tions. Two sets of data associated with a. wind-driven overturn event in August have been excluded from this summary. In these samples,nutrient-rich bottom water was broughtto the surfaceand turnover times were very long and rates of uptake and regeneration were low. We have usedthe regenerationdata to compute rnediar values of the total heterotrophic supply of ammonium and phosphatein the water column. We assumedthat the surface rates were representativeof a 5 m surface mixed layer and that the bottom rates represented a 5 rn bottom layer. The rates obtained mmol/m /d! are shown in Table 2 alongwith data on newinputs and cycling of N andP via sediments. The latter two estimates were obtained fram the l982 EPA report on the Chesapeake. The data show that water column recyclingis the largest sourceof N and DissolvedOxygen Processes / gl

P,and emphasize theimportance ofrecycling processes rel ativeto inputsof newN andP. Thedata may also be used to computethe number of cyclesthat inputs of N andp undergobefore export or burial recycling/inputs!. These dataindicate that N andP inputsarerecycled 30-l20 times, mainlyin the watercolumn. This analysis supports the hypothesisgiven above, and suggest that a betterunder standingof theanoxia problem will beachieved when the total supplyof nutrients is better quantified.

nutrient supolv algal bloPlass

anoxia sedimentatlon

total nutrient suool v inputs ~ recyc ling inouts rain + terrestrial new! recvcling sediments + water colum reused!

Figurel. Schematicdiagram of nutrientcycling in Chesa- peake Bay.

DissolvedOxygen Processes / 53

Tablel,. Turnovertiines for major nutrients in Chesapeake Bay ln May andAugust l986

May 1986 August 1986

Ammonium surface layer! 0-9 h 1 h bottomlayer! 18-220 h

ParticulateN surface! 27-61 h bot tom! 79-170 h

Phosphate surface! 0-7 min 39-00 h bot tom! 48-110 min 80-90 h

ParticulateP surface! 0.3-5,6 h 00-50 h bot tow! o->5 h 60-80 h

Table2. Nutrientsupply and recycling inChesapeake Bay

rrmol/m 2/d ~No.c cles N P

Water column 06, 9-0 78 95 22 112 Sediments 0-V3 18 5 6 inputs 2.1 0-080

TOTAL SUPPLY 59.1 9.51 100 100 27 118 ~E~NALOXYGEN DEPLETION AND PHYTOPLANKTON PRODUCTIONIN CHESAPEAKE BAY: PRELIMINARY g.~LTD OF l9g$-g6FIELD STUDKS

ThomasC. hkaiooe Hornpoint Environmental> aboratories,CEPS Universityof Maryland

The responseof estuarinesystems to nutrient enrich- mentis a majorproblem in marineecology which has broad implicationsin terms of both water quality and fisheries. Nutrient enrichment from naturaL or anthropogenic sources can stirnuiate phytoplank ton production, leading to increased fish yields or to decreasedwa ter quality. The degreeto whichone of theseeffects predominates over the other dependsin part on the rates and pathwaysby whicb organicmatter producedby phytoplanktonis metabolizedby heterotrophic componentsof the system. A critical factor here is the extent to which the photosynthetic production of organicmatter is separatedin time and spacefrom the aerobic decomposition of this organic rnatter. 1n general, the more closely phytoplankton production and heterotrophic consumption are coupled, the lower the probability that increases in phytoplankton production wiH. lead to a decline in water quality.

in Chesapeake Bay, it is generally assumed that increasesin anthropogenicnutrient inputs have causedphyto- plankton production to increase and that this increase has lead to a decline in water quality over the last 34 decades, An increase in the temporal and areal extent of oxygen depletion during the summer is believed to be one «at«e of this decline in water quality. This scenario has several weaknesses. I! Cause-and-ef feet linkages between «»««nput, phytoplankton production, and increases in oxygen demand below the pycnocline have not been docu- quantified. ! Inputs of new allochthonous! Dissolved Oxygen Processes/ 55

nutrientsaccount for onlyabout 10% of annualphytoplank- ton productionso that a linear serial relationshipbetween nutri~~tinput and oxygen depletion is unlikely.That is, the fate of nutrientinputs depends more on system characteristics thanon the magnitude of theinput per se. ! Finally, nutrientinput and the fate of thesenutrients depends to a great extenton fresh water runoff and wind-mixing, both of which exhibit large episodic,seasonal, and interannual variations. Thesevariations modulate the patternsand ratesof nutrientinput and nutrient cycling within the Bay. The dynamicsof the productionand fate of phyto- Planktonbiomass is clearlyof centralimportance to this issue.As a firststep toward pararneterizing therates and Pathwaysby whichorganic rnatter is metabolized,we must determinethe time and space scales on which phytoplankton vary and how these variations are related to variationsin nutrient supplyand oxygen depletion. Theobjective of thispresentation is to describetempo- I a1 variations in phytoplanktonbiomass and their relation- shipto seasonaloxygen depletion of bottomwater. Datawere collected at five stationsalong an east-west transectin thernid-Bay between the Choptank and Patuxent rivers Figure1!. Flowof theSusquehana Riveris the major saurceof newnutrients and of buoyancyin the reachof the Bay. Verticaldistributions of temperature,salinity, nitrate, nitrite, amrnoniurn,phosphate, particulate organic carbon and nitrogen,chlorophyll, and dissolved oxygen were determined at 3-10day intervals during summer 1980 and from I=ebruarythrough September during 1985 and 1986. Phyto- planktonproduction was determined at 7-l,0day intervals. Dissolvedoxygen began to declinebelow the pycnocline in Februaryas soon as temperature began to increase.The rate of decline wasmaximal during May andreached its seasonalminimum in Augustand 3uly during 1985 and 1986 respectively Figure2!. Thesecycles of dissolvedoxygen did not appear to be related to variations in vertical stratification, but did appearto be related ta variationsin phytoplanktonbiomass, Seasonalcycles of chlorophylla 56/ DissolvedQxygen in the Chesapeake Chl!and Chl-specific phytaplank ton production P/Chi! wereout of phase,Chl was highest during March-May when concentrationswere high throughout the watercolumn and P/Chlwas low Figure3a!, P/Chlwas highest during june- Augustwhen Chl was low and concentrated in the surface layer Figure 3b!. Seasonal variations in Chl were directly relatedto seasonalvariations in thevolume transport of the Susquehanna,suggest ing a relationshipbetween nutr ient inputand accumulationsof phytoplankton biomass on a seasonaltime scale Figure0!, Chl contentof the water calumndecreased rapidly during May, primarily as a consequenceof a rapid decline in Chlbelow the euphotic zone and the pycnocline!which coincided with therapid decreasein dissolvedoxygen. These trends indicate that phytoplanktonproduction and grazing are poorly coupled duringspring and closely coupled during suromer. Thus, thereis a,large build-up of phytoplanktonbiomass during March-Aprileven thaugh phytoplankton growth rate was law on the arder of O.l/day!. The rapid declinein phytoplanktonbiomass in May couldreflect an increasein mortality rate due ta exposure ta lowoxygen concentration below the euphotic zone. Such anincrease in mortalitycauld cause a rapidincrease in BOD leadingto furtherreductions in dissolved oxygen. Thus, a positivefeedback could develop between oxygen depletion andBOD as oxygen concentration declines. This could make the differencebetween hypaxia and anoxia. Simplemass balancecalculations indicate that the magnitudeof the springaccumulation of phytaplankton biomass could determine the extent of summeroxygen depletion. Sincesea- sanalvariations in Chl andriver flaw werecarrelated and freshwater runoffis the majorsource of newnutrients, this mightbe thecritical link between nutrient input and oxygen depletion in ChesapeakeBay. 31 4 10'

38 3S'

3e' 30'

re' 30'

F'igUre l. Study area and the ocation of the CHOP-PAX transect. BOTTOM LAYER OXYbEN Station 03

l00

JUL lAN DAY 01985 + !986

Figure 2. Dissolved oxygen concentrations below the pycnocline in l985 and 1986. WATER COLUMN CHLOROPHYLL Transect Mean 50

400

300

200

100

0 0 0 80 }20 160 200 240 280 JULIAN DAY ' }985 }986 Figure 3a. Watercolumn chlorophyll a concentrationin 19S5 and 1986.

PRODUCT1VITY/CHLOROPHYLL Transect Mean

x 30

0 120 160 200 240 280 JULIAN DAY + 1985 ~ 1986 Figure 3b. Chlorophyll-specificphytoplankton production in 1985 and 1986. CHLO~OPHVLLVS,SUSQUEHANNA FLOW Seasonal Means

160 1 140 130 120 110 100 x $0 80 10 60 50 40 30 20 10 0 E7 M3/DAY Figure4. Relationshipsbetween seasonally averaged chlorophylla concentrations and the dischargeof the SusquehannaRiver. 50URCESOF BIOCHEMICALOXYGEN DEMAND IN THE CHESAPEAKEBAY: THE ROLE OF MACROPHYTE DETRITUSAND DECOMPOSITION PROCESSES

3osephC. Zieman Department of Environmental Sciences University of Virginia

StephenA. Macko Department of Earth Sciences Memorial University of Newfoundland

Aaron L Mills Department of Environmental Sciences University of Virginia

Theoverall objective of this studyis to determinethe relative contribution of macrophyte detritus decomposition to the depletion of oxygen and the sequesteringof nutrients in the lower ChesapeakeBay. Thoughthe decompositi.onof massiveamounts of phytoplankton biomass produced by elevated nutrient levels is believed to be a major cause of anoxicconditions in the Bay, the role of rnacrophytelitter in the Bay has not yet been adequatelyevaluated. In addition, the effect of macrophyte litter on excessiveanthropo- genically-derivednutrients has not been studied. This study .»l employ a series of field and laboratory experiments to deter mine:

.ate of decomposition of the major types of ma phyte material entering the Bay and the changesin composition during decay. 2. Rate of oxygen consumption of the decaying macrophyte material, the enhancementof this rate by elevated nutrient levels, and the effect of this process on local stable carbon isotope ratios. 62/ Dissolved Oxygenin the Chesapeake

3 Composition of the major types of macrophyte material lower Chesapeake Bay, and the variation in prnpositionof this material and its resulting detritus betweendifferent drainagebasins entering the Bay.

Contribution of macrophyte-derived material to suspendedand sedimentary particulate matter and to the dissolvedcarbon dioxide in the different drainagebasins of the lower ChesapeakeBay by multiple stable isotope analysis. proportion of oxygen consumption by suspendedand sedimentary particulate matter that is attributable to macrophyte derived material.

Methods During the initial stagesof this project we have concentrated our efforts on three sites in the York River estuary having differing salinity and inorganic nutrient levels in orderto establisha rangeof BODand nutrient valuestypi- cal for sites with low anthropogenicinput. Thesesites will later be comparedto sitescharacterized by stronganthro- pogenic inputs or with rnaxirnal oceanic influence.

The major emphasis has been on autochthonous material from the dominant seagrass,Zostera marina and salt marsh plant,~Sartina alternillora. ~uercussp., the dominant terrestrialmacrophyte in the area, was used during the 1986 experimentsas a representativeof allochthonousorganic material.These three species reflect a rangein amountof structuralmaterial e,g., celluloses, hemicelluloses, lignins! and nitrogen concentration. plantmaterial was collected as close as possible to the timeof naturalsenescence of the plant and was placed i» rnmmesh bags of nitexnylon or fiberglassreinforced P~C- Wet/dryweight ratios were estimated according to proceduresof Zieman975! to providelevels of starting materialwithout artificially drying the freshsamples. ' itter bagswere attached to thesediment surface at Mum- «« lsland strong riverine influence, lowest salinity, high- Dissolved Oxygen Proce sses / 63

est particulates!,Aliens Island moderateriverine influence, moderate salinity, moderate particulates!, and Cuinea Point weakriverine influence, highest salinity, lowest particulates!, Triplicatesamples were withdrawn at approximately two week intervals for the first eight weeks, and at four week intervals subsequently. The decompositionstudies were initiated in 3uly 1985 Allen'sIsland site only!, April 1986and 3une l986. A winter decompositionstudy was started in 3anuary l987. At samplecollection times, the litter bagswere re- trieved, rinsed with sea water to washaway sedimentand extraneousmaterial, and returned to the laboratorywhere the litter remainingin the bagswas sorted and any animals removed. Weighedsubsamples were taken for BOD determinations,nutrient flux measurements,microbial abundance andbacterial productivity. After aliquotsfor otheranalyses were removed,the plant material wasweighed, acid-washed in 10%HC I to removecarbonates, and lyophilized. Weighed subsamplesof the freeze-driedmaterial were analyzed for carbon and nitrogencontent and ash-freedry weight. Subsampleswere also removed for analysisof crudeprotein, crudesoluble carbohydrates, crude fiber, amino acid composition,and stableisotope ratios of carbon,nitrogen and sulf ur analysisin progress!. To determine the rates of oxygen consumptionof decomposingm acrophytes, s ubsamples f or BOD analysis were removedand placed in bottles containingaerated filtered seawater.Initial and final dissolved oxygen levels were determinedafter 6 hoursof incubationin flowing seawater at ambient temperatures.Dissolved oxygen levels were measuredusing an Orbisphereoxygen probe/meter system. The effects of decomposingmacrophytes on poolsof dissolvednutrients were assessed by placinga subsampleof decomposedplant materialin bottlescontaining either a! filtered water from the site, or b! filtered water to which a.mrnonia and phosphate were added. Initial and final nutrient concentrations were measured in each bottle and rates of nutrient uptake and release determined, Nutrient 1«d oxygenin theChesapeake Disso ve weredone according to the methods of Grasshofet analyses were al. 9a3!. > cter<>labundance was determined using the acridine exacter i count technique and bacterial productivity using H-thymidineincorporation into bacterial Theabundance of fungal hyphae was estimated using jones~orison agar-filmtechnique.

Results p.stimatesof DecayRates Therate andthe extent of dry weightloss was similar for~5gp ghana and Zostera. The carbon/nitrogen ratio de- creasedin ~Sartina litter but changedlittle in Zostera. Changesirithe total microbial bacteria + fungi!and bacterial biomasswere consistentwith dry weightloss ancl C/N ratios. The total microbial biomassassociated with the detritusnever exceeded1% of the total detrital mass. The ratio of bacterial to fungal biomass ranged from l:1 to Early increasesin bacterialproductivity with Zostera and~Sartina litter werecoincident with the period of rapid weightloss, presumablyas a resultof leachingof soluble plant materials. Subsequentrates of productivity were lower, re flee ting bacterial utilization o f the remaining part iculate material rather than readily leached so1uble I materials. Growth rates and rates of carbon incorporation werehighly variable throughoutthe course of decornposi- tion.

Nutr ient Fluxes The potential for ~Sartina,Quercus and Zostera litter to sequester or release ammonium and phosphate from or to the water column was measured at ambient and enriched concentrations of nitrogen and phosphorus. In the samples analyzed to date, no apparent differences were observed among sites or plants. Decomposing rnacrophyte litter released arrirnonium to the water column at both the ambient and enriched concentrations while phosphate was either removedor unchanged. Dissolved Oxygen Processes / 65

Oxygen Consumption

No differences in oxygen consumption were detected among the sites for any plant material. However, Zpstera litter exhibited a greater biological oxygen demandat all three sites than either ~Sartina or Quercus,which showed similar patterns of oxygen consumption.

Remaining Project Activities

ln 1987 samplingand decompositionexperiments will be conductedin the 3arnesRiver, a river with a stronganthro- pogenic input, and on Virginia's eastern shore, an area with maximal oceanic influence and minimal humani rnpact. These sampling stations will be located at establishedBay research stations where physical and chemical water quality measurements are taken at regular intervals so that seasonal interactions of temperature, salinity, dissolvednutrients and oxygen, and particulate concentration on decomposition can be examined.

Based on the results of these studies, a predictive model will be developed that will be used to estimate the influence of macrophyte derived carbon on the carbon budget of the lower ChesapeakeBay, and the contribution of this carbonto the anoxiaproblem existing in theBay.

References

Grassbof,K., M. Ehrhardt, K. Krernling. 1983.Methods of Seawater Analysis. Verlag Chemic GmbH. Weinheirn, FOR. 419 pp.

Ziernan, 3.C. 1975. Quantitative and dynamicaspects of the ecology of turtle grass, Thalassia testudinum. Estuar. Res. 1: 541-562.S My feelingis that oak leavestend to be very Mountrefractory for: a ndthat some of thatmaterial might reside from year to yearear on the bottom. ls that consistentwith your findings? Blum: Yes, oak leavesretain their integrity for a much l er period.S artinaand Zostera decay at a muchfaster ratethan Quercus. l think what might determine how long materialhangs around is how much physicalaction the leavesare exposedto how muchgrinding up into small par t icles. barber: Do you suspectthat the decay of macropbyte detritusmight make a significantcontribution to the setup and maintenanceof anoxia/hypoxiain the Bay? If you do a backwf-the-envelopecalculation if you took all of the macrophytematerial anddumped it into the Bay in the deep channel and rot it there, would you see the signal?

Blurn: That's difficult to answer, because at this time we don't have an idea on how much macrophyte material there is in the Bay. We need better estimates on how much is really there. But in localized areas where there are extensive grass beds, it could be significant, or where you have a ~Sartina marsh with material washing in.

Garber: My reason for asking is that Chesapeake Bay is generally characterized as a plankton-based system.

Blum: Right, and we don't dispute that at all, We believe that it is phytoplankton-based, but we want to know what contribution the macrophytes make. Qne of the most important things is what happens to the nutrients and the nutrient cycling during macrophyte decomposition. For «ample~ phosphate uptake, possible nutrient limitations, and so forth. Dissolved Oxygen Processes / 67

Mills: I would like to speculate on that if the data show something different in the next year, then I will come back and tell you something just the opposite -- but I think that Linda's correct that macrophyte detritus can be extremely' important in localized areas. lf it is being trapped in some spot, you can soak up tremendous amounts of oxygen very large oxygen demands are associated with that decaying material. For the whole Bay, looking at the numbers we saw this morning, I am not convinced now it makes a significant quantitative contribution to the overall oxygen depletion problem.

Garber: I think also from our historical perspective the contribution is an interesting one if you look at the history of SAVs in the Bay, the area which they cover fluctua- tess violently, from very large to almost infinitesimally small, so whether that contribution is significant dependson looking at some of these historical trends.

Mills: The only problem with the calculation you want us to make is the distributional pattern. Some areas are readily scoured with no detrital buildup!, and in some areas there are pretty thick mats so we are going to have to get some idea of the actual distribution of the material as it sits on the sediment surface.

Question: Are there any time periods when the detritus is released?

Blum: There are two time periods: Zostera shows a bian- nual cycle, it will be senescing soon and will be senescing againin the fall. ~gartina of coursesenesces in the fall and washesoff the marshes in spring.

