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1986

Estuarine zooplankton community structure in stratified and well- mixed environments (York River, Virginia, Chesapeake Bay)

James Edward Price College of William and Mary - Virginia Institute of Marine Science

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Recommended Citation Price, James Edward, "Estuarine zooplankton community structure in stratified and well-mixed environments (York River, Virginia, Chesapeake Bay)" (1986). Dissertations, Theses, and Masters Projects. Paper 1539616816. https://dx.doi.org/doi:10.25773/v5-abrg-br58

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8708824

Price, James Edward

ESTUAR1NE ZOOPLANKTON COMMUNITY STRUCTURE IN STRATIFIED AND WELL-MIXED ENVIRONMENTS

The College of William and Mary PH.D. 1986

University Microfilms

International300 N. Zeeb Road, Ann Arbor, Ml 48106

ESTUARINE ZOOPLANKTON COMMUNITY STRUCTURE IN

STRATIFIED AND WELL-MIXED ENVIRONMENTS

A Dissertation

Presented to

The Faculty of the School of Marine Science

The College of William and Mary in Virginia

In Partial Fulfillment

Of the Requirements for the Degree of

Doctor of Philosophy

by

James E. Price

1986 APPROVAL SHEET

This dissertation is submitted in partial fulfillment of

the requirements for the degree of

Doctor of Philosophy

James E. Price

Approved, December 1986 7 ^ „

George C. <0rant Committee Chain

fert J

Leonard W. Haas

Michael P. Weinstein

Chris S." Welch --- *=>

ii TABLE OF CONTENTS

Page

ACKNOWLEDGMENTS ...... iv

LIST OF TABLES ...... v

LIST OF FIGURES ...... vii

ABSTRACT ...... ix

INTRODUCTION ...... 2

METHODS AND MATERIALS ...... 6

RESULTS ...... 12

DISCUSSION ...... 54

CONCLUSIONS ...... 73

APPENDIX A ...... 75

APPENDIX B ...... 79

BIBLIOGRAPHY ...... 83

VITA ...... 92

iii ACKNOWLEDGMENTS

I wish to thank Dr. George Grant for his support, cooperation and patience with this project. His guidance and assistance with the numerous taxonomic and ecological aspects were a constant source of encouragement, George was consistently supportive by promoting scientific discovery and providing financial assistance during my studies.

I also wish to recognize the other members of my committee, Dr. Larry

Haas, Dr. Robert Diaz, Dr. Chris Welch and Dr. Michael Weinstein for their critical review of all aspects of this project and for their patience.

X extend a special appreciation to all the past members of the former Planktology Department for their cooperation and assistance with the field work and their participation in the many extended and informative discussions. Special regards to John Olney for his unbounded interest in plankton ecology and for offering numerous and usually informative critiques.

Finally, I acknowledge my wife, Mayson Moore Price, for her constant support, encouragement, patience, and assistance during this endeavor. '

iv LIST OF TABLES

Page

Mean surface (lm) to bottom (18ra) differences and ranges for salinity, temperature and dissolved oxygen during each of the sampling periods .... 14 _3 Density (no. m ) for species which had a mean density of a least 1 m for any cluster group during sampling period I ...... 17 _3 Density (no. m ) for sgecies which had a mean density of a least 1 m for any sampling stratum during sampling period 1 ...... 19

Relative abundance for dominant species in sampling period I comparing the cluster group and sampling stratum classifications ...... 20 -3 Density (no. m ) for sgecies which had a mean density of at least 1 m for any cluster group during sampling period II ...... 23 _3 Density (no. m ) for sggcies which had a mean density of at least 1 m for any sampling stratum during sampling period II ...... 24

Relative abundance for dominant species in sampling period II comparing the cluster group and sampling stratum classifications ...... 25 -3 Density (no. m ) for species which had a mean density greater than 1 m for any cluster group during sampling period III ...... 28 _3 Density (no. m 23^or species with a mean density greater than 1 m for any sampling stratum during sampling period III ...... , 29

Relative abundance for dominant species in sampling period III comparing the cluster group and sampling stratum classifications ...... , 30

v LIST OF TABLES (continued)

Table Page

11. Comparison of significant tests of log densities and relative abundance (arcsine transformed) between cluster groups and sampling strata for selected dominant species ...... 31

12. Comparison of densities for dominant zooplankton species between upper and lower strata during the day and n i g h t ...... 52

13. Comparison of densities for dominant zooplankton species between day and night for upper and lower sampling strata ...... 53

14. Selected water oxygen values in ml 1 * for each of the sampling periods ...... 71

vl LIST OF FIGURES

Page

Location of sampling site ...... 7

Vertical profile of mean temperature (°C), salinity ( /oo) and dissolved oxygen (ml 1 ) . . . 13