Mountford; To Malone! Your data shows this late spring phytoplankton "check" in both of the years I notice in Larry Haas' data that in his DO scatter plots temperature >s DO!, the lows show a birnodality around 23C. I wonder if thosetwo are related, the reduction in phytoplankton"rain" into deeper water and a subsequent reduction in oxygen demand in the water column? 6g / DissolvedOxygen in the Chesapeake

4'hat we have seen consistently between years and between stations is tha.t the crash in phytoplankton always occurs at around the same time and always when DO gets low, Thoughthe effects of low dissolvedoxygen on alg>f mortality in the darkare not known-- virtually nothinghas been done I think a good case could be make that they don't survive very well, and there is probably a very rapid mor tali t y. Tuttle: Could I add something to that? In l985, before the phytoplanktoncrash, there wasa bacterialcrash, which is the oppositeof what you might expect the other pointis that the phytoplankton crash was accompanied by a de crease in water column BOD, too. jonas: Dropsto less than half of what it was earlier, Question: You have turnover rates for orthophosphorusand arnmoniurn, and made some speculation on nutrient Lirnit- ation~ ~ from them. Do you have anything for nitrite or ni trate, and wouldn't that be important also in terms of the potential for limitation?

Fisher: I didn't measure anything with nitrate with the set of data I showed here -- there are other data that I have on nitrate uptake in the Bay, but very few observations on nitrification in the water column. I think 3im McCarthy has done some work on the rates of nitrification in the water column. But in terms of nitrogen limitation, ammonium is the preferred form for phytoplankton nitrate is consumed, but if you are going to Lookat limitation and the processe<, the main source of nitrate is river inflow on an annual basi>, I think. The main source of ammonium is in situ recycling- a minor contribution from rain or terrestrial inputs. lf yoU look at the two on an annual basis, the total amount of ammoniummade available via recycling processesis far io excess of the amount of nitrate introduced via river input. Kernp: But, then again, at levels where ammoniumis Lieit- ing> previous studies have shown that phytoplankton will switch to nitrate if it's available. Dissolved Oxygen Processes / 6g

Comment: New nitrogen is ultimately going to be added to that ammonia flux -- the nitrate gets taken up by the phyto planktonand it ultimately enters the ammonia flux does all the nitrate coming in end up increasing the ammonium flux?

Fisher: That's right virtually all the nitrate coming into Chesapeake Bay is consumed either directly by phytopiankton or denitrified in sediments. Virtually none of the nitrate escapes out onto the shelf.

Haas: So the question is, is it better to keep it cycling around in the ammonia pool rather than settling out? Is that the control that needs to be made?

Answer: That is one way to look at it, but it may not be amenable to management.

Haas: It may be amenable in a sense that we talk about nutrient inputs changing rates of production. Again I go back to the idea that nutrient inputs may change species composition. And if your species are small cyanobacteria or microbial food web, versus diatoms....

Coing back to your differences in standing stock in '80 and '~5 why there was more in one year than another? If it's a grazing factor, if the grazing is different between years it may be because species availabie for grazing are different. This goes back to those ideas of nutrient inputs changing species composition, which could be just as important as changesin rates of productivity,

Fisher: But those changes could also be due to things such as temperature, salinity, depths of mixing which can influence species composition, and so forth.

Haas: True, there are many other factors besides nutrients that could do it.

ountford: I want to ask you about the situation in t 17thCentury -- presumablyright nOWwe are SortOf trying 70/ Disso}vedOxygen inthe Chesapea'ke to "starveout" phytoplankton in Chesapeake Bayin orderto improveconditions. Would your nitrogen and phosphorus recyclerates have been ~extreme/ high under oligothrophic conditionsin the 17thcentury? What does that meanfor thespecies composition Larry was talking about? Fisher: %e, that is sort of like doingan evolutionary experiment is thatwhat you are asking? If youlook at a trophicgradient when you go offshore from a shelfsitu ationto bluewater, which is essent>allywhat you' re trying to doin timehere the evidenceis that nitrogenstanding cropsdecrease and turnover rates increase as youmove from eutrophicto oligotrophic,so presumablywhat was happeninginthe 17th century was that you had lower inputs, lowerstanding crops and more intensive recycling of that aterial than occurs now. Mountford:So the recyclingrate maybe an "emergentpro- perty"of thephytoplankton community that we may want to keepan eye on maybeeven manage for direction or is that too much? Fisher: Not sure I want to answerthat one! D'Elia: Theimportant thing to makeclear hereis the differencebetween new production and production driven by nutrientregeneration. Ultimately, in the linksto hypoxia, newproduction isof concern something that results in the accumulationof organicmatter whichcan be movedsome placeand be concentrated, decay, and use oxygen. Recycl- ing,can occur without any net change whatever in surface waters oxygenwill stayexactly the sameand the system canspin as fastas it canwithout any effect on hypoxia whenconsumption proceeds apace with production. Mountford:But recyclingrates go downas newproduction goes Up? Fisher: In relative terms, yes, but in actual terms it may not. Chrisand I werediscussing this earlier -- the pointI wantto makeis not that youconsider recycling in termsol oxygenproblems in theBay recyclingterms per se are not Dissolved Oxygen Processes / 71 what drives hypoxia, It is the inputs getting into the surface layer, either from above, laterally, or from below the py cnocline, because that's what drives the production of organic material in surface waters and the amount of parti- cles dropping back across the pycnocline. If you look in the context of total supply, the recycling in the water column is really a very important process. Some of the organic mate rial which crosses the pycnocline is degraded in the water as well as in the sediments and then comes back up again [as nutrients]. You have to think of recycling in terms of all the processeswhich make N and P available for phytoplank ton. I wouM just like to add that 3ay Taft did estimates of N and P regeneration in the water column based on oxygen respiration rates. He applied an R/Q ratio and Redfield Stoichiometry and computed P values right on top of the numbers I measured directly. His N figures were off by a factor of 2. This actually turned out to be a useful exercise, to take the respiration data and go through all those calcu lations, as it comes out not very different from the actual measured values. But what they failed to do in that document was to put all that into context of "what are all the processes?" they only looked at inputs.

D'Ella: You need to look at recycling rates in the water column to understand what happens to particulate fractions; differential rates of N and P cycling can do a lot to parti- tion out nutrients and ultimately affect the fate of allochthonous nutrient inputs.

Newell: From a zoologist's point of view, I calculated that prior to the major harvest of oysters in about 1880, there were enough oysters in the Chesapeake Bay to cycle the entire volume of the Chesapeake Bay once every six days. The current existing oyster population can only do this once every 180 days. This suggests that oysters were very important nutrient recycling agents. 3ohn Smith reported that he could see the bottom of the Bay when he first explored it in The 17th century. Oysters are also very important for biodeposition and for taking sediments out of suspension. In terms of our over-harvesting a resource, that could also be affecting the environment directly. 7>/ DissolvedOxygen in theChesapeake

O'Elia:I thinkthat is thesingle most Understudied thing that we really need to get a grasp on. Tuttle: WhatI think we shoulddo is stronglyconsider aquaculture on rafts;it won'tdo any good to putthem on thebottom these days, but wemay be ableto affect things byputting them on top for awhile. I don'tthink it is as"off the ceiling" as it may sound. 3onas: Whatyou' re doingby putting oysterson rafts is gettingparticulate rnatter to the bottom a !otfaster how that is goingto impact DO is anotherquestion. Newell; Of coursethe oystersare taking out somenutrients for their owngrowth, so if youare harvestingthem you' re taking out nutrients. Question:Tom, given the link youhave mentioned between respirationand nutrient recycling, do youhave information on nutrLentrecycling in the water columnbelow the pycnocline andfor sedimentregeneration to makea "guesstimate" of the relative contribution of water column vs. sediments to oxygen demand? Fisher: The data that are available could be used to esti- rnate ammonimun and phosphate production per square meter in the water column below the pycnocli ne and compare that with what'shappening in the sediments. The problemis the limited numberof samples:can you let one samplerepresent the wholepycnocline? There may be local structure and hot spots that you don't see... but you can make an order of magnitudeestimate of that, althoughI haven't done it yet ~ Boynton: There were somedifferences in anoxia between the various years studied. But looking at it the other way, the differences between years are pretty trivial. Every year oxygen decreases a lot; some years actually gets to zero true chemical zero. In most years it becomeshighly hypoxic in any case. Given the amount of oyxgen you have to take out of the water to get it to hypoxic conditions, isn't it reasonable to assume that, in terms of the amount of bio- D issolved Oxygen Processes / 73

logical material available to create anoxia, we' re saturated, The real question is: why is there ~an oxyge~ down there? If we have biological saturation in terms of oxygen- consurning capabilities, then the explanation for why we have any variation from year to year has to "revolve around" the physics. I hate to say this, I really do, but that's the factor -- there may be some subtle features which we are not used to looking for that are key factors in generating this year-to-year variability.

Boicourt: If we are going to distinguish the effects of man, an impartant taxpayer question in terms of nutrient controls, we have to know the answer to that question very carefully, well enough to predict year-to-year variation. It' s nice that we do have this variation as a signal to help us track things down. We know there are lots of physical processes to get oxygen down there, and we know there' s consumption, the question is what's the dynamic balance? We could essentially "get the physics out" so the biologists could then come in and work out the rates and processes and thereby determine the effect of nutrients on the system. I think that we' re a lot closer to doing that physically than we were even a few years ago. A simple model like I showed this morning could be cranked out on the computer and could separate out the rates measured as a fir st-order e stima te.

Comment: If we're trying to monitor the response of the system to controls, maybe we shouldn't be measuring Do~ since that is the interplay between the physical factors and the cansumptive factors. Maybe we should just worry about those sourceswe can control, such as oxygendemand we could just measure water column oxygen demand,sediment oxygen demand as indicators of whether we're making progress. The measuring of DO is just a confounding factor in this whole sorting out of the physical versus the anthropogenic factors.

Jonas: In a minute I' ll put up some data showing respiration rates measured against some of these parameters and bacterial biomass in particular; the correlations are quite striking. Hugh Ducklow brought it up earlier this is one 7<~ ~>ssolvedOxygen in the Chesapeake variablethat Msn'tbeen measured in the past, part ly for technicalreasons, partly because it's very manpower-inten- veto dp butthe relation to oxygenconsumption is there, Kernp:Probably the most provocative thing we' ve seenall day!s Tpm Malone's five-point regression between riverf low andchlorophyll concentrations. I really like it becauseit' s consistentwith somethingBoyn ton and l have beensaying beforebut. ~ ~while it's a nicestory, is it real? Malone p,t least over that segment o f the Bay, the re are generallymore nutrients in termsof phosphorusand nitro- genthan you would expect to haveany effect, at leaston growthrate. Nowthe yield questionis anotherstory; and our data suggestthat there is some relation between nutrient input and yield, which would hark back to the new nutrients/new production issue. But there is no obvious relationship between the nutrients and any of the biological parametersthat were measured.When you seevariations in physicalfieMs that you wouldexpect to affect nutrient flux, you see a phytoplanktonresponse or a bacterial respanseor whatever, but there is no experimental basis for expecting that relationship. Scott Nixon sees a similar relationship in the IVlERL mesocosms. Those were seasonally averaged figures -- freshwater input and chlorophyll concentrations which were used in the regression. CHE5APEAKEBAY DISSOLVED OXYGEN DYN!gg[gS: ROLES OF PHYTOPLANKTON AND IJ1ICROHETEROTROPHS

Robert B 3onas George Mason University

Objectives

The overall objective of this work was to describe the microbiological, both autotrophic and heterotrophic, nutrient and physical hydrodynamic relationships over seasonal cycles with sufficient resolution in time and space to determine the conditions influencing oxygen depletion across the affected area of the Chesapeake Bay. To attain this goal we are conducting a field investigation with the following objectives;

l. Assess the importance of phytoplankton production as a source of organic matter to bottom water.

Evaluate the significance of phytoplankton production over the shallow flanks of the main channel relative to production in the channel itself as a source of organic rnatter.

3. Determine the importance of heterotrophic micro- organisms in the water column and their associated metabolic processes as consumers of organic matter and dissolved oxygen, and establish how the microheterotroph community varies in relation to phytoplankton production, organic inputs, biochemicaloxygen demand BOD! and dissolvedoxygen concentrations.

4 Establish how variations in vertical water colum~ stra tif i cation over seasona1 cycles influence relationships. 76 / Dissolved Oxygen in the Chesapeake

5 Identify the southernboundary of the oxygen-depleted z one i A sizeable data base has been developed for the upper, mesohafineportion af the Bay through cooperative research efforts fumed by the Maryland Sea C'rant program, the Fnvironmental Protection Agency, George Mason University the University of Maryland. bio like data were pre viausly availablefrom that portion af the Bay south of the Potomac River. Therefore, we concentrated our investig ation on the processesoccurring in the Virginia pol tion Df the lower! Bay. The objectives of this focused effort are twofold:

I. Establish the interannual variability in the processes driving oxygendepletian in the lower Bay,

2. Determine if there are major differences in the trophic dynamics driving the process of oxygen depletion in the upper and lower Bay by developing a data base sufficient for comparison to that which has been, and is currently being deveiaped for the Maryland upper! mesohaline portion of the Bay.

During f986 we completed I5 cruises, between Febru- ary and December, in which we occupied stations along 0 transects arrayed across the main axis of the Chesapeake Bay. Transects are located at the Chesapeake Bay bridge, between Dares Beach and the Choptank River, at the mouth of the Patuxent River, and off the mouth of the Great Wicomico River, south of the Potomac River. In addition we participated in twa 30-haur anchor stations in May, ~ad on» in August I986 along the Dares Beach/Choptank River transect. During most of the f986 cruises we occupied ~I least IS stations and collected samples far the entire suite of parameters described in the original proposal.

Vert ical profiles of temperature, salinit y, dissolved oxygen, chlorophyll a, and bacterial abundance were made at each station. Phytaplankton production, bacterial pro- DissolvedOxygen Processes / 77

duction,bacterial metabolism of aminoacids phytoplankton potcinhydrolysate! and glucose,water column oxygen consumptionand biochemicaloxygen demand were also estimated,along with dissolvedinorganic nutrients pho~ phate,nitrate, nitrite, ammoniumand silicate!, particulate organiccarbon and nitrogen,and phytoplankton species compositionwere also measured at these stations,

preliminaryResults Takingthe goals of this investigationinto account,this descriptionof results will concentrateon a comparisonof heterotrophicprocesses in the waters along the southern mosttransect lower! with thosein the more northerly regions.

With regard to dissolved oxygen DO! levels, anoxic conditions developed in the upper rnesohaline portion of the Bay by the third week of 3une 1986, By the first week of 3uly, hydrogen sul fide was detee ted in upper Bay deep water. Anoxic conditions spreadsouth during 3uly, reaching the area off Point-No-Point north of the Potomac!by 3uly 22. By August 7 anoxic conditions existed from the Anna- polisBay Bridge ta below the GreatWicomico River. Along the southernmost transect anoxic conditions existed from the mainstern channel toward the west at depths greater than 7 m. The upper boundary of the anoxic zone was shallowerat the southern transect m! than at the Bay Bridge2 rn!. The anoxic zone was closely associatedwith theintense salinity gradient whichexisted during, l986. At the southern transect, water to the east of the mainstem channelwhich was weakly stratified remainedoxygenated throughout the season. We confirmed the occurrence of midwaterminima in oxygenconcentration and evenmid- water anoxia at several of the main axis deep stations. This »y suggestthat water column oxygen consumptionassocia- «d with recent organic carboninputs, which could accumu- lateat thepycnocline, is a significantfactor driving oxygen depletion.

A»n l985, the ChesapeakeBay streamf low was mark- edlybelow average in l986. However,indications are that, 78/ DissolvedOxygen in theChesapeake becauseof earlyspring runoff, the watercolumn remain d stronglystratified during that summer. During late Ag t a seriesof stormspassed over the Bay area. Thestrength of salinity stratificationwas reducedand bpttpm throughoutthe Bay was reoxygenated. With regardto heterotrophicparameters specifically wehad previously shown that theChesapeake Bay support, an unprecedentedlyhighbacterial biomass >10 million cellsper rnl! duringthe late summer.During 19g6, bac terial abundancesthroughout the Bayrose from abput0 9 x 10cells/ml in latewinter to morethan lp x 106 cells/+! by mid-May.Bacterial abundances in the waterabove the pycnoclineexceeded the 10million per ml levelfirst in the upperBay and then in the lowerBay about a monthlater. Byearly August bacterial abundances greater than 2p x 106 cells/ml were commonthroughout the entire researcharea BayBr idge, 3g x 10;6. G reatWicomico River, 27 x lp 6 cells/ml!. Abundancesdeclined throughout the late summer andfall, reaching1-2 x lp6 cells/mlthroughout the mid-Bay by December, High levels of bacterial production estimatedfrom thymidineincorporation - TdR! and metabolic activity aminoacid AATR and glucose GLTR turnoverrates! were associated with increases in bacterial abundance. By mid-Mayglucose turnover rates in the upperBay exceeded 20%/h and TdR approached 150 pmol/1"h February rates were: GLTR 1-996h, TdR g-00 mol/1+h!. By early 3une GLTR greater than 90%/h were observed from the Bay Bridgeto the Creat WicornicoRiver especiallyalong the western side of the Bay. At the same time the highest AATR values 1096/h!were observednear the western shore in the lower Bay. TdR continuedto increasein the upper levelsof the water columnduring the summer. By late 3uly valuesof 500 to gpp pmol/1+h,some of the highest values ever recordedfor a naturalaquatic system, were foundboth in the northern and southern parts of the research area. TdR valueswere highestalong the westernside of the Bay. Interestingly,Tdk valuesin the areaoff the patuxentRiver, in the rniddle of the study area, were only about 2~O DissolvedOxygen Processes / 79

p /1 1 + h o r 1 se s M et ab o1 ci r a te s a nd p r od uc ti on r at es high throughoutAugust over the entire region. Duringthis period the highest AATR 0596/h,GLTR 37%/h! rates occurred along the western side of the The heterotrophicprocesses, utilizing bothparticulate and dissolvedorganic rnatter and driving oxygendepleti on, are of a si rnilar order of magnitude throughoutthe mesohalineportion of ChesapeakeBay during the summer. Bacterial production and metabolic rates declined jn early September in the lower Bay and about 2 later in the upper Bay. By December TdR was less than 2p pmol/1+hr and GLTR and AATR were 1%/h or less.

There are a number of trends which appear to be consistent with regard to these parameters. High rates of heterotrophic activity develop first in the northern Bay and later in the southern Bay. The highest rates are consistently found along the western side of the Bay. Under summer conditions very high rates of heterotrophic activity and bacterial abundances occur throughout the area of the Bay affected by anoxic and hypoxic conditions. Under the highly stratified summer conditions mid-water maxima for microbial heterotrophic activity and bacterial production often occur in association with the pycnocline. A higher resolution depth profile of these parameters is needed to establish the importance of this phenomenon with regard to integrated water column oxygen consumption. In terms of organic inputs to the system we measured biochemicaloxygen demand BOD! in the Bay, This parameter provides a very useful estimate of the amount of easily rnetabolizable organic carbon available to support heterotrophic metabolism and to drive oxygen consurnp- Over the entire year BOD levels ranged from about 0 >~.~ mg/1 in the surface waters. During FebruaryBOD in the surface waters of the lower Bay .8 mg/1! was more thantwice that in the upperBay .0 mg/1!, althoughthere appearsto have been an accumulation of BOD at the pycnocline in the upper Bay. Mid-water rnaxirnaof BOD were commonduring the spring but not during the summer, Throughoutthe highly stratified portionof 1986 june- August!,BOD rapidly declined with depth. We speculate >0/ pissolvrdClxygen in th«he ap thatthe high rates ofbacterial rn«a I~sm rapidly utilize thisorganic matter especially inthe pycnocline region. In latesp ing ~D levels inthe northern Bay mg/I! general lyexceeded thosein the southern area I rng/l!. Near th MyBridge surface ~p levelswere o monlygreater than ymg/ I arKl orcasionally reached ~-"g/ l. Duringrnid. summer,however, highest values occurred in the southern gayespecially along the western flank. BODyaiues exceeded7 mg/ I for a period o fseve ral weeksdur jng 3uly/augustinthe surface waters along the western side of theGreat Wi~orairo iseof the extendedcruise scheduleduring 19fi6, dataanalysis is still not complete.In-depth comparisons pf primaryproduction and heterotrophic plocesses remain to becompleted, We limited this preliminary abstract to the heterotrophiccomponents, while nutrient and phytoplankton dynamicsduring the l9S6 season are presented elsewhere, ~CURIAL CARBON POOLS AND FLUXES IN C~PEAKE BAY PLANKTON

H~ Ducklow and EmUy Peele Hornpoint Envif onrnental Laboratories, CEE5 university of Maryland

ge havebeen investigating the distributionof bacterial abundanceand production in the water columnof the mid- /ay regionsince summer l980. Theprincipal emphasis of ourwork has changed somewhat each year, but taking the threeyears together, a large data set hasbeen obtained and somegeneralizations about the importance of bacteria in the planktonsystem of the rnid-Bay can be made. In this abstractwe will summarizesome of our findings,point out theirrelevance ta the anoxiaproblem, and outline directions for future research. Most of the discussionwhich follows is derivedfrom analysesof the 198W85data sets. Theconclu- sionsreported here are preliminary.

Our data set is based on measurementsof bacterial abundancevia acridine orangedirect countsmade on an epifluorescencemicroscope, and of bacterialproduction via incorporationof tritiated thyrnidineinto TCA-insolublecell fractions.These are the most widely recognized and util- izedmethods available for measuringthese parameters. For thedata reported here, we assumethat eachbacterial cell 4s 20fg of carbon,and that 2 X lp'8 cellsare produced for «chmole of thymidineincorporated. There is muchsup- Portin theliterature for thesevalues, but it isimportant to Ourconclusions to note that in this casethese assumptions l«d to conservativeestimates of bacterialbiomass and production.

«r samplinghas shown that bacterialabundance is low inthe winter and early spring ca. l-3 millioncells/ml! then 82/ DissolvedOxygen in the Chesapeake risesto a peakin latesummer. Upwards of 20million cells/mlare occasionally observed onthe flanks of themid- Bayin june-September,In our combined I'984-85 data sets n=I >00samples!, abou t half the abundancevalues greaterthan 7 millioncells/rnl. This i»«able because valuesof 5-7 million cells/ml are generallythe highest valuesreported inother well-sampled estuaries e.g., Wright andCoffin, l983!. Patterns of thymidineincorporation are similar. Bothabundance and thymidineincorporation ar< slightlyhigher near the Bay Bridge than further south higheron the flanks than in thernid-channel, andhigher in the surface layerthan in the bottom,especially under stratifiedconditions when there is lowoxygen in thebottom layer. In general,the temporal and spatial patterns corres pondto thosefor phytoplanktonbiomass and production, suggestingthat bacterial metabolisrn is supported by the productsof in situphytoplankton production during the sam~ year. These~>ndings are describedin greaterdetail Maloneet al. l986! and in our reports to the EPAChesa peake Bay Program. Comparisonof the bacterialand phytoplanktondata setsshows that from late springthrough the summer,bacterial biomassand production are very high throughoutthe regionfrom the Bay Bridge to thePotomac, i.e., the region impactedby anoxia. Usingthe conversionfactors rnen- tionedabove, and assuming a carborv.chlorophyllratio of $0, we estimate that bacterial biomassand production are about equalto phytopianktonvalues in manysamples. In ourl984 data set n=400!,bacterial biomassis greater than phytoplanktonbiomass by up to a factorof 3 in abouthalf the samples.This means that the bacteria are a highlysignifi- cantpool of carbonin theChesapeake Bay plankton. In the rnid-Bayregion, more carbon appears to be flowingfrorA phytoplanktonto bacteria than in otherestuaries, and thi»s unequivocally true in comparison to the ocean. This conclusionis supportedby the data on productior rates. Most studies in the literature show that in lake~ estuaries, and in the coastal and open sea, bacterial Pro- duction is about 5-30%of daily primary productio" Ducklow,l983!, It is generallysupposed that bacteria»'~ DissolvedOxygen Processes / 83

conversionefficiencies of around50%, suggesting that about lo 60% of the total primary production eventually flows through the bacteria. In this regard, the situation in Chesa- peakeBay, now supported by a large body of data, is some what extreme. In our combined f984-86 data sets, bacterial productionin the euphotic zone aloneaverages about if096 of primary production. This is hard to explain even if bacteria are highly efficient, unlessthey dependto a great extent on allochthonous carbon. Yet several lines of reason- ing lead us to believe that this is not the case. First, as we mentioned above, bacterial variability corresponds in general to phytoplankton patterns. Furthermore it is well- established that most of the particulate organic carbon in the rnid and lower Bay is derived from in situ production. We also know that the dissolved compounds used by planktonic bacteria have turnover times of a few hours at most. These considerations make it seem unlikely that bacteria are supported largely by remote sources of organic matter. This leaves three alternatives: I! our measurerrients are wrong and our assumptions are incorrect, ! high bacterial prodiKtion is a consequence of accamposition of accurnula- ted phytoplankton biomass on scales of weeks to months in late spring or, ! the plankton system in Chesapeake Bay hasbeen modified in such a way that a very large amountof primary production is cycled through the bacteria by mech- anisms we do not yet understand.

Without going into much detail here, we wiII state not too surprisingly! that we lean toward the latter explanations, while leaving open the possibility that our data may require reinterpretation as more data come in. However, we note again that we have used conservative assumptions in converting our data to carbon units. Even using the most conservativefactors, we estimate that on the average70% of the primary production is metabolizedby bacteria on a daily basis.

1986 we began to analyze experimentally the as- sumptionswe have used for making our estimates of bac- «rial production. For examplewe havebeen makinginde- pendentmeasurements of bacterial growthrates in low and highlevels of oxygen.In generalthe preliminary findings gg/ DissolvedOxygen in theChesapeake supporttheidea that bacterial production isvery high. havealso begun to investigatethe effect of anoxiaon the incorporationof thymidine into DNA, R'NA and protein in bacterialcells. Ourfindings show that in lowoxygen condi tlonsfsubstantially lesst hymidine is i ncorpora ted into DN comparedto samples with saturated oxygen. This thatgreat care must be taken when interpreting thymidin datafrom the bottom layer in surnrner.Our estimatesof bottomlayer bacterialproduction in summertake effectinto account,and showthat ir july-September bacterialproduction is onlyabout 5-20% of the overlying primaryproduction. We conclude that hypoxia and anoxia havea profoundeffect on bacterial metabolisrn, and thus probablyon the patterns of carbonflux in theplankton, whichare otherwise bacteria-dominated. Whatis the importanceof thesedata for understanding anoxia?W'e have shown that the ChesapeakeBay appears to benotable in that a verylarge amount of carbonis diverted throughthe bacteria,and that bacteriaare oneof the largestsingle pools of biomass inthe plankton. Estimates of biomass-specificoxygen utilization rates vary widely, but eventhe lowerones, when combined with our data,show that mostof the measuredwater columnoxygen demand could be met by bacteria alone. Weare not yet sure of thevalidity of ourestimates of bacterialcarbon flux, except to saywith certaintythat they areas high or higher than in mostother systems studied to date. Similarly, we do not understand the processessup- porting,this veryhigh bacterial production. But wecan statesafely that understanding theprocesses generating and maintaininganoxia in ChesapeakeBay requires continued researchon watercolumn processes in generaland bacterial processesin particular. Duringthe next year or two,we plan to fill in poorly sampledtimes of the yearand to investigatein great« detailthe factorsinfluencing our assumptionsabout b>c- terial carbonfluxes, including the e f ectsof loweredoxygef' onbacterial physiology. We also plan to try somemodellif'g approachestosee if theyshed light on the ecological mech anismscapable of supportinglarge bacterial production. Dissolved Oxygen Processes/ g5

RefefeAces Ducklow,H.W. 1983. Productionand fate of bacteria in the oceans. BioScience 33:394-501

Malone, T.C., W.M. Kemp, H.W. Duckow, W.R. Boynton, g.H, Tuttle and R.H. jonas. 1986. Lateral variation ia the production and fate of phytoplankton in a partially stratified estuary. Mar. Ecol. Prog. Ser. 32:149-16p. Wright,R,T. and R.B. Coffin. 1983. Planktonic bacteria in estuaries and coastal waters of northern Massachu- setts: spatial and temporal distribution. Mar. Ecol. Prog. Ser. 11:205-16, IMPLICATIONSOFMICROZOOPLANKTQN GRAZING ON CARBONFLUX AND ANOXIA IN CHE iAPEAKE BAY alevinG Qeliflerand David C. Brownlee BenedictEstuarine Research Laboratory The Academyof Natural Sciences

Lawrence%. Harding,Jr. ChesapeakeBay Institute The johns HopkinsUniversity

Aspart of theNCAA-Maryland andVirginia Sea Grant EPAsponsored multi-investigator collaborative research programon anoxia in mid-ChesapeakeBayin l986,we initiateda programto determine the role of rapidlygrowing microzooplanktonin grazing phytoplankton and bacteria io rnid-ChesapeakeBay. Theprogram was designed around severalassumptions: l! after stratification, cross-Bay "tilting"of thepycnocline causes upwelling of nutrient-rich, lowoxygen waters at oneside of theBay which results in highphytoplankton biomass and/or productivity in these shallowareas; ! phytoplanktonresponse is relatively rapid, theoreticallyfavoring rnicroheterotrophs thatcould rapidly utilizethe rapidly growing autotrophic assemblages; and! phytoplanktonnot grazed might sink to sub-pycnocline rnicroaerobic or anoxic waters, perpe tua ting continued oxygendemand and anoxia in mid-channelwaters. ln May,after stratification, but prior to developmentof anox ic conditions, cross-baytransec ts were madefroe 38 23'to 38 58'N.Temperature, salinity and density as weIi as in vivo fluorescenceand dissolved oxygen levels were de- terminedthroughout the water column in 3-5stations/transect. Therewas little evidenceof any cross-baytilt in t~e pycnoclineand concentrations of chlorophyllin surface waters were low .0-9.7 y g/I! and similar from east-to- westat a givenlatitude. Chlorophyllconcentrations did increasewith depthand up-Bay, however, primarily due to DissolvedOxygen Processes highconcentrations of the dinoflagellateprorocentrum and Ch ~Cl II . 51- f hi maximawere observed at andbelow the pycnoclineand can beattributed to up-Bay transport of Prorocentrum. Sincethe May cruisesdid not coincidewith anycross baytilt of the pycnocline,rnicrozooplankton grazing exper- imentswere conductedwith planktonassemblages assocj atedwith the sub-surfacechlorophyll maxima "seed" popu lationsfor surface bloomsaccompanying tilting! and,as a contrast,chlorophyll-poorer waters from mid-channelareas planktonassociated with non-tilting conditions!.

[ndividual microzooplankton species were isolated and transferred to tubes containing whole water from the col- lectiondepth. Using single or dual label radioisotopetech- niques, rnicrozooplankton grazing rates Q/individual/h! on autotrophsand bacteria were determined. Microzooplank- ton carbon demand was estimated for each species from the productof grazing rate and the available phytoplanktonor bacterial carbon. Total carbon demand for the microzooplankton community was estimated from the rates measured in the study and rnicrozooplanktondensities. Finally, the quantity of carbon remaining after microzooplanktongrazing was estimated from the difference between phytoplanktonbiomass and rnicrozooplanktonrequirements. Phytoplanktondistributions in the Bay in May generally re flee ted the contr ibutions of Prorocentrum and ~CI Ih. Pt y pl k 1 gag pp mately625-1801 pg/1 in the samplesemployed in the graz- ingexperiments, with total cell densities from 7-t5.2 x 1o/l. Microzooplanktondensities ranged from 771-31029 ~rivi«als/1. Ciliates were dominant though rotifers, «pepod nauplii, and polychaete larvae were also present note:heterotrophic flagellates were not enumerated!. Largemicrozooplankton were most numerous at station 8>~C8 ~8'N! in early and late May,coinciding with high- chlorophyll concentrations 6-28 vg/1! and Proro- centrumdensities i6.6 and 6.9 x tp6/t, respectivetyi;total 88/ DissolvedOxygen in theChesapeake rnicrozooplanktondensities were 2312 and 4505individ uals/I,respectively. High Prorocentrum densities were associatedwith highdensities of severalof thelarger taxa, tnciudingpoiychaete larvae and rotifers while the lowest dinoflageliatedensity .6 x 105 cells/I! encounteredat station840H 8 40'N!was associatedwith the lowest numbersof the largest rnicrozooplankton taxa <10/I!. ln May,rnicroooOpiankto grazing experiments wefQ conductedwith ten "populations,"including rotifers ~5nchaetasp.l,polychaete larvae, copepod nauplii, an its i s bich I h* ' dpi inata.Grazing rates for taxafeeding on phytoplankton were «ow,ranging from 0.01 ul/h forT. acuminatato 19.17>i/h for polychaetelarvae. Two S nchaetaspecies had rates ranging,from 0. 34-9. 74 p I/individualb. Theoligotrich species,Strombidium, was typified by ratesfrom 0.05-1,00 tfi/individualh. Copepodnauplii, at <46/I, cleared1.45 pl/naupiius/h. Microzooplanktongrazing on bacteriawas alsolow althoughseveral taxa had higher grazing rates on the thymidine-labeledparticulates. Grazing rates from all experi- mentsranged from 0.08-3.64 vl/individual/h. The tintinnid T. acuminata,one ~Snchaeta species and one of twoStrom- bidiurnpopulations, was typified by grazingrates of 0, !8, 0.53and 0.38tel/individual/h, respectively, or rates 8, 1.2 and8.4 times the grazing rates measured for thesespecies feeding on phytoplankton. ln late August,cross-bay transects at 38 30'Nrevealed higherchlorophyll as in vivofluorescence! in surfacewaters on the westernshore. However,there was little tilt in isopycnalscross-bay and low dissolved oxygen was encountered at depths>5.5 rn! on the westernshore only on the 2th. Di fhgdl *, pi v By~G&i i,c and~pof krikos sp., dominated phytoplankton biomass in samplesfrom thewestern shore, reaching 1 x 10 «lis/I Dinofiagellatedensities in mid-Bayand on theeastern shor~ were0.2 and0.1 x 106 cells/I, respectively. Total microzooplanktondensities followed a similarcross-bay trend with Dissolved OxygenProcesses / 89 highestdensities on the western shore >9500individuals/1!, dominatedby tintinnids 880-5320/1!, oHgotrichs280 412p/1!,"other" ciliates 480/1! and equal contributions -19<282/1!of rotifers and copepodnauplii. Microzooplank- ton densitiesin stations to the east rangedfrom 29pl-6452 individuals/1with decreasingcontributions for all groups.

Grazing experiments were run with three microzooplankton taxa, the rotifer S nchaeta st lata, a tintinnid ciliate Favella sp. and an oligotrlc ci iate La oea. Experi- ments were conducted with samples from the western shore and mid-Bay, representing high and low phytoplankton concentrations. The rotifer derived much of its carbon from bacteria. Grazing, rates were 4-1p times higher on thymidine-labeledparticulate material versus 14 C-phyto plankton resulting in bacterial carbon forming 61% and 34%, of total carbon ingested. As an herbivore, S ~stlata con- surnedonly 0.33 and 0.03 pg C/1/d in the western and mid- channel stations, respectively. Favella sp., recognized as a major predator on dinoflagellates, preferred phytoplankton with grazing rates of 2.06 and 1.77 yl/individual/h, respectively, resulting in the removal of 1179 and 392 pg C/1/day. Bacterial intake, with grazing rates of 0.07-0.09 vl/ind/h, was very low at >0.013 ygC/1/d. I.aboea sp. was characterized by the highest grazing rates encountered in the study, The oligotrich grazed phytoplankton andbacteria at 31.16 and 6.30 p 1/ ind/h, respect i vel y, in the western shore, di no f l age 1 late-r i ch s tat ion whi le rates of 2.88 and 3.56 yl/ind/h were obtained in the mid- channel, dinoflagellate-poor station. The rates for the western shore population are somewhat in doubt and thus the lower rates from the mid-channel station were used in all calculations. In mid-Bay, Laboea would remove 12.26 0.72 p gC/1/d from the phytoplankton and bacterioplankton, respec tively.

Total carbon demand by May and August microzoo- planktonassemblages was estimated from the productsof "average"grazing rates and densities assuming no net growth in either plankton component. In May, microzoo- planktonherbivory would remove 13-55% of phytoplankton qp/ DissolvedOxygen in the Chesapeake

In August,microzooplankton would consume 21- $3~o fthe available phyto plank ton carbon.These data ly thatthe smalle st herbivores in the Bay could be the most

Zooplankton-AnoxiaInteractions

Zooplankton, the dominant herbivare gro~ in ChesapeakeBay, may be adversely affected by anoxic bottom waters because anoxia would limit the normal diel vertical migration behavior of the dominant copepod species; reduce the amount of "available" habitat, thereby increasingcompetition for food and susceptability to predation; and patentially reduce zooplankton recruitment as a result of copepod eggs sinking into the anaxic bottom waters where they would die. In addition to being adversely af f ected by anoxia, zooplanktonmay contribute both directly and indirectly to thedepletion of oxygenin the bottomwaters of Chesapeake Bay. It has been observed in other marine habitats that zooplanktonaggregate closely aboveoxygen minimum or anoxiclayers. Zooplanktonwhich accur at the oxyclinecan beover an order of magnitudehigher than densities found in otherparts of the water column. In ChesapeakeBay during spring,summer and fall when the feedingactivity ami respirationof zooplanktonis highest,aggregations of zoo- Planktonabove the oxycline wouldresult in a sinkfar oxy- Ren Zooplanktoncould thus reducethe diffusionof oxygen '~tothe bottom watersthereby contributing to the maintenanceof anoxia in the Bay. Zooplanktonmay also indirectly contribute to bo initiation and maintenance af anoxia in bottom 92t DissolvedOxygen in theChesapeake

their lack, or reducedutilization, of phytopiankton.Un- grazedphytppiankton production would sink to thebottom waters where its decompositionwould reduce oxygencan centrations. "Uncoupled"phytoplankton-zooplankton inter- actionswould occur whenthere was a rapid increasein phytoplanktonproduction perhaps due to anepisodic input pf nutrientsbecause of wind mixing or pycnoclinetilting! which is faster than increasesin zooplankton productionand grazingas a resultof a bloomof a phytoplanktonspecies k y Py Pl k P ~Ph y P, Ceratiurnsp. andwhen zooplankton populations are reduced by predatorssuch as ctenophores. All of thesesources of 1'Iuncoupling ~ zooplankton g razingf romphy topiank ton production can be important during the period of anoxia in Chesapeake Bay.

project Description As a componentof the Marylandand Virginia SeaGrant dissolvedoxygen program, we are examining the role Of zooplanktonin the onset and maintenanceof anoxiain ChesapeakeBay. Our sampling strategy has been to rnea sure the vertical distribution of zooplankton biomass, abundanceand grazingon scalesof hours-daysduring seasonal extremes of freshwater flow. To date we have conducted these measurements in May 1986, prior to the onset of anoxia, and in August 1986 when the water column was stronglystratified and the bottom watersanoxic. During the 1987field programwe will measurezooplankton abundanceand grazingduring two-weekperiods in March,May and August. Additional measurementsduring the 1987 researchprogram will include estimates of zooplankton nitrogen excretion, in situ oxygen consumption by zoo- planktonand a laboratorystudy on the effects of anoxiaon the hatching success of copepod eggs. Specificobjectives of our researchprogram include: 1. Determiningthe diel, fine-scaledistribution pattern 0< zooplanktonin relation to the density structureof th< water column,oxygen concentration and phytpplankton biomass. DissolvedOxygen Processes / 93

Measuringthe in situ ingestionrate of zooplanktonto determinehow muchof the daily phytoplanktonproduction is grazed by zooplankton. 3. Measuringthe in situ oxygenconsumption of zooplankton so that their direct contributionto oxygendepletion can be estimated.