Cluster dendrograms for collections from the initial stratified sampling period (period I) . . . 16

Cluster dendrograms for collections from the well-mixed sampling period (period II) ...... 21

Cluster dendrograms for collections from the restratified sampling period (period II) ...... 26

Density of Acartia tonsa adults in the upper and lower strata for each sampling period ...... 33

Density of Acartia tonsa late copepodids in the upper and lower strata for each sampling period ...... 34

Acartia tonsa adults, as a percentage of the total sample, in the upper and lower strata for each sampling period ...... 35

Acartia tonsa late copepodids, as a percentage of the total sample, in the upper and lower strata for each sampling period ...... 36

Density of Fseudodiaptomus coronatus adults, in the upper and lower strata for each sampling period ...... 37

Density of Pseudodiaptomus coronatus copepodids in the upper and lower strata for each sampling period ...... 38

Density of Neomysis americana in the upper and lower strata for each sampling period ...... 40

Density of Leucon americanus In the upper and lower strata for each sampling period ...... 41

vii LIST OF FIGURES (continued)

Figure Page

14. Density of Sagitta tenuis in the upper and lower strata for each sampling period ...... 42

15. Length frequencies as a percentage of the total stratum for night collections of Sagitta tenuis ...... 43

16. Density of Anchoa spp. in the upper and lower strata for each sampling p e r i o d ...... 45

17. Percentage of Acartia tonsa adults in the upper stratum during each sampling period ...... 46

18. Percentage of Acartia tonsa late copepodids In the upper stratum during each sampling period ...... 47

19. Percentage of Pseudodiaptomus coronatus adults in the upper stratum during each sampling period ...... 48

20. Percentage of Neomysls amerlcana in the upper stratum during each sampling period ...... 49

21. Percentage of Sagitta tenuis in the upper stratum during each stratum ...... 50

vili ABSTRACT

The community structure and vertical distribution patterns of mesozooplankton in the lower York River, Virginia were examined during August, 1978. Samples were collected in the upper and lower water strata before, during and after destratification of the water column due to tidal mixing.

In the stratified pre-mixed water column, distinct zooplankton communities were identified above and below the pycnocline. Pronounced diurnal vertical migrations of Acartia tonsa (all stages) and Pseudodiaptomus coronatus were documented. During the well-mixed hydrographic regime, the previously identified communities were not as well defined. There was a greater homogeneity of species assemblages and less pronounced differentiation of the vertical distributions for the dominant species during all light conditions. Following restratification, the species assemblages and diurnal vertical distributions closely resembled those identified in the well-mixed sampling period rather than the communities and patterns observed from the initial stratified environment.

The homogeneous nature of the zooplankton communities in the well-mixed environment is attributed to the absence of water column stratification. The inability of the zooplankton to reestablish vertically distinct assemblages following restratification of the water column is proposed to be a result of low oxygen concentrations.

ix ESTUARINE ZOOPLANKTON COMMUNITY STRUCTURE IN

STRATIFIED AND WELL-MIXED ENVIRONMENTS INTRODUCTION

Traditionally, studies of zooplankton communities in estuaries have been largely confined to descriptions of seasonal or spatial patterns of species composition and densities (Herman et al., 1968;

Gardner, 1977; Jacobs, 1978; Knatz, 1978; Maurer et al,, 1978; Stepien et a l ., 1981; Turner, 1982; Carter and Dadswell, 1983). Other investigations which examined shorter temporal or spatial scales have described estuarine communities and selected species distributions as being randomly organized with respect to depth (Williams et^ al., 1968;

Sage and Herman, 1972; Thayer et a l . , 1974; Hopkins, 1977; Kremer and

Nixon, 1978).

Numerous studies of estuarine mesozooplankton have incorporated ancillary physical data and were primarily designed to examine selected species or species groups. These studies have attributed the observed zooplankton distributions to the effects of tidal advection or tidal currents (Stickney and Knowles, 1975; Lee and McAlice, 1979; Alldredge and Hamner, 1980; Woolridge and Erasmus, 1980; Gagnon and Lacroix,

1982, 1983), behavioral reponses for enhanced reproductive success (see

Sulkin, 1984 for a review), vertical migrations (Fulton, 1984a;

McConnaughey and Sulkin, 1984), or recruitment strategies (Barlow,

1955; Jacobs, 1968; Sandifer, 1975; Trinast, 1975; Seliger et al.,

2 3

1982). These have provided extensive data and interpretations relating

estuarine processes and plankton processes; however, examinations, both

temporally and vertically, of entire zooplankton populations are

minimal.