Measuring the nitrogen excretion rage of various size- groups of zooplankton.

5. Determining the density of gelatinouszooplankton and applying Published values of their consumption rates so that we can estimate their role in reducing the phytoplankton-consuming zooplankton copepods!.

Examining the effect of oxygen concentration on the hatching success and development times of copepod eggs

Results to Date

We are currently enumerating the zooplankton samples that were collected during the 1986 field season. The biomass of daytime zooplankton >60 Um! above and below the pycnocHne at the mid-Bay station illustrates the effect of anoxia Figure 1!. ln May Figure la!, althoughthere was less dissolved oxygen in the bottom water, greater amounts of zooplankton were present below the pycnocline. Zoo- plankton biomass ranged from 2.8 - 3,2 mg C/m 3. in surface 3.' waters and from 3.8 - 8.0 mg C/m in the bottom waters of the rnid-Bay station. This pattern of vertical abundancewas reversedin August when the bottom waters vere anoxic Figurelb!. On 8/11 when there wasa sharpoxycline and no d tecgable oxygen below 12m, surface zooplanktonwere over 7 times higher than concentrationsfound in the bottom Over the August study period 8/10-8/27! the dis- Mlvedoxygen content of the bottom water of the mid-Bay stationincreased likely as a consequenceof wind-induced predation near the western shore,th}}s releasingthe phytoplanktonfrom grazingpressure. If t»s Dissolved Oxygen Processes / 95

royes correct there may be more organic Pdeposition ".. as phyto-detritvs!near the vresternshore as compared to the easternforay C O00 < c~ I$ o~ ~ 0 g 0 0 o ">L

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Figure2. Densitiesof copepodnauplii at the mid-Bay station in May andAugust. Shadedbars representnauplii abovethe oxycline;open bars representnauplii belowthe ox ycline. 6o2 3

MAY STATIONS

6o mE I2 3

AUGUST STATIONS

Figure3. Biomass displacement volume = cc/m3 ! of gelatinouszooplankton >500ym! over a two-week period in May upper! and August lower!, 1986. Sampbng transect across ChesapeakeBay off Little ChoptankRiver = Eastern,3 = Mid-Bay, 1 = Western. CONTRI5U'nONOFSULFUR CYC1 fNG TO ANOXIA IN CHESAPEAKEBAY

3onH. Tuttlc, Eric E. Rodeo and Charles L. Divan ChesapeakeBiological Laboratory, CEES University of Maryland

Sulfurcycling in estuariesis a dynamicprocess involv ingboth the water column and sediments. It is comprised of two key reactions:the reductionof sulfateto sulfide, catalyzedby obligatelyanaerobic bacteria and fueled by organiccarbon from phytoplankton production; and sulfide oxidation,which consumes oxygen and may occurbiolog- icallyor abiologicaily.We report bere preliminary results of our 1986 studies to: l. Determinethe contributionof sulfide oxidationto oxygenconsumption at the 02/H2Sinterface during water column anoxia. 2, Determinewhether water columnsulfide oxidation is biologicallycatalyzed or is a strictly abiologicalre- action. 3, Estimate sulfide flux from sediments to the water column. 4. Relate depth-integratedsediment sulf ate reduction rates to hydrogensulfide flux from sedimentsto the water column. Determinethe factors controlling sulfate reductioni' the water column and in Bay sediments. Anoxiain the Bayin summer1986 was widespread i4 themesohaline portion of theBay, but persistentanoxia did not occuruntil mid-julyand lasted in our studyarea ~> Dissolved Oxygen Processes/ lp1 lateral transect at the Choptank River! only until rnid August. Becausesulfate reduction in the water columncan occur only during anoxic events, measurements of water column sulfate reduction were confined to August. column sulfide concentrations during 1986 were about two fold higherthan in 1980and 1985,reaching levelsas high as 3q>M. Water column sulfate reduction rates were ~ to ten-fold higher than observed in previous years with the highestrates l2-20 mmol S2- /m 2 /d! found at the deepest mid-channel stations. High integrated water column rates were a function of both increased depth of the anoxic zone at these stations and higher rates of sulfate reduction jn bottom waters near the sediments. Water column sulfate reduction seems to be limited by available bacterial carbon and energy sources. Addition of lactate to sulfide-bearing waters increased the rate of sulfate reduction by as much as twenty-fold.

Despite high rates of water column sulfate reduction, sedimentsstiff dominatedtotal sulfide yroduction. Mean August rates were about 55 mrnol S /m /d. ln agreement with previous years, sediment sulfate reduction was strongly influencedby temperaturewith a Q~o of aboutthree. However, there also appeared to be an influence of carbon flux, particularly at the shallowest CP2, Zmax = 8-12 m! of the two sites studied. Sulfate reduction rates were usually higher at the shallow station than at a deeper, mid-channel site CP3, Zrnax = 16-2Q rn. Nevertheless, depletion of sulfate with depth was most pronounced at CP3, probably due to the tendency of the sediments at this station to remainanoxic during the summer. During August, sediment sulfate reduction at CP3 was likely limited by sulfate below the 4-6 crn horizon. Sulfate was virtually undetectableat 8- lp cm. Pore water sulfide concentrations increased during the summer,reaching a maximum of about ll mM in mid- August. Pore water sulfide concentrations at CP2 also increasedduring the summer but remained more than an orderof magnitude lower than at CP3, reflecting the fact that surficlal sediments at the former site were seldom, if ever, anoxic. Total reduced sulfur in the sediments ranged from 5p to>050 mM. LO2/ Dissolved Oxygen in theChesapeake

Preliminaryexperiments to assesscarbon flow through sedimentsulfate reduction were doneat CP3 in Augost Acetateturnover was extremely rapid with turnovertin,e, as low as 4 min.and increasing gradually with depthin th< sediment.Lactate turnover was also high in the 0 4 cr horizon,but decreasedrapidly below 4 cm. Acetaterespir ation wasmarkedly higher than lactate respirationbelow 2 cm,suggesting that acetateis preferredover lactate as > carbon and energy source by the sediment rnicro flora Assessmentof pare water organicacid concentrationsQy GC-KC determinationscurrently in progress!will be re quiredto fully evaluatethese data. A series of measurementsof sulfide oxidationin the water column were made during the period o f August anoxia. Althoughthe resultsof these determinationshave not yet causedus to modifyour previousestimates of 9 mg 02/i/dconsumed during sulfide oxidation, thedata suggest that maximal rates of sulfide oxidation occur within a narrowrange of sulfideconcentrations .5 to 1.5yM 5 2- Thus,significant rates of sulfideoxidation may be confined to a verynarrow depth band

The primary objectives of this project were to measure and compare rates of oxygen consumption by water-column and benthic processes and to estimate the effect of these processeson turnover of oxygen pools in the bottom water of Chesapeake Bay during spring and summer. Secondary objectivesinclude: l! measuring flux of sulfide from sedimentsto anoxic overlying water during surnrner; ! parti- tiorling water-coLumn respiration into ecologically relevant size groups; ! estimating the role of benthic nutrient recycling in maintaining high rates of plankton production duringlate spring and summer; ! comparing measurements Of oxygen and nutrient fluxes across the sediment-water interface using shipboard incubated Intact cores versus in situ chambers. Duringthe~kiendar year 1986water-column aod benthic respiration rneasurernents were made at two stations Cp2,lO rn; Qp3 25 m! In mid-ChesapeakeBay 8 34-3'N! duringtwelve cruises on the following dates: Mar 27; Apr 4; Apr ll; Apr l.8; Apr 25; May 2' May 14; May 18; May 22; 30~l5; Aug 10-15; Aug 25 26.' Nut ient fluxes across the iment su~fa~e were also ~~a~~re sedimentnitrification rates were estimated by N-S«v technique! on selected occasions. On ten dates water- columnrespiration was partitioned into three size-groupsby 104/ DissolvedOxygen in the Chesapeake pre-filtration; <3 pm,bacteria and protozoa; 3 64pm phytoplanktonand small zooplankton; >64 pm, largezoo plankton.Sediment-water fluxes of oxygenand nutrients were measuredsimultaneously using both in situ chambers and shipboard-incubatedintact sedimentcores on four occasionsin Mayand August. Fluxes of sulfidefrom sedi mentswere measuredat CP3 twice duringanoxic summer conditions. Benthicmacrofaunal communities were sample throughspring and summer conditions, and animal abun danceand diversity havebeen estimated. Althoughdata analyses are still in progress,varioUs resultsare presentlyavailable. Some implications of these results have been considered;however, fuil statistical analysesare incomplete. Benthic oxygen consumption rates wereconsistently higher at the10 rn station CP2! compared to thedeeper station CP3, 25 m!. Ratesat CP2increased steadilyfrom ca. 1.0g 02/m2/d in April to ca. 1.7g 02/m2 /d inAugust. During the same period benthic oxygen consumptionat CP3decreased from about0.8 to 0.4g 02/mjd. Thismarked difference in rates and seasonal trendsTnay be attributable,in part, to the substantially lowerabundance of rnacrofauna at CP3 in spring,with popU- lationsdecreasing at the onset of hypoxiaand being elimin- atedduring surnrner anoxia. However, measurements of sulfatereduction by Tuttle et al.,personal communication! suggestthat there is considerablemetabolic activity in the deepersediments during this period. Direct measurements of sulfideflux fromsediments at CP3during August revealed rates whichare stoichiometr ically equivalentto the highestoxygen fluxes measured during this study ca. 1.S g 02/fn2 /d!. Thus,it appears thatsulfide release from sediments and subsequentoxidation in the water columnrepre- sentsan important oxygen consuming process which has not beenpreviously considered in mostoxygen budgets. Ratesof benthicammonium regeneration were sufficient to supply30-60% of thenitrogen required by phyto- planktonfor steady-state growth inspring and summer, l> general,ammonium recycling rates f olioweda seasonal patternwhich appears tobe strongly correlated to tempera- Dissolved Oxygen Processes / 105

However,preliminary analysesindicate that ammoni qmrecycling rates are directly proportional to contempora neousrates of particulatenitrogen deposition if one applies a temperature-dependentlag-time between deposition and regeneration.Rates of particulatenitrogen deposition exceededregeneration rates by a factorof 2-3in spring,but thetwo processes came into balancein August.During the spring,nitrogen loss via denitrification was estimated to be aboutone-half of the total nitrogen cycling; however, denitrificationapproached zero in summer,presumably a resultof low redoxconditions and associated loss of macro faunalpopulations and nitrification. Respirationrates in surfacewaters increased steadily from early spring to late summer with warmingwater temperatures.Rates increased from ca. 0.0l mgO /h in Aprilto ca. 0.02in May to 0.035in August.Oxygen con- sumptionrates in bottom-watersduring spring and at the pycnociineduring summer were similar to ratesin the surfacelayer, Pre-filtered respirationrate measurements indicatedthat the organisms< 3um consistentlyaccounted for about45-70% oi the total oxygenconsumption in surface waters and 70-100% of consumptionin bottom and pycnoclinewaters. The 3-6Q ijm size-fractionaccounted for mostof the remainingrespiration with a fewexceptions in latespring and summer when m tabolisme associated with largerparticles appeared to contribute10-20% of the total respiration.During one August cruise, pelagic respiration wasmeasured 0-5 timesspaced over a 20h period.Signifi- cantdiel cycles were revealed with highest rates occurring in morning,and markedly reduced values in lateafternoon

Fisher: I had been asked earli er about ammoniumregener- ationin the water column and in the sediment. Mike Kemp justprovided some numbers for ammoniumregeneration and I just made a back-of-the-paper calculation to make sure that oxygen consumption in the water column and the sedi- rgents was at least the same order of magnitude, if you accept his assumptions. If you make the same computations for ammonium regeneration you get numbers which are againthe same order of magnitude. That is, there is some sort of equal partitioning of the ammonium production per squaremeter in the subpycnoclinalwaters and in sediments, or within a factor of 2 to 5 of the same order of magnitude, So that goes along with the story that the matrix is showing for oxygen.

Mountford: Mike, am I reading you correctly, that the sedimentsare not serving as a major cumulative sink, but that you are getting processing of what comes in, that is a reasonable steady state over the period of an annual cycle?

Kemp; Yes, that's my feeling. There is some burial, and there is this lag, particularly between deposition that occurs in the spring or in the late winter. There is a lot of phytoplankton production occurring in the later winter/early spring. Some of that is deposited, and it's consumed at a very low rate in the sediments. There is very little accumulation in the sediments. In fact, according to Scott Nixon's nutrient budget of the Bay, I guess there is not enough.

Boynton: The particulate carbon profiles look pretty straight,once you get down belowthe surfacelayer. Itis mostly a surface phenomenon, and seasonally up to an annual period I think most of what gets there goes away; lags during cold seasons,less lags during the very warm seasons. !OS/ DissolvedOxygen in the Chesapeake

Mountford:So, once we are ableto establishmore oiigo trophiccanditions in the Bay, then you wouldn't have some terriblylong memory to contendwi.th. Boynton:My initial feeling was that the sedimentswould' [havea memory]but based on experiments we have done, l think that for nitrogenanyway there is not a longmemory, At thispoint, I woulddefend that paint of viewmore than! would defend the opposite one. Mauntford: Whatabout phosphorus? Boynton:I don'tknow. I'rmam waiting for someoneto discovera gaseousphase and we would beoff to the races. Kemp:! shouldadd a disclaimerto that, Kent Wait Boyntonand I publisheda paper a fewyears ago that said that therewas an excessamount of carbonleft overin the fall thatdidn't get consumed. We conceived that this might providefood for spring recruitmentof benthic invertebrates,We published that basedon somereasonably goodestimates, butnow! doubt that it in factholds. I think it is mostlywhat is producedin the springand the summer that supportsthe secondaryproduction. Malone: What! want to know is once we addthese heterotrophicprocesses up, can we still considerthis a phytoplankton-driven system? Ducklow: Onecomment once you see all this carbongo intothe bacteria, people may ask how there can be any left overfor anythingelse. Which makes the point that a lotof whatthe bacteriaget comesvia other heterotrophicprocesses,especially grazing. They are not at all mutually exclusive,and they aren't simply additive. Tuttle: But if it doesn'tcome directly from the phytoplankton,it's evenharder to justifybecause it means it is goingup a coupleof trophic levels and still comingmostly throughthe bacteria. The most efficient way to explain« is that it's coming directly from the phytoplankton. DissolvedOxygen Processes / 109

Ducklow:I don't think there is a lptof evidencethat a large p t of thebacterial production is comingthat way Tuttle: 1 know. And if your bacteriaare takingover 100' andso are the microzooplankton,and you still get hypoxiain thesediments and we still catcha fewfish, we all bettergp to flush our toilets to keep the systemgoing becausethe phytoplankton aren't doing it!

Comment: It seems Like we have two problems: Malpne is showing data for accumulation of phytoplankton below the pycnocline the accumulation of phytoplankton productioncreating an oxygen demand. On the other hand, lppking at the grazing numbers, we are apparently consum ing too muchof the phytoplankton. The bacteria and micro zooplanktonare sometimes consuming greater than 10096,

Ducklow: But not in the spring [general agreement].

Sellner; When I just went through these numbers, the total grazing pressure by both our groups but not bacteria would be about 30% in the spring.

Comment: So that would leave 70%. That's still consistent then.

Kemp: That is exactly the respiration that we got for bac ter ia.

Question: jon Tuttle has said that sulfur oxidizers are using Upa lot of oxygen 3on, how much carbon are they fixing?

TUttle: 1 didn't say sulfur oxidizers were doing that. So far purdata suggestthat most of the sulfide oxidationoccuring in the water column is truly chemical. After postulating

Brownlee: And mesozooplankton. Roman;Most of the copepodspecies are omnivores y~ cangrow them on the tintinnids and other microzooplank«n that DaveBrownlee and Kevin Sellnerlook at. Dataan zooplanktonandmicrazooplankton showsort of aninve« relationship. Dissolved Oxygen processes / I I I

What we plan to do this summer are same more cagecultures with thedominant microzoaplankton species andsee if in fact we canget anungrazed maximum growth mare or less. So we would have a better estimate of totalmicrozooplankton turnover demand,versus a doubling time for phytoplanktonof once every two daysor once every day. Question;What stimulated you to plot microzooplankton grazingversus dinoflagellate biomass? 5ellner: We were looking through some of our data and foundFavella as one of the dominants, and Stoeckerat WoodsHole OceanographicInstitute hasreported this ciliate as a dominant grazer in red tides in New Fngland. Therefore,we thought there might be a relationship between dinof lagellate biomass and rnicrozooplankton grazingpressure in theBay. Whenwe tried to establishany atnerrelationship between total rnicrazooplanktondensity andany standingcrop measure,nothing fell out; it was absolutelyrandom. One that did appearto suggestthat theremight be someresponse of the total microzooplankton communitywas biomassof the dinoflagellates.

Haas: Were you counting cyanobacteria? Sellner;Yes, but not with epifluorescencetechniques. Haas:In lower ChesapeakeBay onecan abservecoccoid cyambacteria at almost a million per milliliter substant ial numbers. 5eliner: Yes, we have values like that, but we never stressedthem because, again, we are stretchingthe limits of ourtaxonomists... hard to seeand identify! Mountford:Do microzooplanktoneat picoplanktono, mare specifically, cyanobacteria? Brownlee:We found significant grazing on bacteria as well as phytoplankton.game species,such as 5 ncha«a showeda higherclearance rate on bacteriat an P yt p I L2/ DissolvedOxygen in the Chesapeake ton in termsof ingestion,it was greater than SP+pn bacteria. Most other speciesconcentrated on phytopiank ton. Sellner: It wasalmost always autotrophic-dominated, in terms of ingested carbon. Mountford: So in that sense the picoplankton or cyanobac teria may be sort of waste production?" Sellner: Well, we are goingto do somesize fractionationto actuallylook at the prey for the variousmicrozooplankton, for instance the tintinnids Dave Brownlee was talking about, to geta betteridea of whateach size group is eating. Nixon: Hasanyone put togethera goodinventory of alloch thonouscarbon inputs into the Bay? I have not seenone. l don't think USCSor EPA did carbon in their studies. A surpr ising omission. Malone: I think it would be pretty Low, Scott, relative to autochthonous production.