Numerous studies have documented the distributional ecology or

seasonal occurrences for the majority of the dominant zooplankton

species or species groups encountered in the lower Chesapeake Bay and

include copepods (Heinle, 1969a,b, 1974; Burrell, 1972), decapods

(Sandifer, 1973, 1975), fish eggs and larvae (Olney, 1983), cladocerans

(Bosch and Taylor, 1973; Bryan, 1977), chaetognaths (Grant, 1977;

Canino and Grant, 1985), Callinectes sapldus (McConaugha et al., 1983;

Provenzano et al., 1983), Chrysaora quinquecirrha (Felgenbaum and

Kelly, 1984), Crassostrea virginica larvae (Seliger et al., 1982) and

Mnemiopsis leidyi (Burrell and Van Engel, 1976). Corresponding

community analysis of zooplankton populations in the lower Chesapeake

Bay and its subestuaries is largely restricted to areal surveys

(Jacobs, 1978; Grant and Olney, 1979, 1983).

Although these studies from the Chesapeake Bay provided extensive

data identifying seasonal occurrences and abundances or regional

distributions of zooplankton species, only limited data were presented

to suggest that the observed distributions were influenced by tidal

currents or circulation patterns or could be attributed to behavioral

migration patterns. Evidence provided by research in other estuaries

illustrate the importance of considering synoptic sampling of both

zooplankton and physical data when investigating estuarine mesozooplankton populations. Despite these data, there were no known 4

research efforts in the Chesapeake Bay designed to test hypotheses relating crustacean mesozooplankton communities to hydrographic structures or anticipated physical events.

Haas (1977) and later Hayward et al. (1982) documented that several moderately stratified subestuaries of the Chesapeake Bay predictably alternated between a stratified and vertically homogeneous hydrographic state. This short-lived spring tide mixing event lasted approximately 3-4 d. The well-mixed water column resulted in an increase in the mixed layer depth, homogeneous distribution of nutrients and oxygen, alteration of phytoplankton species composition and an increase in overall water column productivity. The tidal mixing phenomenon has been documented in other estuaries and used to explain observed patterns of phytoplankton (Sinclair, 1978; Sinclair et al.,

1980) and zooplankton (Gagnon and Lacroix, 1982; Maranda and Lacroix,

1983) distributions in the St. Lawrence estuary.

These studies suggest that tidal mixing, through its effects on stratification, redistribution of nutrients, and mixed-layer depths, plays an important role in the structure and functioning of planktonic communities. The extent and effects of these interactions on higher trophic levels (i.e. zooplankton and ichthyoplankton) and possible alterations in species distributions are largely unknown.

The predictability of tidal mixing in the lower York River prompted a research effort to be conducted during August, 1978. This study examined the relationships between mesozooplankton species and species groups and the physical environment in the lower York River.

Sampling was conducted before, during and after tidal mixing and was 5

designed to address the following questions: 1) Do distinct zooplankton assemblages exist in the upper and lower water strata during stratified conditions? 2) Is the distinction, temporally or spatially, between these assemblages obscured or eliminated during a spring tide mixing event? and 3) What is the extent of the reformation of these previously defined assemblages following the restratification of the water column. METHODS and MATERIALS

Based on previous research in the lower York River (Haas,

1977) and predicted tide heights, destratification was anticipated to commence shortly after 19 August, 1978, and persist for 3-4 d.

Zooplankton samples were collected in the channel at the mouth of the

York River (37° 15' 4" N. Lat,, 76° 23' 28" W. Long; Figure 1).

This station was occupied for 26 h during each of the following periods: 8-9 August, neap tide, stratified condition; 22-23 August, spring tide, destratified condition; 30-31 August, neap-tide restratified condition. These sampling dates will be referred to as sampling periods I, II and III, respectively.

Sample collection

Ancillary data (e.g. salinity, temperature, dissolved oxygen, chlorophyll a_, fluorescence) were collected at 1 m intervals every 3 h during the sampling periods from a stationary platform in the lower

York River. Salinity and temperature were measured with a YSI model 33

S-C-T meter and dissolved oxygen was measured with a YSI model 51A salinity compensated oxygen meter. During sampling period III, data collection on the fixed platform was terminated prior to the completion

6 7

Figure 1. Sampling location in the lower Chesapeake Bay. Sampling site was at the mouth of the York River. * * 4 T A H *

/

I ■ 76* 20'

Figure 1 8

of the zooplankton sampling. Data presented for this period were

collected during the preceding 12 h, A complete description of the

sampling procedures and analytical techniques is provided by Haas et^

al. (1981a,b).