Nixon: But we haven't done it yet; it is hard to know. Malone; We could probably estimate that, given river discharge rates, DOC levels, TOC, etc. Kern@The Biggs and FLerner article oncarbon budgets 4one way back gave estimates on a rough level. Malone: That was all POC, though. But if the POC was mostlyautochthonous, it's hardto seethat the DOC.ould be allochthonous it's more labile. Houde: I'd like to suggestan idea that we might w»t to consider,especially in light of the paperstomorrow. Th«e is a controversygoing on aboutthe eutrophicationof t< Great Lakes, and the causesof that -- this centers on ."~t is essentiallythe conceptwe havebeen talking aboutt«~y that nutrient inputs stimulate primary productionor increases in algal biomass and sedimentation and»yge" DissolvedOxygen Processes I l3 de+and-- the "bottom-up" concept versus the "top downlI conceptthat alterations in grazingpressure harvesting of Oystersor fish is essentiallydecoupling primary produc tjonfrom secondaryproduction, and what you thenget is + higheraccumulation of algal biomasswith or without changesin the nutrientloading rate. There'sunderstandably a controversyin the Great Lakesbetween the fisherypeople whothink it's removalof the fish that'scausing the greening Of the lakes, and the nutrient chemists who think it s the nutrient loading, We should think about that in the context pf the papers tomorrow for example, Roger Newell raised the issueof oyster grazing and effects of decreasingoyster populations we might want to consider whether the ac cumulationsof algal biomass leading to hypoxia in deep watersis the result of increasedphytoplankton productivity andbiomass or the result of the decoupling of that produc tion to secondary productivity. There is the interesting potential link between water quality anf fishery yields: if you increase the nutrient loading you can divert that into higher fish production or you can decrease water quality, dependingon whether that is coupled or decoupled.

Tuttle: I think you' ve hit on something here. It looks to me as if there has been a basic change in the ecosystem oi ChesapeakeBay. Now, one possible explanation for that is that there are too few oysters or too few striped bass or whatever, and what we' ve done is simply throw it into this phytoplankton-bacterial loop.

Newell: But if 8096 of the production is by species smalle~ than 3 microns, a lot of rnacroinvertebrates can't filter such smallparticles efficiently, perhaps only micropredators ran capitalize on those small cells.

Sellner; Interestingly enough,the largest productivity max is when the small cells dominate. 3onas:You bandied about this concept of a positive fmd backloop. Once you knock the oxygendown to a certain level,it might have this impact so that evenif it's not wipingthem out, it is selectingcertain groups, and all of a suddenyou have flipped it over into this phytoplankton- ] IO / Dissolved Oxygen in the Chesapeake production,microheterotroph-respiration systemwh h f all intentsand purposes l~ks like a waste waterer tr reatment t plant. It is an agedtrophic community from the bact pointof view,which is verygood at utilizingoxygen ac ter>al producingsome sludge. And t h at is whatwe are looking at. Comment: As muchas I hate to say it, it is like an alt«. ate steadystate for an ~osystem You can either go t08 respiration systemor you accumulate phytoplank ton biomass on the bottom and it consumes oxygen there or you ln !a~~ sendit througha trophic structure that results in oyster> and fish and so on, which is what we' re all trying to do.

gonas: Hugh, are you familiar with any systematic bacterial abundance historical records for the Great Lakes? In term> of eutrophication there versus a place like the Bay?

Ducklow: "History" for good bacterial abundance measurements is the last ten years. But interestingly enough, @on 5cavia at the Great Lakes lab is doing the same work in Lake Michigan, which is a cleaner, more oligotrophic system, and he has the same problem with bacterial production versusphytoplankton that we do here. The absolute levels are lower, but he is having trouble finding out where it comes from as well, Biolopca1Effects of Hypoxia DEPICTING FUNCTIONAL CHANGES IN THE CHESAPEAKE ECOSYSTEM

ftobert E. Ulanowicz ChesapeakeBiological Laboratory, CEES University of Maryland

The usual method of studying ecosystems' response to perturbations such as hypoxia has been to create simulation models of the system dynamics. Models require copious data on species stocks and intercompartrnental flows. lt is possibleto extract much information useful for deciding management issues from the structure of the exchanges itself, without having to invoke the manifold a priori assumptions required for simulation. One modeler has likened simulation modeling to studying the "physiology" of the ecosystem, whereas flow analysis is akin to inspecting the system's"anatomy" Figure I!.

Flow analysis can be made at several hierarchical levels, For example, one may calculate the total exchanges between any pair of species over all direct and indirect pathways. In this manner, one may portray the "extended diets" of species of interest. For example, the striped bass is knownto directly ingest bay anchovies,menhaden, crabs and alewives. But these prey in turn consume a host of otherinvertebrates and plants, someof whom feed on still others, etc. Using matrix and vector operations it becomes possibleto calculate the extent to which any organism « interestdepends upon any other compartmentfor direct and indirect sustenance Table l!. Although adult striped bass do not feed on zooplankton directly, the latter item has beenincorporated into about 67% of the striped bass prey. The extendeddiets of adult striped bass and bluefish are seento diverge in that bluefish are indirectly more dependent upon benthic materials and organisms than are the > l8 / Dissolved Oxygen in the Chesapeake striped bass. Hence, bluefish diets are more likely to be impactedby anoxicevents.

Similar matrix operations allow one to determine the average trophic distance over which each feeding organism obtains its food Table 2!. in the Chesapeake system, despite the existence of somefeeding pathways with as many as eight trophic links, no carnivore feeds, on the average,at trophic level 0 or higher Table 3!. If this assignmentrepre- sents "the apportionment of integral trophic levels among the species,"then it is interesting to note that the inverse operationis also possible. That is, knowing the various trophic pathways along which food reaches a particular species,one can divide the activity of that speciesamong the integer trophic levels in proportionto the intensitiesof the pathwaysof various lengths. The end result is to trans- form the arbitrarily complicated network of exchangesinto a "straight chain"of ever-decreasingtransfers the clas- sical Lindeman trophic pyramid. The effects of stresses suchas hypoxiaare most likely to be exhibited as changesin the uppertrophic elements of the chain. Any abruptchange in the trophic assignmentof a particular specieswould probablyindicate a strain on that organism. Oneof the outgrowths of the trophic aggregationexercise in ChesapeakeBay is the revelation that detritivory and saprophagyexceed herbivory by ninefold, thus underscoring the potential of anoxic events to modulateheterotrophic productivity in the Bay. Control in the ecosystem is usually indicated by feed- backcycles of materialand energy. Such cycles inherent in the web of exchangescan be enumeratedand extracted from the networkusing an appropriatebacktracking algorithm, Thepattern of feedbackin the Chesapeakesystem is bipartitewith recycleamong the pelagicspecies decoupled from feedback among the benthic and nektonic components. The entire suite of filter-feeding organisms engagein no feedback,but ratherperform the functionof shuntingmaterial and energy from onedomain of control« the other.

I l8 Biological Effects of Hypoxia / ll9

Finally, it is possible to characterize the development stageof the overall network usingtechniques borrowed from information theory and flow analysis. In particular, if one has access to the configuration of the system at two or ~ore different times, it becomespossible to quantitatively verify the existence of heretofore qualitative phenomena suchas eutrophication and ecosystem "health." A preliminaryquantification of carbonexchange among the 3> major components of the rnesohaline ecosystem during each season has been made. Work is currently underway to compare the structure of the Chesapeake network with a similar study of the Baltic 5ea being con ducted by the ASKOI laboratory of the University of Stockholm, DOC0

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Table 3: Trophic rankingsand average trophic levels of the major components of the Chesapeakernesohaline ecosystem

Phytoplankton l.oo 19. Nereis 3.00 2. Dissolved Organic Carbon l.00 20. Macoma spp. 3.00 3. SuspendedPOC I,00 2 l. Crustacean Sediinent POC l.00 Deposi t Feeders 3.X f. Benthic Diatoms l.00 22. C tcnophores 3.01 6. SuspendedPOC Bacteria 2.00 23. Fish Larvae !. 7. Sediinent POC Bacteria 2.00 Alevri e and Blue g, Free Bacteria 2.00 Herring l. 9. Misc. SuspensionFeeders 2.09 Shad 3.34 l0. ~Ma arenaria 2.09 26. Blue Crab !.fl ll. Oysters 2.09 27. Sea Nettle !.fit l 2. Zooplankton 2.34 2g- Hogchoker !A9 Clliates 2.58 29. Weakfish !.97 l4. Mciolauria 2.67 30. Croaker 400 Menliadt n 2.89 3l. Spot 4.GO l6. l!ay Anchovy 2.97 32. Whi te Perch l1. Heterotrophic 33. S tri ped Bass 400 Microf lagellates 3.00 34. Sum m cr F loun de r 4.l9 lg. Misc. Polychaetes !.00 3f. Bluefish 4.64 gg!I.OGICALMONITORING OF SELECTEDOYSTER SARS~ THE LOWERCHOPTANK REER

~ p. Christmas and Stephen3. Jordan TidewaterAdministration MarylandDepartment of Natural Resources

As Part of a cooperative project to investigate the poss'bleimpacts of anoxic and hypoxic water on Chopta k River oyster survival, six oyster bars were monitored weekly duringthe summer of l986. To determine survival rates, 60 sometimesfewer! adult oysters were collected by dredging and classified as either live, newly dead new box!, dying gaper!,or formerly departed old box!. presenceand abundance of yearling spat and selected taxa of fouling organisms were recorded from a subsample of l0 oysters weekly at each bar Figure l!. Temperature, conductivity, pH, dissolvedoxygen DO! and secchi depth were measuredat the surface, mid-depth, and bottom of the water column.

Total mortality, averaged over all sampling periods, ranged from 3.3% on Sandy Hill bar to 3~.2% on France bar Figure l!. There were significant differences in percent mortality among bars, with those closest to the confluence of the Choptank River and Chesapeake Bay having the highest proportions of both old and new boxes. Levels of new mortality were low, with no clear mortality "events" havingoccurred during the summer. However, abundance and viability of spat correlated positively with the viability «adult oysters. Variations in the abundance and viability of fou4ng organisms were observed, but these data requir'e further analysis.

No occurrences of anoxia were recorded, a~ hyl xia was minimal, with the lowest Do values betwe~ >" PM. was strongly correlated with pH Figure 2!, but not eIther salinity ~ temperature, suggestingthat Do v~ia- 126/ DissolvedOxygen in the Chesapeake tiOnSWere driyen primarily by phOtoSynthesisin the rela- tivelyshallow -6 m! watersover the oyster bars, levels of old mortalities at the downstream bars mayay hay ave reSulted frOr hyPOxia during Previous summerS from diseaseor other causes. This study was designedto short term mortality events which did not significantdegree during the summerof l9$6 Moqjtorj will continue in the summer of 1987 with the addition of an oysterpathology component. Complementary hydrographic studies conductedby DNR and the University of Maryland Horn Point Environmental Laboratories, will continue investigatethe hydrodynamicaspects of DO dynamicsin the lower Choptank River. QC d

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g L V I 3 gl a v Xgyrympy e&t~aeg glSCU55ION yeoman:How persistent was that low oxygen feature in the upperreaches of the Choptank?

3ordan: The continuous monitoring was not Long-term last summer,but a preliminary look at data from grid surveys indicates that it's reasonably persistent. ln 1984 I had essentially anecdotal evidence that DO was extremely low, 0.3 mgf'1in shallow water, at 9-5 m depth. l think what we're looking at are very short-term events that could potentiallyimpact the shallows. That'swhat we're trying to find....

Newell: There is good evidence that adult oysters can withstand 5-7 days of anoxia or longer, depending on the temperature. I don't think anything has been presented so far at this session that suggestsanoxic events persist for this long in shallow water, even on the WesternShore, in areaswhere you' ve got extensive oyster bars today.

Question: Did you look at fouling organisms, bryozoans, etc., which might be more sensitiveto low DO to seeif there was any evidence of mortality?

3ordan: We did look at fouling organisms and the haven'tbeen analyzed yet. We also looked at year-oldspat aridthere was a pretty goodcorrelation with spatsurvival sndDO in this downstreamarea. So~somethinis impacting t"ese downstream bars and it looks like a DO problem. whetherlow DO is the cause,or simply interacting with diseaseor some other stress, we don't know. Newell: That of course is the major question. There are severaldiseases, such as MSX,which are positivelycorrela- tedwith salinity and could also be affecting the oysters. Question:Do you know anyone who has been able to distin- g«» specific causes of mortality in bivaLves? 130/ DissolvedOxygen in the Chesapeake

3ordan;This would be difficult to dofrom any field mo, toringprogram because you don't actually see them die arenow looking for habitatfactors which could potentially impactresources, andto do that, we go out in the field and lookfor correlations.When we find correlations, then itt timeto supportmore detailed work and look at causeand effect. Kennedy:When you go out and bring in somedead oysters andfind relativelylarge live barnacles on their shell, then it'sprobably not oxygen, it's probably disease. Question:Could it be H2S? Kennedy:But would we expect the fouling community tobe lesssensitive to thisthan the oyster?In otherwords, if you havea community,and the oysterbed community is a very diverseone, and if all that'sdead is theoyster, then I thnk you're lookingat anoyster-related problem, not an ecosystem problem. 3ordan:That's why we are goingto look very hardat our foulingdata, using multivariate techniquest todiscriminate amongthe bars and their foulingcommunities. If wesee healthy, old f oulingcommunities associated with dead oysters,then it is probablyan oyster problem, not oxygen. Question:What is "oLdmortality?" 3ordan:This is a prettystandard technique in Maryland. Youdredge the sample,and looking only at articulated shells,if there'sfouling inside the shell,that's an "old rnor- tality." !f theshell is cleanthat's a "newmortality."

Question: No real time scale then? 3ordan:No, but for thisstudy we consider that themortal- ty wasfrom about a yearbefore. I hadn'tplanned to get into temporaltrends, but therewas some indication that we sawhigher levels of newmortality early in theseason, in 3une,and increasing numbers of old mortalityLater. T»s Biological Effects of Hypoxia/ l3l

thatperhaps there was a mortalityevent in thespring beforewe got out there, and there'sbeen a suggestionthat could have been due to disease.

For an oyster that is operating anaerobically, log can it go without depleting its glycogen? These xic events aren't continuous, but can be frequent and there may not be much reaeration in between. geweli: An oyster held out of water can stay closed "P > d ys or so at high temperatures, for 2-3 weeksat low tem- A couple of days of reaeration between anaerobic events should be OK; they have sufficient nutrient reserves to withstand a day or so of anoxia with a few days aeration in between. It is obviously going to be a stress eventually, because these energy reserves should be used for gametogenesis.So you have a possibility of impacting the wholepopulation by affecting reproductive success. At one time it was thought that bivalves in general had a very inefficient anaerobic rnetabolisrn, but now we know that is rot true. In fact, it is highly efficient and highly evolved, so we must not think that anaerobiosis is necessarily any great stress.

3ordan: In fact, intertidal oysters spend a great deal of time closed up, and do just fine. Mann: lf you believe the work the Dutch have done with mussels,and in fact also shown in some of Charlotte Man- gurn's work, the oyster functions to some extent anaero- bicaliy all of the time, irrespective of the oxygenenviron- ment. And that brings up anotherquestion which has not reallybeen addressed by anyonein this groupyet; that is, a significantpart of the benthosconsists of organismswhich areevolved to function in low oxygenenvironments, and if theDutch work can in any waybe transferred to therest of the animals that live in the benthos, there is a suggestion thata part of that benthosfunctions, in part,anaerobically all the time. Thenwhat you are gettinginto is carbonand »«ogencycling that hasnothing to dowith oxygen cycling So when we considertrying to do massbalances functioningin terms of N or C, andassuming that oxygen l32 / DissolvedOxygen in the Chesapeake runsin parallel,we may in fact be makinga mistakeonce weget into thebenthos. Comment;That getsback to what we were talking about yesterday:ofQ~ there and iscomPlete a bigdi anoxia,f ference becausebet ween a concentrationjust a small pf Q,l ping/l>s still enoughto providesome aerobic respiration for the benthos. Newell:l don'twant people to think that lowO2 andhy poxicconditions aren't a stresson the oyster theyare An oystercan withstand it, but its feedingactivities are compromisedduring this time, and according to its overall energybudget, it's going to be cornprornised.That is why it's important to considerthe effects into the next gener- ation. Mackiernan: I think that in some of the REMOTScamera work that wasdone in the Bay for the Corpsof Engineers, thephotos showed that in areaswhere there were consider- ablerepeated anoxic events the benthic communitywas essentiallylimited to opportunisticspecies which recruited in the winter, and there were no normal deep-livingpoly- chaetesand such. Obviouslythere is at least gradual attri- tion of the benthos. Kernp:You can see that alonga water-columndepth gradient anywhere in the mesohaline Bay. Mackiernan: True. ln fact some people have looked at old cores from areas that are now anoxic in most summers and foundlarge Macoma valves, which being rather a long-lived bivalve, you wouldnot expect to have survivedif anoxia were a common occurrence in the past. But no one has really pursued that.

Newell: l would like to ask Bob Ulanowicz a question. He seemedto show that the suspensionfeeders were the important links between the water column processes and the highertrophic levels,such things as ~Maand Macoma.What were the links from those bivalves into tbe higher levels? BiologicalEffects of Hypoxia] l33

Ulanowicz:The link betweenthe filter-feedingbivalves were primarilyblue crab andalso such nektonic spe [es as hogchoker,spot, white perchand so forth. Newell: Thatjust emphasizesthe importance of thesuspen- sion feedinginvertebrates in the overallnutrient cycling within the Bay.

C,omment: In aerobic waters....

Ulanowiczt Also, they are taking a lot of suspendedpot- out of the water column and putting it into the sediments. A major function.

Kemp; Well, from what I know in the main areaof the Bay in MarYland! which goes anoxic, Macorna baithica is the dominant species by biomass, and here at ieast it ts not a susPension feeder, it is a deposit feeder. Where or wh t are all of these suspension feeders, besides oysters? And ~Ma in the fresher areas.

3ordan; Barnacles....

Kernp: But that is part of the oyster community. Most of the area that we are talking about is soft-bot tomed carnmunit y.

C:ornment: Well, there are amphipods, polychaetes....

PUrceil: I have a question for Bob have you tried to incorporate certain complex life histories into this model at all so that you have both planktonic and benthic stages af these animals, or does that complicate the model so that lt is unworkabiea

Ulanowicz: They are combined,with the exception of fish larvae, which we did distinguish brause of their different feeding patterns and their importance later on. That still cornpiicateswhat you have,because if the feedinghabits of life stages are different, then your componenthere will h«e a more complicated multiplicity of inputs. I will say that what you are looking at is just for the summer.We are 130 / Dissolved Oxygen in the Chesapeake now essentially making seasonal snapshots for the four seasonsso we will have some idea of the temporal variation in this particular network. I think Gail Mackiernanasked earlier what would happen if you removed species "23" or "x" or..., There are a couple of things you couM do. Yoq could look along row 23 for large numbers in that ro~, indicatinga strongdirect or indirect independenceon that species, which would immediately alert you to a potential problem. Thereis alsoa form of sensitivity analysiswhich can be run on the topographical model alone we have thosealgorithms available. So if "23" no longer feedspq "13," we can determine quantitatively how that affects the network. I will also add that most of the software is available for the IBM PC, and is rather easy to use. jonas: This is a mesohaline model could you put same geographiclimits on what you think it applies to?

Ulanowicz: It's approximately from the surface 6 ppt iso- haline in the north down to about the surface 18 ppt iso- haline. Geographically, that's a little bit below Pooles !sland to just below the mouth of the Potomac. It's about roughly half the surface area of the Bay waters.

Comment: There was a recent study done in Europe showing that Macorna balthica can either suspension or deposit feed quite well.

Kemp: But I don't think it does here. Comment: Under highly turbid conditions they can switch over to suspensionfeeding. I want to make a point that in the lower Bay'shigher saline waters,we don't have significant Macoma balthica populations. So during low DO events we essentially lose all our fauna they are just completely wiped out. So the benthic community in deep basins where they havesignificant hypoxic eventsconsists of very shallow dwelling, short-lived organisms, no perennials at all. Kernp: We have the same situation in the areas Bob Ul anowicz was describing. Biological Effects of Hypoxia/ ling

Malone: Bob, how did you come up with the exogenous organicinputs. Ulanpwicz:Let me defer to Dr. Baird, whoessentially put together.

We just tried to balance that particular component and since it had to come from somewhere else, and was not generatedwithin the mesohalinearea, we assumedit hadtp come from outside.

Malone: In other words, the heterotrophic demand exceeded t"e primary Productivity inputs and so you put that info the balance? That's interesting in the context of what we were discussingyesterday.

3onas; In that context from yesterday!,in that DOC pool one of the questions is how much of that pool is labile biologically, and how much of it is that refractory yellow stuff" floating around out there. How might that drive the system?

Baird: We don't really know.

Kernp: But that doesn't really matter with this model, exceptfor calculating turnover rates or the like.

Ulanowicz: The labile portion is seen in that transfer from pools 2 to 6 for the most part. The entire stock is listed under"DOC," 189,560 rngC/rn in the water column. The turnover rate is rather slow; most of the turnover being due to the labile fraction. There is some smearing due to the aggregation of the labile and nonlabile fractions. ~onas: It would suggest, looking at that rate, that some- thinglike half the DOC might be labile. I haveno ideaif that'sa high number. Ulanowicz:Remember this is per summer,per 92 «ys~ "' per day. 5o you are tal.king about half being t"r" three months. lp6 / DissolvedOxygen in the Chesapeake ponas: I'm just lookingat that box on DOC and lookingat that rate up to pool6 the bacteria. If we think the BayIs an unusualsystem, how much of that DOC is labile comparedto thetotal that is there,and is the Bayreally very different than other estuaries in that regard? that proportionatebasis of dissolvedmatter whichpresu> ably has to move through this microtheterotroph community, Is that a driving force? I don't have a simple answer, Newell: I would like to get back to Bob Ulanowicz's point aboUt the DOC. Perhaps because the estuary flushed, the refractory DOC is being pushed out onto the shelf so you only have DOC that is relatively labile and fairly new getting recycled. So maybe you are right maybe it is fairly labile. 3onas: Some of the turnover rates these rates do faye someuncertainty! for dissolved organic pools -- glucosefo example are in the order of two hoursfor turningover the wholepooL on the basisof somedata that we have. Thereis more than one explanation for this, but they do appear to be very high. Up until 1986, our amino acid turnovers we were using this mixed amino acid composition that sup- posedlyresembles a phytoplanktonprotein hydrolysate were always much lower than the glucose pool, which sur- prised us initially because we would assume that the Bay looked more like an ocean system where traditionally the peoplewho play this gamethink aminoacids are a preferred pool. But in the Bay we are leaning towards carbohydrates. That is why I brought up yesterday the question that we may really need to get some information on amino acid composition and quantitation, as well as carbohydrates in that dissolved organic pool. Ulanowicz: Approposyour earlier commentas to howthis compared with other estuaries, I am not an expert on DOC dynamics,but we are endeavoringto contrast this network with a very similar one in the Baltic that the ASKS lab Is developing,and hopefully in a few months will be able to say something about the section of the network with respect« BiologicalEffects of Hypoxia/ l37 the turnover rates of DOC pools here and in the Baltic and also their relative importance- From the point of view of the heterotrophicmicro- communi tywe have in our heads that most DOC is fractory»yellowstuff" whichis metabolizedby bacteria, very ~lowly, and probably doesn't constitute in ow oxygen a great demand at all. But when ran some of the calculationswith theseBOD experi- ts we have done and then back-calculatedto carbohy- drateavailable in that dissolvedpool, they wereshowing up mg/1 range about 1 rng/l which in my experience ms high and 1 am just interested in just how much that labile DOC pool drives the sYstem.

Kemp: 3ust a comment 1 would be curious in seeingthose data -- but just using the standard digestion analysis for DOC and then doing mixing diagrams, DOC behavesby and large conservatively. lf you look at time series for a single station over the course of a year, it doesn't changemuch. jonas: Right! The point is that it is mostly refractory material, condensed, high nitrogen, humics if we are in fresh water probably from phytoplankton, aliphatic stuff condensed in the Bay, and it is not labile, not available, and it's very constant. Frankly, lMC is a silly thing to measure if you are interested in these driving forces, but the problem we run into in this bacterial side, it looks as if some of the dissolvedmaterial is driving the system in soroe cases, particularly in the deep water in the summertime, where the proportion of the total BOD -- the labile C esti~ate- sometimesup to 9Q96will pass through that GF/F filter. Now what that is exactly is still a question in my mind, but it lookslike there is a high level of what we are functionally calling "dissolved." And 1 am just interested in that propor- tionatedriving force. MeasuringDOC in an analytical sense Is probably not a useful measure for any of these rate process-oriented things.

anowicz; 1 have some other figures for Y~- that 95,QQQfigure as being reasonable the " pu the DOC to the free bacteria then t"a I35 / Dissolved Oxygen in the Chesapeake proximately 7-12% of the indirect diets of your filter-feed jng fish nekton! which seemto have the highest sensitivities to that particular transfer. I can tell you that from how this network is now constructed. If you take issue with that particular flow, why that could change.

Question: Taking this up to a higher level, I would be curious about the data you present on sea nettles because jt is my impression that we know quite little about their feeding rate and their excretion rate, yet at the end of your talk when you look at the recycling controls, they were the dominant organisms in terms of feedback in the pelagic part of the water column. How did you get that data? Was this by difference, or...?

Baird: Nell, that was mainly Dave Cargo's data that he has collected over quite a number of years. And the values incorporated into this model are based on an average of the last three years' numbers. There is information available on their feeding, ingestion, and respiration rates and so forth, That is how that compartment evolved. ~PLUEh1CEOF LOW OXYGEN TENSIONSON LARVAE POST SETTLEMENT STAGES OF THE OYSTER gghSSO5TREA VIRGINICA roger Mann,Brian Meehanatxf Julia 5. Ranjer yifginiaInstitute of Marine Science

Victor 5 Kenhedy, Roger I.E Newell and gilliam F. Van Hevkelem Horn point Environmental Laboratories, CEES University of Maryland

Our interests in the influence of low dissolved oxygen tensions on the early life stages of the American oyster Crassostrea~rir inica were stimulated by consideration of two scenarios. Oyster larvae occur in the tributary estuaries of Chesapeake Bay during a period of the year when low dissolvedoxygen levels have also been recorded. The literature strongly suggests that larval retention in estuaries is influenced by active depth regulation of those larvae at or near the level of no net motion. If this is indeed the case, then larvae may be exposed to lowered oxygen tensions. The literature, including contributions by Pls of this project, alsosuggests that the lipid-protein based energy metabolism of bivalve larvae is incapable of supporting an anaerobic energy metabo Iisrn.

Two questions from this "larval scenario" arise: I! is the swimming behavi or in flu enced by low dissolved oxygen tensions,i.e., do larvae close and sink to the bottom wher~ theymay be eaten by benthic organisms,or do they actively avoidlow dissolvedoxygen and therebycompromise their optimaldepth regulatory mechanism for retention?and, ! canlarvae maintain swimmingactivity underreduced dissolvedoxygen tensions, i.e., do they continue t«unc« " aerobicallyat the sameor reducedrate, or do they indeed "avesome anaerobic capability? 14O/ DissolvedQxygen in the Chesapeake

Thesecond scenario involves the settlingand early post settlementstages of theoyster. When larvae have matured to thepoint of beingable to settle,they are called "compe tent." Thesettling metamorphosing! stages called pedi veligers!have limited crawling ability for onlya shorttitne hours!,after whichthey cementthemselves to the sub strateand remainsedentary. Again, contributions by the plssuggest that these stages gradually undergo a transition from lipid-proteinto carbohydrate-proteinbased energy metabolism.The latter is appropriatefor anaerobicrnetab olism. Thetime courseof this transitionis not adequately documented.The question arises: if early postsettlement oystersare exposed to low dissolvedoxygen, for howlong canthey survive given their inferred!limited capabilityfor anaerobic metabolism? With this as background,we defined severalobjectives to examine these questions or scenarios:

l. To examinethe influence of low oxygen on ontogenetic changesin swimmingrate and behaviorof larvae, beginningwith recentlyspawned individuals.

2. To determine the influence of low oxygen on behavior and settlement success in competent pediveliger larvae. 3. To quantify the influence of low oxygen on the aerobic metabolisrn and survival of post settlement juveniles.

4. To document the acquisition of the post-settlement capability to func tion anaerobically. 5. To comparequantitatively the contributionsof aerobic and anaerobiccomponents of energy metabolismfrom early larval to post-settlement life stages. Our progressin meeting theseobjectives is as follows. BiologicalEffects of Hypoxia/

At the University of Maryland's Horn Point Fnviron- mental Laboratories, we have begun to examine the rateof swimmingof larvaeof fivedifferent size ranges Og-N, SS-L09, 109-177, 177-210, >210 pm maximum dimension! exposed to oxygen-saturated seawater approximatelyL5 ppt salinity!,50% and 2p% oxygen saturatedwater. Observationswere madeat regular intervals for up to 20 h after introduction of the Larvae into the controlled oxygen regime. Exposureto 20%of full oxygen saturation for 20 h does indeed reduce swimming activity, with the pediveliger perhapsthe most sensitive stage.

In addition, a number of experiments were completed to investigate geotactic behavior of these five size classes of swimming larvae at HPEL. We had anticipated that larvae would close their valves in the presence of Low oxygen levels, thus sinking to the bottom. However, recent data show that larvae except the smallest or youngest 40 qm! tended to swim upwardin our dark- ened experimental chambers to a greater extent than did control larvae in fully oxygenated conditions. There was a tendency for this negative geotactic response swirnrning up! to be slightly more pronouncedin water of 20% oxygen saturation than at %%. Thus, although their swimming rates did slow somewhat by 20 h, larvae did not stop swimming as expected upon first exposure to low oxygen, but appeared to respond in a fashion that would bring them up into presumably more oxygenated water. We plan further replicates of these experiments in 1987.

At the Virginia Institute of Marine 5cience we have examinedchanges in swimmingactivity of similar size ranges of larvae as they are exposed to stepwise decreasesin percentage saturation af oxygenover short periods; for example, 100% to 60% to 40% to 20% to 9% over a 0-h period, The useof a flow-throughcham- ber avoided exposure of the larvae to an air-water interface. These studies were carried out at 2> ppt salinity. In all instances we observed the larvae to swimactively at all oxygentensions even 9%i!. Wedid lyZ / DissolvedOxygen in the Chesapeake

behavior is underway or planned using a motion ana lyzer recentlyacquired at HPFL.

2. The findings above emphasize the need to focus on the pediveligerstage. At VIMSin late 1986,two experi ments were completed in which competent pediveligers were provided with suitable substrate in a matrix experimentof three oxygen saturation levels lppy 20% or 5% at 25 ppt salinity! and three temporal sce narios one, three and five days exposure at the original oxygen level followed by a change to ippfIf, saturation!. The objective was to examine both survival and settlement success under these regimes. Data are presentlyunder examination, and we hopeto reporton them at the 3anuary 1988 meeting. Again, further replicates are plannedfor completion in 1987.

3. We have been somewhat perplexed by the results from experiments designed to examine the third objective. While we have successfully generated a curve relating weightto oxygenconsumption rate VO2!in post-set- tlernent oysters of 1-6 weeks age post settlement!, we have made four attempts to examine post-settlement survival at 100% and 20% oxygen levels with confounding results. In two instances oysters survived with apparently no differences at both levels, whereasin the other experiments almost total mortalities were quickly observed -3 days! at the lower oxygen level, with some additional mortality also being observed in the 100% controls. Ma.terials and methods were identical in all experiments, clearly indicating the magnitudeof natural variability in these studies.

The enzymes alanopine and strombine dehydrogerfase havebeen resolved for pediveligerlarvae pooledlots of 10 individuals! by thin layer polyacrylamide gels. Th< finding of ADH andSDH ~su ests capability to function !!y! d y ! g measure of this ability see 5!.

5. We plan to combine tt! above with this objective i» study of total heat production microcalor"~y! BiologicalEffects of Hypoxiaf 103

respiratoryheat production in collaboration with Dr. 3ohnWiddows at the Institute of MarineEnvironmental Research,Plymouth, U.K., in the spring/summerof l987.The combination ofcalorimetry andrespirometry is theonly defnitive method of d mmstratingfunc- tionalanaerobiosis in these early life stages. pletionof thestudy, Widdows will travelto University of Marylandand VIMS to undertakecomplementary feedingrate studiesand, with the PIs,use the resultant data to generatean energybudget for the early life stagesof theoyster under saturated and reduced oxy- genconditions. We can provide details of theproposed methods to interested parties. In summary,our data are obviouslynot yet complete, botour initial ideas on the inability of larvae to tolerate low oxygenfor at least short exposures hours! may need to be modified.Some tolerance is evident,although whether or notthe larvaecontinue to functionaerobically during this periodis an openquestion. Clearly, the pediveligerstage appearssusceptible to low oxygen stress, and we will can- tinueto focus on this settlement stagein studiesplanned for 19S7. EFFECTSOF LO% DISSOLVEDOXYGEN IN THE CHESAPEAKE BAY ON DENSITY, DISTRIBUTION AND RECRUITMENT OF AN IMPORTANT BENTHIC FISH SPECIES

Denise I.. Breitburg Academy of Natural Sciences Benedict Estuarine Research Laboratory

Although extensive summer oxygen depletion is thought to be one of the most important factors decreasing the capacity of the Chesapeake Bay to support economically important fisheries resources Taft et al., 1980; EPA, 1982; Officer et al l984!, little is actually known about the impact of hypoxic conditions on population dynamics of Bay fishes. Because of their limited ability to exploit the water column habitat, benthic fishes should be among the most severely impacted by hypoxic bottom waters. During 19811 am examining the abundance, distribution and recruitment of a benthic fish species, the naked goby Gobiosomabosci!, in relation to dissolved oxygen levels in nearshore oysterbeds in the mainstem Bay. The naked goby is among the most important prey species in the diet of commercially important piscivores, lt can comprise 50% of the total, and the majority of the fish component, of the diet of juvenile striped bass Wass and %right, 1969; Markle and Grant, 1970!. Naked goby larvae are also among the most abundant fish larvae in the Bay, frequently ranking first or secondin abundance in summer collections conducted in Chesapeake Bay tributaries and nearshore waters Wakefield, et al,, l977; ANSP, 1981; Norrnandeau Associates, 1981; Gallagher and Currence, 1982; Shenker et al., 1983!. The potential impact of these larvae on the zooplankton on which they prey, theref ore, is considerable.

Field observations indicate that naked gobies migra« inshore as dissolvedoxygen levels decline. Several conse- Biological Effects of Hypoxia/ 105 quencescould result from suchmovement: ! becausethey afe dependent on oyster shells, etc., for shelter, extensive movement m ay g reatly increase predation ra tes on the se fish; ! where shallow, sheltered habitat is not continuous yith deeperhabitat, mortality couldbe extremelyhigh as fishare forced to either remain in low oxygenareas or enter habitatwithout suitable shelter; ! considerablereproduc- tioncould be lost as nests containing eggsin affected areas areeither abandonedor suffer high ratesof eggmortality dueto inadequateoxygen concentrations; ! nestingactivi- ties in shallow areas may be disrupted as larger species e.ii.,the toadfish, ~Osanustau! or more dominantgohy individualsinvade inshore sites; ! becauseoxygen levels fluctuate as the pycnocline tilts, the combination of low adultpopulations and suitable shelter in deeperareas may resultin high larval settlement during periodsof adequate oxygenlevels followed by high post-settlementmortality as oxygenlevels drop. All of these effects of hypoxiacould influenceproduction and biomassof the nakedgoby in the Bay,and thus impact both the goby'spredators and prey.

References

The Academy of Natural Sciences, Philadelphia. 1981. Chalk Point 316 demonstration of thermal entrainment and impingernent impacts on the Patuxent River in accordance with COMAR 08.05.00.13. Vol. IIL EnvironmentalProtection Agency. 1982. ChesapeakeBay Program Technical Studies: A Synthesis. U.S. EPA, Philadelphia, PA.