The study required effective sampling above and below the

discontinuity layer during sampling periods I and III; therefore, it

was critical to select sampling depths to 1) minimize the temperature

and salinity variations within each depth strata, and 2) maximize the

temperature and salinity differences between the two discrete-depth

samples. Temperature and salinity data collected from the stationary

platform immediately prior to the first sampling period indicated the

1-6 and 10-17 m depth intervals would optimally satisfy these

requirements. Data compatability and consistency required that the

sampling during the well-mixed (destratified) sampling period (period

II) be conducted using the same previously defined depth Intervals.

Prior to sampling during period III, temperature and salinity data

indicated the same 1-6 and 10-17 m depth intervals would satisfy the

sampling criteria. Gear limitations precluded the concurrent

collections in the 7-9 m depth interval.

Collections were made utilizing a multiple net discrete-depth

sampler (Hopkins et al., 1973) activated by a General Oceanics messenger operated closing mechanism. The gear was fitted with two 60

cm Nitex plankton nets (202 um mesh) and an internally mounted

calibrated General Oceanics flowmeter. The final configuration,

considering an estimated 15° departure from perpendicular while

sampling, had an effective cross sectional filtering area of 0.22 9

-2 m . Additional data were collected with a i m WHOI neuston sampler

(333 um mesh) fitted with a calibrated General Oceanics flowmeter. All

samples were collected with a vessel speed of 1.5-2.0 knots and were

collected adjacent to the stationary platform.

The subsurface gear was initially deployed in a stepped

oblique manner in the upper depth interval. After fishing the upper

stratum, the first net was closed, which automatically opened the

second net, and the gear was immediately lowered to the second sampling

stratum. This process took an estimated 3-5 s. Subsurface samples 3 typically filtered 30-100 m of water.

Rather than randomly collect samples at predetermined time

intervals, samples were collected during all maximum currents, high and

low water levels and slack conditions during each sampling period.

These times would permit a preliminary examination of potential effects

of tidal currents on species densities and occurrences. Additional

samples were collected at sunrise and sunset when time permitted.

Collections were concentrated with a 110 jam Nitex screen and

immediately preserved with 5% buffered formalin in seawater. Prior to

preservation, all Aurelia aurita and Chrysaora quinquecirrha were

removed, rinsed, enumerated and discarded. An abundance of Mnemiopsis leidyi, and subsequent clogging of the concentrating screens, required prescreening during sampling periods XI and III. The prescreening process included an initial sieving and rinsing of the samples through a 0.25 cm nylon mesh. The concentrated ctenophores were stained with a chromic acid solution (Gosner, 1971), enumerated and discarded.

All collections were initially examined for uncommon or large 10

organisms (e.g. medusae, mysids, fish larvae) and then abundant taxa

were sorted from successively greater aliquots using a Burrell et^ al.

(1974) subsampling apparatus. All specimens were identified to the

lowest possible taxonomic category.

Copepodids were identified to species and enumerated as two

classes for Acartia tonsa (late copepodid; stage IV-V: early copepodid;

stage III) and grouped as one class (copepodid) for Pseudodiaptomus

coronatus. The lower densities observed for the smaller forms (e.g.

early copepodid stages, small unid. mollusca, unld. polychaetes) was

due to extrusion through the 202 um mesh; however, I assumed these

errors were constant for all samples.

Anchoa spp. and Sagitta tenuis were measured with an ocular micrometer and data recorded in 1 mm intervals. Anchoa spp. lengths were recorded as notocord length (NL) for preflexion larvae and

standard length (SL) for postflexion larvae, while Sagitta tenuis were measured for total length.

Data analysis

3 Abundances were standardized to numbers per m . The high positive correlation between the mean and variance of zooplankton samples required densities to be l°gjQ (x+1) transformed which reduced the dominance of the common taxa and resulted in a log-normal distribution.

The principle method for the analysis of community structure was cluster analysis, both normal and inverse, using the logjQ 11

transformed data. All taxa which occurred in more than 2 samples for the particular sampling period were included. This greatly reduced the distortion commonly encountered with occurrences of infrequently collected taxa (Clifford and Stephenson, 1975). The normal analysis provided a grouping of samples according to their similarity in species composition and abundance while the inverse analysis provided a classification of species according to their site of occurrence. These clustering procedures utilized the Bray-Curtis similarity index and a flexible linkage method (B = -0.25) (Boesch, 1977). The resulting dendrograms were examined for natural separations where the similarity within groups exceeded the between group similarities.

All statistical comparisons were performed following procedures from Sokal and Rohlf (1981) and Zar (1984) and performed using SPSSX (Nle et^ al., 1983), ORDIFLEX (Gauch, 1977) or user written programs. RESULTS

Hydrography

The vertical profiles of mean salinity, temperature and dissolved oxygen for each sampling period and surface to bottom differences and ranges are presented in Figure 2 and Table 1, respectively. During sampling period I, the water column was moderately stratified.