Gallagher,R.p. and L.E, Currence. 1982. Ichthyoplan"t» and macroplankton studies in the vicinity of Calvert Cliffs Nuclear power plant, 1981. PreparedbY the Academyof Natural Sciences,Philadel.phia-

Markle,D.F. and G.C. Grant. 1970. The summer food hab- its of youngmf-year striped bass in three Virginia Rivers. Ches. Sci. 11:50-50 146/ DissolvedOxygen in the Chesapeake

NormandeauAssociates. 1981. Plankton studies conducted at H.A. WagnerGenerating Station, 3ulySepte~p,r 1980. Officer,C.B., R.B. Biggs, 3.L. Taft, L.E. Cronin,M.A- Tyler and W.R. Boynton. 1984. ChesapeakeBay anoxia: origin, developmentand significance.Science 223:22 27. She»er>3.M., D.3. Hepner,P.E. Frere, L.E, Currenceand W.W. Wakefield. 1983. Upriver migration and abundanceof nakedgoby Gobiosornabosci! larvae in the Patuxent River estuary, Maryland. Estuaries 6:3<42. Taft, 3.L., W.R.Taylor, E.O. Hartwig and R. Loftus. 1980. Seasonaloxygen depletion in ChesapeakeBay. Estuar- ies 3: 242-247. Wakefield, W.W., L.F. Berseth and S,L. Zeger. 1977. Ichthyoplanktonentrainment studies. In Morgantovm Station and the Potomac estuary: a 316 environmental demonstration.pp, 16-1-16-145,Vol. III. TheAcademy of Natural Sciences, Philadelphia, Wass,M.L. andT.D. Wright. 1969. Coastalwetlands of Virginia.Interim report to the Governorand GeneraI Assembly.Spec. Rep. Applied Mar. Sci. andOcean. Enid. No. IG. ~y pplCHOVY ECOLOGY IN MID-CHESAPEAKEBAy

~d D. Horde, Edward 3. Chesney, 3r. and Timothy A. Newberger chesapeakeBiological Laboratory, CEES University of Maryland

The bay anchovy Anchoa mitchilli! is the most numer- Ousand ubiquitous fish in the Chesapeake Bay. It is a major consumerof plankton and is an important food of predator fish. Its population ecology in mid-Chesapeake Bay, in- cludingabundance, age, growth, mortality, foods, fecundity, spawningand recruitment patterns, is being studied with Maryland Sea Grant support. Anchovy and ooplankton distributions relative to a tidal front near the mouth of the Patuxent River were the focus of sampling in 1986. Trawl sampling will continue in 1987 but, in addition, laboratory experimentson oxygen tolerances of anchovy adults, eggs and larvae will be determined. Future research objectives will include ration estimation and production estimates in the laboratory under hypoxic and adequate oxygen conditions. Results will be important to predict how low oxygen conditionscan affect anchovy production/biomass and how impactof this abundant planktivore on zooplanktonfood can be altered by a hypoxic environment in Chesapeake Bay. A transect beginning in the mouth of the Patuxent Riverand extending offshore 0 kminto the Baywas sampled from june to November 1986 with a 16-foot trawl. Catch per unit effort CPUE! varied greatly but meanCPUE increasedfrom 200 to 1,025individuals per 10-mintrawl «w between3uly and September,the increasereflecting recruitmentof young-of-the year anchovythat first occur- redin August. Preliminary aging,basecl on annuliin oto- hths,indicates at leastthree year-classes+, I+ andII+! in thepopulation, Length-frequency distributions are multi- indicating the presence of two or more year- lpga/ Dissolved Qxygen in the Chesapeake classes.Spawning by anchoviesis serial and not all females ovulate each day. Hydration of oocytes, ovulation and spawningappear to occuronly at night. A transient,tidal front just offshore of the Patuxent River mouth, which developson ebbtides and disperses on the flood,frequently had large concentrationsof surface-schoolinganchovies near it. The frontal region was mapped, its hydrography charted,and both anchoviesand zooplanktonwere sampled near it to determine if the convergent feature concentrates organismssuitable as anchovyprey. The 1986samples and data are being analyzed at present. The 1987research will include02 toleranceexperi- rnents. An adult anchovy population has been established in the laboratory. We will attempt to induce spawningby the captivepopulation to produceeggs and larvae for the toler- anceexperiments, Life stage-specific02 tolerancelimits will be estimated. In the field we will sample eggs, larvae and adults in parts of the Bay where anoxia prevails to determine their distribution, abundance and condition relative to those from our standard sampling area, where anoxia is not believed to occur commonly. Ultimately,we wishto knowif law O2 levelsin Chesa- peakeBay can or have impactedanchovy consumption and production. Extensionof our scheduledlaboratory research to include feeding rates, growth determinations, and meta- boUcrates undervariable temperatureand 02 conditions will allow us ! to estimate the impact of anchovies on the planktoncommunity; ! to determinethe potential effects low 02 levelson anchovyphysiology and population biology;and ! to modelthe probableeffect of low 02 on plankton-anchovy-predator fish f ood webs. Question: Roger, on the rnicrocalorirnetry system, since a Lot of the energy budgets that I have seen basedon phyto plankton have involved estimates of the number of cells, is this system adaptable to putting in a known numberof cells and getting, for example, the caloric value of a specific diatom?

Mann: You don't combust them in this system to get their heat their caloric content what you are doing is putting the living larvae, in our case, in sea water and then measuring the heat increase in the sea water due to the metabolic heat output of the larvae.

Question: How do you maintain low oxygen?

Mann: These are sealed containers, and initially we nitrogen-flow the water down to as low DO as we can, and then aerate to get it up to the level we want. We add the food, and we have essentially a siphoning system where we put larvae, usually in one or two mls of water in the bottom of our container, and then fill by siphon until it overflows. As it is overflowing we put parafilrn over the top, then a screw- top on that, so we end ~ with a sealed container with the larvae and the food, which is then maintained in the dark. The only thing that is really going to make a mess of your maintenance of oxygen tension are leaks in the system, which you take care of by having a dualed-sealedsystem, or respiration by the animals in it. If you look at larval respiration rates in the literature, we are workinghere with about 400 larvae in a container that is about 170 mls, and even if you leave them for the full 8-day period, the impact of everybody respiring at maximum rate would be a couple of percent.

Question: What about bacteria, wail effects, and that kind of thing -- do you filter the sea water to reduce those initially? i5p / Dissolved Oxygen in the Chesapeake

Mann: The water has been one micron-filtered to start with. Comment: That wouldprobably not take out the bacteria, 5Jlann:It is not going to take out all the bacteria, no. Question: But over 8 days, you don't see a significant decrease in oxygen tension?

Mann: We have never maintained them 8 days; everything gets a water changeevery 2 days. Question:If you ever study actual anoxia for short periods, is there likely to be a difference between nitrogen-produced anoxiaand naturally occurring anoxia that has H2Sin the water? What does that mean for these kind of experiments? Newell: That is a good point that is something that we need to work closely with you and others who are really lookingat this. Becausewhen we started our experiments, we didn't know exactly what the distribution of anoxic totallyanoxic with H2S! water was, opposed to justhypoxic water. Once that information is more fully documented we can in the future actually look at effects of environmentally realistic levels of sulfide on the metabolism. Potentially, sulfide can be very important in poisoning the invertebrates. 3onas: To give an idea of a seasonalcycle, when the water appearsto be truly anoxic basedon Winkler titrations early on in the season, the water column does not necessarily contain detectable sulfide. According to 3on Tuttle's work, essentially all this sulfide is coming from the sediments, not water column processes. As time goes on and these anoxic conditions persist, then the sulfide coming from the sediments does saturate, it moves up in the water column you saw the gradients yesterday. Very rarely has he seenco- existing sulfide and oxygen. It happens,but it's very transient, and right near that pycnocline. It's probablya dynamic phenonemon. But in the advective oscillations that we are talking about,you can get substantialconcentrations of sulfide being pushed up into these areas. Biological Effects of Hypoxia / 15l

Comment: But the other point is that if you are concerned with the sediment/water interface, that is where sulfide should appear the soonest, after anoxia.

3onas: Absolutely. And there are other reduced sulfur intermediates that people just wave their arms about; they are there who knows if they are of any consequence, though the sulfide itself could certainly be a poison directly.

Newell: Our initial experiments will really be looking at the pure anoxic side the dissolved oxygen but we need to address the sulfide.

3onas: One other thing, kinetically, is that sulfide, if it is pulsed in to an aerobic environment, does not disappear immediately. This year, because we had anoxia and were trying to do these BOOs, we wanted to get rid of the sulfide becausewe wanted to know the organic component,not the inorganic component, of oxygen demand. And one could not get rid of it even by agitating it, by holding it for hours- six hours we still wouM have sulfide in the water. 3on Tuttle tells me that is a salinity-related phenomenon. Well, it does obviously oxidize chemically, but not instantaneously. The rates are reasonably slow, and might have an impact on organisms,

Mann: Are you getting any experimental or survey work, or any type of mapping of these levels of sulfide so that we can then use them?

3onas: That was not part of the orginial objective, but we did institute sulfide analyses, so we have some data for the past year across that range of Bay from below the Potomac up to the Bay Bridge. One of 3on's students came out with us to do this.

Kemp: I think that there is enoughdata for you to establish a frame of reference for such experiments, but I think the' point that Bob 3onas makes is that sulfide is really tough to regulate because unlike a lot of other rnetabolites you have a spontaneous oxidation, so regulating constant concentrations is a good trick experimentally. ly2/ Dissolved ~oxygen inthe Chesapeake

ou; But you yo know it is goingto be a problem,because ~me residual. for hours. If Tuttle's data there has to be be correct so for subsequentyear continue to e those highh I leve vels s, g" we have seenanywhere el this past surnrner-mer very, very high levels of sulfide d f theyh going are goin to persis o six or eighthours, even Io levels,1 I, ththey aregoing to bevery toxic metaboticatly. My understanding,at least of the chemicalhalf- 'Comment'l'f f hydrogensulfide, is on the orderof I-2 hoursfor half-tife.life o So if you are starting with IOprnicrornolar mix& in, it mightstick arounda bit; if you start with 3 or 10 micromolar,that is pretty low consideringwhen you mentsare havingM around I l00 rnicromolar. 3ordan: You can maintain reasonably stable sulfide solu tions in the laboratory over a few hours, particularly if you have water that is low in oxygen to start with.

Newell: 3on was say>ng that you get quite good elect od for sulfide have you used theme 3ordan: My experience is that they were totally uselessin the natural systems you might be able to use them in the laboratory. The problem is that in nature, those electrodes only measure the free sulfide ion, so unless you are at pH l3, the free sulfide ion is present in extraordinarily low concentrations even if the probe nominally will get down to those numbers, there is so much noise that it is almost impossible to react ~

3onas: Ac tually, the detee tion scheme that they have developed -- that Chuck Divan has been working on is a sp~trophotornetric technique and is tested in various solu- tions. It is fairly straightforward, and they actually can preserve the sulfide, and so the analysis does not have to happenimmediately.

3ordan: You can do those on shipboard. I » BiologicalEffects of Hypoxia/ ly3

Newell: So it is easy in an experimental situation to monitor sulfide levels? 3onas: 5ure it would not be a problem.

Purcell: l was wondering if w'e find the larvae of the ancho vy andthe oyster in the hypoxicwaters - l haven'tseen any in si.tu information on tha,t- l also don't really knowhow discrete theseboundaries are if it is an abruptjump into hypoxic waters or if it is a gradual transition, But l was wondering if the experiments with oyster larvae, for example, looked at gradients or at how the larvae behave at discontinuities.

Mann: We haven't tried DO discontinuities, but we tried salinity discontinuities, and we can make those work quite nicely. We can set up salinity discontinuities which the animals will swim through totally ineffective -- and others where they treat them rather like brick walls. With that basis as a control, l see no reason physically why we can't build a DO discontinuity superimposed over a salinity discontinuity. So in terms of how do they behave at DO discontinuities, we don't know, but practically, I think we can address that question.

3onas: When you were treating larvae with different DOs, did you see anything there?

Mann: No, the experimental set-up is as follows: A long, tall chamber sits in front of the video machine. When you irrigate it to changeDQ, you totally changethe volumein the chamber over about five minutes. This gives you a vertical velocity through the chamber in the order of G.l mm/sec, which is lower than the swimming speed of the animal. lf you sit and watch the video machine, you see a general upward movement of your whole population. As soon as you stop the peristaltic pump and essentiallylock in the oxygen throughoutyour whole chamber,and we have done dye studiesto make sure that we get adequateex- change,these animalswill cascadedown through the chamber and within 10 or l5 secondsthey are swimmingagain Soin termsof whetherthey movethrough a discontinuity don'tsee that that is goingto affect themmarkedly over a period of time. l54 / Dissolved Oxygen in the Chesapeake

purcell: What about the in situ question -- do you find them in situ in the low oxygen waters?

Mann: One of the real problems l have in addressing this questionis that oyster biologists have not traditionally gone out and worked in the field and looked at oyster larvae, because it is an extremely difficult thing to do easily. And on those occasions where individuals have tried to do this, it has usually involved hordes of graduate students simply because of the problem of looking at all the samples. We have tried to look at some of the molluscan samples that were collected in field studies.

Roman: We have been counting oyster larvae.

Question: Did you count oyster larvae or bivalve larvae?

Roman: Bivalve larvae.

Kennedy: See, once you have gotten them you have to identify them, and it is only until they get to be about 200 urn or more that you can tell them apart.

Mann: Basically, what we are trying to do at the moment is to use specific gravity gradients to separate out the detritus from the bivalve larvae to give us a first cut. To give you an idea of the manpower involved, in late l985 with Bob Byrne we did a study of a frontal system in the 3ames River where we looked at a transect with six stations on it. We did this transect over 3 tidal cycles, so we ended up with 30- 00 bottles of plankton. To sort those out and enumerate all the oyster larvae took something like nine man-monthsof energy. And so trying to go out and take plankton samples and hoping to find larvae in the first instance, and then trying to sort them out to be sure you are actually looking at oyster larvae versus other things is just an horrendous proposition, and I don't think it is even practical to try to answer that definitely in the field until you have the plankton sorting problem under control. Biological Effects of Hypoxia/ L55

Newell: 3ennypurcell hasa goodpoint, becausewe needn't necessarilybe concernedabout what hypoxia does to oyste larvae we could look at any bivalve larvae and whetherthey are living they shouldbe affectedin the sameway. But I don't knowif anyoneis gettingany plank ton samplesthere wel}., you are, Michael. Roman: The few times we went out in August, we collected >60micron plankton,which shouldinclude bivalve larvae, from below the pycnoclinein the anoxic layer so we could geta roughcut as to whetherbivalve larvae were there or not. Still, there is a whalequestion about whether the distribution of really low 0 water is anywherenear where the presenceof oysterlarvae 2 matters if thereare no bars in the deeptrough zone, then do wecare if thereare oyster larvae there? Newell: Well, these larvae are in the water columnfor threeweeks, and are carriedessentially passively with the water flow so they can movegreat distances, and can be carriedthrough many types of water.If thatwater happens to be eitherhypoxic or anoxic,even if there isn'tan oyster bar within 5 milesof there, they can still be affected. They nevercome out the other end it is like a blackhole, they just fall into it and never come out. Purcell; I wasgoing to askEd Houde, what about the anchovy eggs and larvae? Houde: We shoulddo somesampling probably along that Chop-paxtransect during the summerto Lookat t»s ~n relationto ouroxygen tolerance work. Penny Dalton is goingto defenda Master'sthesis tomorrow where she Looked at theegg and larval distribution of the Bay Anchovy based on CaLvert Cliffs power plant collections, and her ba»c conclusionwas that at the transectoff CalvertCliffs! wh«" extendedout into the deeptrough, the eggswere mo st abundantoffshore and in the bottomtows, which were done with a sled, onemeter off the bottom. Theanchovy eKgs wereat 22-25C,which you might expect to hatchin about ~ he Chesapeake I 56/ DissolvedOxyge»n t"

.n the water column is about one 24hours~ so theirthei duration >n the weredead or alivewhen cauht butthe numbers of smalllarvae were also ed ' h th bottombut rno«orless equally di5trib utedd thoughouth h t the th water column-co Fisheggs generally float, the anchovy eggs wouuld float if the salinity were 25 ppt or h>gher, but l suspectect what w a has happenedis that they have been spawneded 'in thet e mimidwater, wa near the surface,and have sunkdown into that 20-21 ppt w>t«near thebottom. What theimplications are, I don'tknow. She has oxygen data only for 1972,'7l, '76and '77. There is a significantnegative correlationbetween egg abundance and oxygenlevels -- a simplecorrelation. You can be misledby thesethings, bot there is one. Question:What is the size range of the preyitems for an adult mature anchovy? What is the lower size limit of their food? Are they particulate feeders, or filter-feeders?

Houde: I don't know what the lower limit is, but they are basicallyfeeding on copepods, copepod-sized organisms. Anchovies are basically part iculate f eeders, zooplankton feeders.

Question: What about the larval forms, the critical first feeding? Is there such a thing for these? At that stage, what do they eat?

Houde: Well, people talk about critical first feeding, but I don't like to use the term myself. They are eating copepod nauplii or probably large dinoflagellates, and data that 5ellner and Roman showed us yesterday indicates that this is pretty common stuff. hlARYLAND STOCK ASSESSMENTSTUDf~

Har icy Speir Tidewater Administration Maryland Department of Natural Resources

The Estuarine Fisheries Pragram of Maryland DNR is presently studying the stock characteristicsof sevenspecies of finfish and blue crab. Striped bass, American shad, alewife herring and blueback herring studies ar supported either by federal Anadromous Fish Act funds or Waliop- Breaux funds. Blue crab, yellaw perch, white perch and weakfish studies are funded under the cooperative state- federal Chesapeake Bay Stock Assessment Committee.

The short-term objectives of these studies are to develop or maintain indices of yearly reproduction, develop relative measures of the size of the adult stocks, determine the seasonal age and sex compasition,and monitor growth rates of target species. In addition to detaileddata on the target species,each project alsa recordsdata on the presence and abundance of other species in the samplinggear. For the most part, these community data have not been examined. Target speciesdata shouldeventually yield estimates af the proportions caught by man, natural mortality, relative abundanceof breedersand annualrecruit- ment of adults to the populatian. The iong-termgoal is to fit the datainto a rational schemefor managingthe harvestsof thesestocks. Man- agernentplans, which require such data, are in P«Para l " or havebeen prepared for shad,striped bass, herring, white perch, yeHow perch and weakfish. Althoughroutine environmental data e-g.,deeP thh anda temperature!are taken, it is»t » ob!ect've samplingprograms to examinethe habitatand poPu ulation 158/ Dissolved Oxygen in the Chesapeake interactions in detail. Obviously the program is concerned with the effects of potentially lethal habitat conditions on stock because "natural" mortality must be sorted out of estimates of total mortality so that levels of fishing mortality can be estimated for managementplanning.

Hypoxia can potentially affect growth, migration and distribution; it can also produce direct mortality. Kith the exception of striped bass, there has been little published work on the potential effects of hypoxia on our target species. The speculative hypothesis on the response of striped bass to hypoxia Coutant 1985! suggests the potential for a significant complication in establishing a management plan for the Chesapeake Bay which would fit under the present coastal management framework. Management of other target species may also be complicated by hypoxia problems.

Exchange of data and ideas between habitat quality researchers and stock assessors is a necessity, and the Estu- arine Fisheries Program is willing to make its data available.

References

Coutant, C.C. 1985. Striped bass, temperature and dissolved oxygen: a speculative hypothesis for environmental risk. Trans. Am. Fish Soc. 110:31. QUAN11FYINGTHE SEVERITYOF HYPOXIC EFFECTS

M.T. Boswell and G.P. Patil Pennsylvania State University

3oel S. O' Connor NOAA, Ocean Assessments Division

We want to emphasize that quantitative assessmentsof hypoxiacan be carried further than they have been. This requires more useful quantification of hypoxic effects and, through collaboration between scientists and decision- makers,explicit interpretation of theseeffects. Webelieve that one or more formal indices of hypoxic effects can help structure these improvements to better define and assess hypoxic ef f ec ts. Assessmentof hypoxia entails three elements: 1. Knowledgeof dissolvedoxygen concentration DO! fields, generally through measurement. 2. Knowledgeof DO dynamics,to understandwhat could be done about hypoxia, 3. Knowledgeof hypoxic effects and their social importance, to help decide if anything need be done.