Following the period of destratification, which commenced on 20 August and persisted until 24 August, the water column restratified with a greater surface to bottom difference and a more pronounced profile.

During each of the stratified periods (I and XIX), the halocline and thermocline coincided and there was no indication of a secondary discontinuity layer. Detailed presentations of the physical, nutrient and phytoplankton results for this tidally-induced mixing event may be found in Webb and D'Elia (1980) and Haas et al. (1981a,b).

Community Structure

A total of 127 taxa were identified, including 85 to the species level, and are itemized for the subsurface and surface samples

(Appendix A and B, respectively). The species occurrences and

12 Figure 2. Vertical profile of mean temperature (°C),_j salinity ( /oo) and dissolved oxygen (ml L ______sampling period I sampling period IX ------sampling period III 09 ro o 00 03 Temperature

fO 03

ccro

00 03 ro o oo 03

ro

C/3 0>

ro r* *<

to 00

Figure 2 00 to 14

Table 1. Mean surface (lm) to bottom (18 m) differences and ranges for salinity, temperature and dissolved oxygen during each of the sampling periods

Sampling Hydrographic period Date state S (°/oo) T (°C) D.O. (mg/1)

I 8-9 Aug. stratified 3.86 2.68 5.4 18.92-23.92 25.0-28.2 1.0-8.6

II 22-23 Aug. well-mixed 0.03 0.30 2.0 19.74-20.08 26.1-27.9 3.8-7.2

III 30-31 Aug. stratified 6.72 3.50 10.0 19.85-27.06 24.0-28.7 0.4-16.0 15

densities concur with published values obtained from previous studies

conducted in the lower Chesapeake Bay (Rupp, 1969; Jacobs, 1978; Grant

and Olney, 1983). In the following presentation, upper and lower refer

to the upper and lower stratum, respectively.

Period I. Numerical classification based on sample attributes

for the samples collected during the initial stratified period identified four major groups with those dominated by day samples exhibiting the greatest separation with respect to stratum (Figure 3).

The most distinctive sample assemblage, group D, was composed entirely of upper day samples. The other assemblage with a high within-group similarity and a consistent sample type was group A, containing all lower day samples. This assemblage of low light samples has a strong similarity with group B (primarily lower night samples). The remaining assemblage (group C) was essentially an undefined mixture of all possible categories, but was dominated by samples collected during low light conditions.

Acartia tonsa (all stages) and Pseudodiaptomus coronatus (adult and copepodid) were consistently the numerically dominant species and their combined densities accounted for approximately 99% of the total

(Table 2). Using the collection groups derived from numerical classification, the highest densities for A. tonsa and JP. coronatus occurred in the mixed collection group (group C) and the lower day assemblage (group A) while the lowest densities occurred in assemblage

D (upper day) (Table 2). The total zooplankton density in addition to the densities of A. tonsa adults and P. coronatus adults were an order Figure 3. Cluster dendrogram for collections from the initial stratified sampling period (period I). SIMILARITY 1.0 .75 .50 .25 0 j------1------1______I______L

A

B

C

D

A all lower day B mostly lower night C mixed upper/lower,day/night D all upper day Figure 3 17

_3 Table 2. Mean and standard error of the densities (no. m ) of all taxa with a mean density greater than 1 ra for each cluster group during sampling period I. Classification is derived from clustering results.

Clustering Groups

Species AB C D **“

Acartia tonsa (adult) 15,689 10,347 20,656 920 +3,218 +2,154 +4,554 +278

Acartia tonsa (late copepodid) 6,513 3,138 7,104 405 +1,129 +571 +1,279 +117

Acartia tonsa (early copepodid) 1,699 656 933 54 +545 +391 +381 +23

Pseudodiaptoaus coronatus (adult) 362 66 571 7 +87 +15 +200 +3 ““ —*

Pseudodiaptomus coronatus (copepodid) 236 42 527 11 +73 +22 +181 +3

Neomysis americana 58 27 141 1 +19 +9 +37 *

barnacle cyprid 53 16 31 14 +15““ +5 +6 +4

unld. mollusca 15 26 24 4 +4 —+61 +5 +3

fish eggs 13 15 10 7 +2 +1 +2 +2

Number of Taxa 48 61 78 43

* < 1.0 18

of magnitude less for assemblage D (upper day). These distributions

concur with the densities derived from light and sampling strata

categories with the minimum densities occurring in the upper day

samples (Table 3). The densities for A. tonsa indicate pronounced

differences in the diurnal densities between strata (Tables 2 and 3).

Densities of other taxa, particularily J?. coronatus, Neomysis americana, Leucon americanus and Sagitta tenuis, indicate a preference for the lower stratum, regardless of light level. The relative abundance of the dominant species was unaltered, whether grouped by classification assemblages or by sampling strata (Table 4), with the exception of the higher proportion of _A. tonsa adults in the upper night stratum.

Period II. The cluster dendrogram resulting from the numerical classification of the samples taken during the period of destratification also indicated the presence of four assemblages

(Figure 4). As in the previous stratified sampling period, separation of the day samples by strata was readily apparent; however, the overall level of distinction was less obvious during period II. The most consistent assemblages were group D (upper day) and group A (lower day) while groups B and C were composed of samples collected during low light conditions. The within-group similarity and identification of lower day and upper day assemblages are comparable to the results for sampling period I. The anticipated greater between-group similarity was observed. This indicates less pronounced temporal or vertical differentiation between the assemblages and a more homogeneous 19

_3 Table 3. Mean and standard error of the densities (no. m ) of all taxa with a mean density greater than 1 m for each sampling strata during sampling period I.

Day Night

Species Upper Lower Upper Lower

Acartia tonsa (adult) 3,574 23,768 19,070 11,810 +1,226 +5,358 +3,297 +1,830

Acartia tonsa (late copepodid) 1,214 8,249 4,839 6,324 +346 +1343 +1,439 +1,448

Acartia tonsa (early copepodid) 124 1,195 673 1,760 +64 +339 +653 +869

Pseudodiaptomus coronatus (adult) 52 711 64 396 +29 +219 +26 +350 ““

Pseudodiaptomus coronatus (copepodid) 45 507 137 513 +23 +222 +62 +291

Neomysis americana 23 133 33 137 +12 +39 +14 +84 M.

barnacle cyprid 14 14 35 26 +3 +15 +20 +2 ““

unid. polychaetes 2 43 9 45 * +13 +2 +9

unid. mollusca 7 19 36 31 +2 +3 +14 +5

Leucon americanus 1 10 9 48 A +5 +5 +35

fish eggs 10 11 12 11 +3 +1 +4 +3

Sagitta tenuis 2 13 7 10 * +3 +4 +3

Number of Taxa 57 74 59 67

* < 1.0 Table 4. Relative abundance for dominant species in sampling period I comparing the cluster group and sampling strata classifications. HQ rH •H to CJ & r-l CO •M 4J u bO C u (fl a (B 0) M nJ fd a •r-f a) n 4-1 o a bO M m 3 OCd o a CO CO cO u CJ • • • • • • • ■w* o n i-H P (0 0 CO Mi* vO H ov VO CM CM CO OO CM Ov CM rH oo CM Os CM CM H4-> •H P o o o a a> CO da cd cd rH M a> cd a u • • • « • • • * • • • ^ v cd a cd * a CJ CO in CM CO vO CO o CO 00 CM in r>. Hcd *H < J 4 P 0 CO M cd • • • • • w' 'w H •H 'd O GU 0) Gu o cu U >» U CM o Os o CO o OV in in i-H O n H H rtf TO t-H •H PH P P a o 6 d o CO CJ o u o c cd d CO cd )0) CO a) d o • • • • • • • • ✓“S ■d I— p cd d r < o CO 00 o 00 s a n i vo o d * d cM PH P P Oa CO cd CO o € d CJ o u a cd d CO Cu o O d #»“ N T> J T P U C 0) cd O CJ a CM O CM o m o o o O m Ho •H S5 cd a CO 00) (0 o £ I M cd

1.0 .75 .50 .25 0 j______I______1______I______L 1 co 1 1 1 1 t 1 1 t 1 o 1 1 1 1 1 -- 1 1 1

A

D

A all lower day B mixed night and lower day C mixed night and lower day D all upper day Figure 4 22

community structure in the absence of stratification.

As in the previous stratified period, all stages of tonsa

dominated the collections with P. coronatus and Neomysis americana

being the next most abundant species. Densities derived from normal

classification assemblages indicated the lower day samples (cluster A)

had the highest densities (Table 5) and were greater than twice those

of the next highest group, the upper day (cluster D). The extremely

low densities in the upper day samples from the stratified period

(period I) were not evident in period II. When densities (Table 6)

with respect to light and depth strata are examined, there were less

pronounced temporal or vertical distinctions for individual species and

a more uniform distribution across criteria during the destratified

period. Rank abundance was essentially identical to Period I; however,

the densities and relative percentages of A. tonsa adults and late

copepodids are similar (Tables 5-7). These data support the numerical classification results and indicate more homogeneous species densities, distributions and community composition in a vertically homogeneous water column.