In the Chesapeake, progress is being made in understandingthe DO fields and their dynamics. However, there appears to be little quantitative understanding, Bay- wide, of socially consequentialeffects of hypoxia e,g., how muchhypoxia limits oysteror soft shell clam production!. This lack of quantitative understanding is marked, even to the extent of disparate professional views as to which effects are most important, We argue that it is important and relatively easy to quantify the severity of at least some 160/ DissolvedOxygen in the Chesapeake

socially consequentialeffects; we use mortality in oysters an illustration. Before indexingother effects e.g-, avoidancebehavior and its population consequences striped bassand blue crabs,or growth reductionin oysters!, additional researchmay well be necessaryto etermine the social significanceof theseeffects or even how to quantify them. Further,if we are to really "help decideif anything need be done" we must go beyondsimply quantifying effects, We musthelp interpret them, We must help decision makersdetermine how much hypoxia is acceptable. Only whengovernments have defined unacceptable hypoxic effectsquantitatively can they establishdefensible goals with regard to hypoxia. Withthe helpof otherinvestigators we havedeveloped an indexof hypoxiceffects primarily for decisionmaking purposes.First we need a readily understandablescale for the index. Weuse the samescale used for other indicesin a seriesdesigned to quantifyseveral marine poUution effects F igure 1!. Theindex scale has three ranges:

no concern

warning range justifies consideration, if "enough area" is affected! 0 alarmrange justifiesaction, if "enough area" is affected! For illustrative purposes,assume the index valueof 10 correspondsto 1096hypoxic-induced adult oystermortality over a summerseason. In practice the lower boundaryof the alarmrange would be negotiatedby the partiescon- cerned,and decided upon by the appropriatedecision makers.

Note that the index does not addresshow mucharea must be affected to justify governmentalconsideration or Biological Ef fee ts o f Hypoxia / l6 I

action. This undoubtedly varies greatly from region to region, and we doubt that it can be usefully defined by an algorithm.

Now we can quantify hypoxic-induced mortality and helpinterpret it for decisionrnaking. WhenDO falls below a thresholdvalue, dependingon temperature, an oxygen deficit begins to build up in the oyster. A low dissolved oxygen episode is defined to occur during the time period when the DO is consistently below this curve, i.e., when oysters are accumulatingoxygen debt. We want to index the accumulation of this oxygen debt as the episode progresses. In order to calculate the index for an hypoxic episode we need three things:

i. Bottom DO and temperature rneasurernents over the episode, measured frequently enough to allow meaning- ful estimates,

2. Authoritative judgment that some percentage of hypoxic-induced oyster mortality is unacceptable. Other endpoints might be chosen; it is important that they be estimable endpoints that are important for environmental decisions. For instance, we could judge that some percent of larval oyster mortality is unacceptable. This might well be a more sensitive measure then adult mortality, as has been suggestedby some experienced investigators.!

3. Finally, we need some quantitative linkage betweenthe DO field and the resulting oyster mortality.

It is not a major problem to measure the DO field in Chesapeake Bay -- not trivial or cheap, but it is more straightforward than other information and decisionsneeded to assess hypoxic effects.

Let's assume we have an authoritative judgment about unacceptable effects, for example, that 1096oyster mortality due to low DO is socially unacceptable. Now, in prin- ciple, we could link DO to oyster mortality in either of two l62 / Dissolved Oxygen in the Chesapeake

ways: either by field observations or from the measured DO concentration and reliable dose/response relationships estimated in the laboratory, i,e., by determining what dose of oxygen deficit causes l0% mortality. It would be very difficuit and very costly to reliably link DO to field estimates of oyster mortality, at least over the entire Bay, so we chose to use laboratory estimates of 10% mortality as functions of DO, temperature and exposure duration. We emphasize that existing dose response data for the oyster are not reliable enough to index all relevant conditions in the Bay. But these data are relatively easy and inexpensive to get.

We have used laboratory dose response data from Bill Stickle, Louisana State University. We can use Stickle's LC10 estimates to determine what environmental conditions will cause an oxygen deficit to build up, and how long it will take to kill 1096 of the oysters Figure 2!. The highest concentration of oxygen at a given temperature that causes p2 deficit is termed the "incipient lethal" DO concentration. Incipient lethal concentrations are specified by the vertical asymptotes of the LC10 curves of Figure 2. These concentrations are plotted in Figure 3. In practice, the curves of Figure 2 should be fitted statistically. A four- parameter hyperbola seems to provide the best fit to this unusually variable family of curves, based upon several data sets for fishes and invertebrates.! Once these things are known then we can calculate the index at each station sampled. Each index value is the sum of daily doses of oxygen deficit:

index = E daily doseof oxygendeficit! episode

Z weight! incipient lethal-environmental DO ! episode

Once the weights are calculated, the daily oxygen deficit is calculated. It is a function of the difference between the incipient lethal DO and the measured DO concentrations. This difference must be weighted in proportion to how fast 10% mortality occurs at the measured temperature and DO. Biological Ef f ects o f Hypoxia / I 63

Figures 0, 5, and 6 give three examples of low DQ episodes and the associated index values. These exa.mples Use Tom Malone's data from his transect of the upper Bay, just below the Choptank River. Observations cover most of the hypoxic season. Bottom waters near the western shore Station l, about 9 m deep! and near mid-channel Station 3, about 25 rn deep! had index values several times greater than the "alarm" range. It is safe to say that these areas were hypoxic enough in I9&6 to cause greater than l0% oyster mortalities where oysters were present. Where oysters were absent, low oxygen precluded oyster beds even if other conditions were adequate. Oxygen declines at the last, near mid-channel example Station 0, about 7 m deep! never fell low enoughto causeoxygen debt, so the indexwas zero.

Comparable index values throughout the Bay would indicate how much of the existing and potential oyster beds suffer "warning" and "alarm" levels of hypoxia. Perhaps even more significantly, since governments are already committed to rernediating hypoxia in the Bay, the indexcan help quantify how much remediation is needed to regain acceptable water quality. As we emphasized,more laboratory measurementsof hypoxic-induced mor tali ty in oysters are n-.cessary f or reliable index values. These are relatively inexpensive measurements to make. Given more reliable LCIO curves, index values could be calculated for all locations at which we know the seasonal DO calculations, thereby characteriz- ing areas of the Bay that are not affected by hypoxiaand those in "warning" and "alarm" rangesof the index. Further, some other hypoxic effects can be quantified and indexed. One could, at minimal cost, quantify growth reduction and mortality of oyster spat and larvae; and, at greater cost and less accuracy,quantify avoidanceby blue crabs and other resource species. The index makes explicit both specific hypoxic effects and their social importance, factors that are typically assessedin vagueand ad hoc ways. Wesuggest that useof 160/ Dissolved Oxygenin the Chesapeake the index would facilitate decisions about remediating hypoxiain more consistentand defensible ways. Some will find the index too complicated. It is possible to define a simplified version, but we are reluctant to simplify by relaxing the tight linkage possible between hypoxiaand effects. If decisionmakersrequire a lesscom- plicatedindex, simpler approaches to interpreting Figures0- 6 are evident. F>gUre1. Scaleand levels of concern for the index. 2

4 5 DISSOLVEDOXYGEN CONCENTRAT ION mg/t,!

Figure 2. Days of exposure to reduced oxygen concentrations, at specified temperature, causing 1096 mortality in the American oyster. The obviously uncertain extrapola- tions are for illustrative purposes only. Each circle represents an LC10 value based upon 20 oysters. Data courtesy of Wil!iam B. Stickle, 3r., Louisiana State University.! 10 20 30 Temperature 'C!

Figure 3. Incipient lethal I.C10! curve for the American oyster; the DO and temperature interactions just sufficient, over indefinitely long exposures, to kill 10% of adult oysters.

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CJ.~ ~/«! [oa] ~ VD DISCUSSION jordan: Were those LCIOs measuredon actively feeding Dysters?

O'Connor: I am not certain. Kennedy:This index, 3oel, is it beingused anywhere, as a managementtool yet, or... whatare the plansfor the future? O'Connor; It is not being usedyet I am shoppingaround for customers,looking for decisionrnakerswho feel they havehypoxia problemsthat this approachcould help resolve.I would like to work with themin actuallyapplying theindex. I feel that there are severalareas where it could beapplied, using this sortof data. Forexample, the surf clam further north around New York. The surf clam data are even less accurate than the oyster data. But one can cake reasonableestimates that probablywouMn't be too far off. You know those incipient lethal curves':ave to be within reasonablysmall margins. For managementpurposes though,I doubt that the oyster data, for instance,would be adequate.That is whatneeds to bedone: define the quanti- tativelinkage between the oxygenfields and the effects more convincingly. Houde: Can you usethe indexto look at thingssuch as production?You mentioned that it couldbe used to lookat theproportion of bluecrabs that disappearedfrom an area. O'Connor: Yes, all you need to do is define somerneasur- ableeffect for which you are willing to specifythat some degreeof that effect is unacceptable,However, the index presumesthat you can measurethe effect, saythe proportion of blue crabs escapinghypoxic areas. It seemsto me that suchavoidance is one of the most difficult of all hypoxic effects to measurereliably overlarge areas. I am not certain,for instance,that blue crab avoidance can be rnea- l72 / DissalvedOxygen in the Ches p

sured practica y, wi'th useful u precision,over hypoxicareas of the Chesapeake. jonas:Along thase lines, you have something like the issue off h' shipp>ng ' l' ive e blue ue crabs for example,the ability survive shipping. Mackiernan;[n l984,a lot of waterrnenreported that the live crabsseemed weak, a lot didn'tsurvive shipping or even gettingto thedock. This was a badanoxia year, so the crabsmay have been very stressed. Question:How sensitive is yourindex to the periodicityof dissolved oxygen rneasurernents? O'Connor: It assumesthat the DO is continuously below that incipientlethal concentration that if yauhave sorne- thinglike diurnalar aperiodicinfusion of oxygenatedwater, it just daesn'tapply, because I just don't knowhow fast di fferent organismscan repay that oxygen debt. With salmonids,they think it is on the order of 24 hours. And I don't know many other organisms for which this measure- rnent has really been made.

Comment: Sa your index would be really most appropriate in the deepbasins, but in areas where you have this tilting occurring, in the shallower areas, you have a problem on how often you go out and measure your dissalved oxygen ! event.

O'Connor: Right, what you could do in a case like that is measure the index over the longest period at which oxygen is cantinously below that incipient lethal concentration. Aixf that would be the conservative estimate of the effect over the whole season.

Question: Yau only need one episode?

O'Connor: Right, but for something like oysters, for all I know it may only take something like 3 hours of oxygenated water and they can repay all of this oxygen debt. Biological Effects of Hypoxia / J 73

Houde: From the point of view of both waterquality and livingresources, I think the ideathat RobertMagnien yesterdayto put somedeployed DO sensors out there would probablymake a lot of sense.You could get an integrated effect of the time exposed to low oxygen. Thomas:The Bay Program'sexploring doing jUst that Boicourt: So are a numberof investigators. Sellner: Larry5anford and now Denise Breitburg and myself havebeen talking with Endecoabout simply getting a long- term conti~uous monitoring of DO. The Hydrolabsthat we weregoing to useare essentiallypoisoned by sulfide not theprobe, but the wholepiece of equipmenthas to besent backevery time sulfidecomes in. Soat $500a popit is a little difficult to get measurements. Breitburg:They will probablybe usefulin veryshallow waterwhere you are not likelyto get thesulfides, and then youcan use the Endeco ones in thedeeper waters where you are going to get somesulfide. Sellner:Actually, it is a greatidea but seemsrather dif- ficultright nowto do it, becausethere is apparentlyonly onecontinuous DO-monitoring piece of equipmentavailable. Houde:Is thereanything specific planned in theCBSAC groupto lookat hypoxic effects on fish, crabs or oysters? Hennemuth:Not immediately, however we are going to pick thewhite perch and maybe one other species, and do some trial assessmentswith the methodologywe have and are developing.And if wecan square away how to do this kind of thing,we will probably move on to priorityspecies that tendto come up. I amnot so sure tha,t since these analyses aremostly retrospective, they require some time series data base.I amnot sure whether the historic data base is ade- quateto do thisfor ~anspecies. It mayvary from one speciesto another we talkabout doing this, but it would dependon where and when the low dissolved oxygen occurs relative to the species. 17' / DissolvedOxygen in the Chesapeake

Houde:I guessyou are right there arenot manydata ta doanything with in a retrospectiveanalysis. Hennernuth:There hasn't been much in 10or 12years of data it is uneven. 3onas:Two thingsrelative to that one is, within these NOAA/SeaGrant DO projects for 1987,Dr. Peter Qepur from GeorgeMason is goingto work on the hypoxicinflu- enceon moltingblue crabs. He believesthey are under severeoxygen stress at this time. During the molting period,they are unable to ventilate,and any stress at that periodmight provide a verysensitive biological measure of the impact.They can't go anywhere,they are waitingand if one of these little oscillations appears to occur and even in the Patuxent, we have seen DO at the surface in the 3.0 mg/1range, up in the mouthof the PatuxentRiver then you might in fact have a severe impact on molting crabs right there. The other thing to note in passing,is that some of Dave Cargo's oxygenwork from 1957 after the incident of mortaLity of blue crabs in crab pots, I think it is a CM. technical report, I don't think it is out in the open literature anywhere there is some data for blue crab survival, and that sort of thing, in relation to dissolved oxygen. SEMINARON HYPOXIA AND RELATEDPROCESSES IN CHESAPEAKE ~ Y

List of ParticiPantg~ A~

Mr. RichardBatiuk Mr. 3ohn F. Christmas EPAChesapeake Bay Program Tidewater Administration 401 Severn Ave. Maryland DNR AnnapolisCity Marina TawesState Off ice Building Annapolis,Maryland 21403 Annapolis,Maryland 21401

Dr. Linda Blum Dr. ChristopherP. PELLa Departmentof Environmental ChesapeakeBiological Laboratory Science Box 38 Universityof Virginia Solomons,Maryland 2068$ Charlottesville, Virginia 22903 Mr. Charles L. Divan Dr. William Boicourt ChesapeakeBiological Laboratory Horn Point Environmental Box 38 Labora tories Solomons,MaryLand 20688 Box 775 Cambridge,Maryland 21613 Mr- Robert D. Doyle Horn Point Environmental Dr. M.T. Bos»ell laboratories Center for Statistical Ecology Box 775 Penn State University Cambridge,Maryland 21613 UniversityPark, Pennsylvanial6802 Dr. HughDuckfo» Dr. %alter R. Boynton Horn Point Environmental ChesapeakeBiological Laboratory Laboratories Box 38 Box 775 Solomons,Maryland 20688 Cambridge,Maryland 21613

Dr. Denise L. Breitburg Dr. Thomas R. Fisher Academy of Natural Sciences Horn Point Environmental Benedict Estuarine Research Laboratories Laboratory Box 775 Benedict, Maryland 20612 Cambridge,Maryland 21613

Dr. David C. Bro»nlee Dr. Leonard V. Haas Academyof Natural Sciences VirginiaLnstitute of MarineScience Benedict Estuarine Research GloucesterPoint, Virginia 23062 Laboratory Benedict, Maryland 20612 Dr. La»rence%. Harding,3r. 3ohnsHopkins University Dr. Ed»ard 3. Chesney, 3r- ChesapeakeBay institute ChesapeakeBiological Laboratory 4800 Atwell Road Box 38 ShadySide, Ma,ryland 20867 Soiomons, Maryland 20688 6 / p3rt>ctpants

IVlr. Richard Hennemuth Dr. Robert E. lULagnien NOAA/NMFS Division of Modeling and Analysis NE F i she r i es Cen ter Maryland DE Water Street 201 West Preston Street WoodsHole, Massachusetts02543 Baltimore, Maryland 2120 l

Mr. Bruce %. Hill Dr. Thomas C- Malone VirginiaInstitute of MarineScience Horn Point Environmental GloucesterPoint, Virginia 23062 Labora tories Box 775 Dr. Edward D. Houde Cambridge,Maryland 2l613 ChesapeakeBiological Laboratory Box 38 Dr. Roger Mann Solomons,Maryland 20688 Virginia Insti tute of MarineScience G loucester Point, V irginia 23062 Mr. Rick 3arrnan Sca Grant Co!!ege Program Dr. 3ohn R. McConaugha University of Maryland Applied Marine Research H..'l. Patterson Laboratory CollegePark, Mary!and 20792 Old Dominton University Norfolk, Virginia 23508-8512 Dr. Robert 8. 3onas Departinent of Biology Dr. Brian Meehan George Mason University V irginiaIns t itute of MarineScience trtr00 University Dr- Gloucester Point, Virginia 23062 Fairfax, Virginia 22030 Dr. Aaron L. Mills Mr. Stephen3. 3ordan Department of Environmentai Tidewater Administration Science Maryland DNR University of Virginia Tawes State Office Building Charlottesville, Maryland 22903 Annapo!is, Maryland 21tr01 Mr. Timothy A. Newberger Dr. %. Qichae! Kemp Chesapeake Biological Laboratory !lorn Point Environmental Box 38 Laboratories Solomons, Maryland 20688 Box 775 Cainbridge, Maryland 2! 613 Dr. Roger IX. Newel! Horn Point Environmental Dr. Victor S.Kennedy Laboratories !horn Point Environmental Box 775 Labor a tories Cambridge, Maryland 2l6 Box 777 Cambridge, Maryland 216 Dr. 3oel O' Connor NOAA Dr. StephenA. Macko Ocean Assessment Division Department of Earth Sciences N/OMA-32 Memorial Universi ty of 1 !F00 Rockville Pike Ne w foun dland Rockville, Maryland 20852 St. 3ohns, Newfoundland, Canada Participants / l77

Pr. G.P. Patil Mr.Robert Stegfrled Center for Statistical Ecology ChesapeakeBay Project PennState University State Water Control Board UniversityPark, Pennsylvania l6802 2l I l Harntlton Street Richmond,Virginia 23230 pr. Emily Peele Horn Point Environmental Mr. HarleySpeir Laboratories Tidewater Admon Box 775 Maryland DNR Cambridge,Maryland 21613 TawesState Office Building Annapolis,Maryland 2I90l Ms. abuliaS. Ranier VirginiaInstitute of MarineScience 'Dr. Robert Summers GloucesterPoint, Virginia 23062 Division of Modelingand Analysis Maryland Dept. of Environment Dr. William Rickards 20 I West Preston Street SeaGrant Coiiege Program Baltimore, Maryland 2I20I Madison House l70 Rugby Road Dr. 3ames Thomas University of Virginia NOAA Estuarine ProgramsOffice Charlottesville, Virginia 22903 3300 Whitehaven Street Washington,D.C. 20235 Mr. Eric E. Roden ChesapeakeBiological Laboratory Dr. Jon H. Tuttle Box 38 ChesapeakeBiologicai Laboratory Solomon, Maryland 20688 Box 38 Solomons,Maryland 20688 Dr. Mike R. Roman Horn Point Environmental Dr. Robert E. Ulanoeicx Laboratories ChesapeakeBiological Laboratory Box 775 Box 38 Cambridge,Maryland 2I613 Solornons,Maryland 20688

Dr. Peter Sampou Dr. WilliamF. VanHeuketum Horn Point Environmental Horn Point Environmental Laboratories Laboratories Box 775 Box 775 Cambridge,Maryland 216I 3 Cambridge,Maryland 2 I613

Dr. Lawrence p. Sanford Dr. JosephC 2.'leman Horn P oint E n vironm en tal Departmentof Environmental Laboratories Science Box 775 Universityof Virginia Cambridge,Maryland 21613 Charlottesville,Virginia 22903

Dr. Kevin G. Seliner Academyof Natural Sciences Benedict Estuarine Research Laboratory Benedict, Maryland 20612