Period III. Numerical classification of collections from the restratified period produced a dendogram which showed distinctive separation based primarily on light and secondarily on stratum (Figure

5). The lower day assemblage (group A), unlike the two previous sampling periods, had the greatest between-group separation. The upper day assemblage (group D), which had the greatest between group separation in sampling periods I and II, was also identified; however, 23

_3 Table 5. Mean and standard error of the densities (go. m ) for all taxa with a mean density greater than 1 m for any cluster group during sampling period II. Classification is derived from clustering results. Cluster Groups Species A B C D Acartia tonsa (adult) 9,955 3,620 3,022 4,016 +2,649 +1481 +704 +885

Acartia tonsa (late copepodid) 5,356 3,453 2,856 4,073 +1,148 +1,654 +821 +654

Acartia tonsa (early copepodid) 153 205 263 255 +46 +36 +118 +64

Pseudodiaptomus coronatus (adult) 222 39 147 10 +96 +12 +28 +7

Pseudodiaptomus coronatus (copepodid) 369 310 276 90 +145 +192 +54 +13

Neoraysis americana 288 39 165 12 +139 +11 +43 +4

barnacle cyprid 12 12 13 19 +4 +5 +5 +3

unid, polychaetes 132 63 119 52 +53 +18 +45 +21

unid. mollusca 135 59 78 29 +60 +19 +27 +7

fish eggs 6 9 7 5 +3 +3 +2 A

barnacle nauplii 22 5 4 12 +14 +2 +3 +5

Sagitta tenuis 24 3 12 4 +4 * +2 *

unid. phronid larvae 19 4 7 4 +5 +1 +2 A

Paracalanus sp. 32 31 13 13 +11 +30 +9 +9 Number of Taxa 70 71 78 80

* < 1.0 24

— 3 Table 6. Mean and standard error of the densities (no. m ) for all taxa with a mean density greater than 1 m for all sampling strata during sampling period II.

Day Night

Species Upper Lower Upper Lower

Acartia tonsa (adult) 4,003 5,761 3,874 4,108 +1,021 +1,722 +484 +2,228

Acartia tonsa (late copepodid) 4,181 3,640 3,339 4,121 +745 +791 +919 +2,250

Acartia tonsa (early copepodid) 267 184 302 180 +72 +31 +179 +37

Pseudodiaptomus coronatus (adult) 12 132 86 117 +7 +52 +32 +45

Pseudodiaptomus coronatus (copepodid) 87 299 156 446 +15 +74 +56 +275 ““

Neomysis americana 13 202 78 64 +5 +69 +21 +18

barnacle cyprld 21 7 25 10 +2 +2 +6 +3 unid. polychaetes 56 114 83 71 +24 +40 +14 +17 unid. mollusca 28 97 85 44

+8 +32 +30 *—+8 fish eggs 5 4 16 7 * +1 +4 +2 barnacle nauplii 14 10 11 1 +5 +7 +5 unid. phronid larvae 3 11 5 7 * +3 +2 +2

Number of Taxa 79 80 73 70

* < 1.0 Table 7. Relative abundance for dominant species in sampling period II comparing the cluster group and sampling strata classifications. C/3 CO 4-1 e bO « P cfl 0 CJ H JC S5 •ft Cfl P 4J CD a bO Q P P o Q. a I

cl CO ol ra !§* hJ 3 P u 3 CO 0) Pi a (0 P s a o a) P a 0) p o p d -et­ -d- r-* o *H Oh CM -3 nr m o \ <3 n i vD i—• n i CM 00 00 CM JJ J4J 4J 033 3 3 3 <0 3 3 3 O O 3 o 3 a « • • • • • • • * 'O rH JJ 3 Hf h c HJJ •H f-H <3 CO Oh 00 o 00 Oh co *3- CO co -d- m CO m 4J M 3 P 3 3 a a • • • * • • • • •SH* F—t T3 3 Pi i3 Pi O CJ Oh < H3 *H o co Om CM vO CO CO oo CO P's o i-H rH Oh JJ JJ P 3 3 3 O ■ • • • • • • • w r— I 3 T T> 3 O cu a o Cu >> CJ CO •a -a •H CO o CM rH 00 H r CO 00 CO a i-H H r H r rH H r u p + CD cu o co 3 o 3 3 3 CO a a o e P CD CJ O P a • • • • • • T3 cO a •a CM •a •H Hfr CO Oh co Oh H t CM o H r o CM Oh a rH 4J p CO OJ o co a o CD O 3 CO o 3 a Jto S a CJ • • • • • ■ • • •a •a o CJ a a: a o 43 o H O CM O CM o o CM o o co CM o o + + + CO p cO a o o a p • • • • • * 0 0 0 0 o o 00 r-s t rH ■8 o o o H r hO rH H r rH § o CO (1) Pu >% • • • { 9 ■ • • * CO o CM o H r p H r o + + a § co o cd H 3 3 • ■ • • * V o 25 Figure 5. Cluster dendrogram for collections from the restratified sampling period (period III), SIMILARITY 1.0 .75 .50 .25 0 J______I______I______1------L

A lower day B mi xed C night, mostly upper D upper day E mixed, lower and night

Figure 5 27

there was a greater than expected affinity with the remaining mixed sample assemblages. Unlike the two previous sampling periods, the remaining three cluster groups were less well defined.

Mean densities (Tables 8 and 9) and relative abundances (Table

10) of the dominant species identify a substantial alteration in species ranking during restratification, particularily in the lower day stratum and cluster group A where the overall density of P. coronatus adults essentially equalled the density of A. tonsa adults during period III. Unlike the two previous sampling periods, the overall density and the density of A. tonsa in the upper day samples exceeded those of lower day samples (Table 9). In contrast, Pseudodiaptomus coronatus did not present a different distributional pattern and exhibited higher overall densities in the lower stratum. The densities for period III are presented as relative abundances (Table 10).

Similar to the results from periods I and II (Tables 4 and 7), A. tonsa dominated all strata and cluster groups; however, the relative percentage of P. coronatus was substantially greater in several criteria during period III (Table 10).

A nonparametric comparison (Kruskal-Wallis, 2 tailed) to test for intergroup differences was performed on the densities and abundances

(arcsine transformed) for each of the cluster and stratum criteria for each sampling period (Table 11). Only those dominant species which had -3 abundances greater than 1.0 m or relative abundances greater than

1.0 % for each sampling period were selected for analysis. 28

~3 Table 8. Mean and standard error of the densities (no. m ) for all taxa with a mean density greater than 1 m for any cluster group during sampling period III. Classification is derived from clustering results.

Cluster Groups

Species A B C D E ““ ““

Acartia tonsa (adult) 2,755 2,387 5,728 4,367 1,336 +1,049 +1,196 +705 +1,120 +217

Acartia tonsa 4,210 2,425 4,668 3,313 1,095 (late copepodid) +2,853 +1,133 +1,097 +950 +263

Acartia tonsa 775 160 500 334 102 (early copepodid) +532 +64 +118 +103 +31

Pseudodiaptomus coronatus 2,271 26 212 144 109 (adult) +1,145 * +98 +47 +35

Pseudodiaptomus coronatus 1,390 26 179 191 58 (copepodid) +556 A +47 +73 +20

Neomysis americana 112 2 48 4 13 +49 +5 +14 +1 +4

barnacle cyprid 8 5 14 17 3 +4 +3 +3 +8 A

unid. polychaetes 864 2 13 37 233 +263 +1 +3 +14 +94

unid. mollusca 74 5 11 10 24 +9 +2 +3 +2 +6

barnacle nauplii 8 2 15 25 5 +3 A +4 +10 +2

Number of Taxa 92 51 74 86 79

* < 1.0 29

_3 Table 9. Mean and standard error of the densities (^o. m ) for all taxa with a mean density greater than 1 m for all sampling strata during sampling period III.

Day Night

Species Upper Lower Upper Lower

Acartia tonsa (adult) 4,201 1,669 4,937 3,806 +976 +419 +1,349 +1,645 ””

Acartia tonsa (late copepodid) 3,343 1,377 4,361 6,269

*—+823 +439 +1,367 +4,916

Acartia tonsa (early copepodid) 320 217 475 1,045 +90 +163 +143 +890

Pseudodiaptomus coronatus (adult) 129 1,668 202 370 +43 +949 +101 +249

Pseudodiaptomus coronatus (copepodid) 170 956 177 362 +66 +480 +49 +313

Neomysls americana h 82 44 36 + 1 +42 +16 +14 barnacle cyprid 16 4 13 10 +7 * +4 +7 unid. polychaetes 33 595 39 439 +13 +208 +28 +350 unid. mollusca 10 51 12 45 +2 +12 +3 +19 ““

Paracalanus sp. 8 18 4 93 +6 +16 +4 +93

Number of Taxa 86 88 74 89

* < 1.0 Table 10. Relative abundance for dominant species in sampling period III comparing the cluster group and sampling strata classifications. rH *rl CO CO j j j j cd cd cd C bo Mt cu 3 5 •G *—£ U SI •H Q O 4-) j j C0 b0 cd >. (U M M O p s D

. Cl •H o| w a Ml co £3 0) O «r co O < in oo m H rH r- rH JJ j j cd M co 3 cd o g u . • • • • • 24 CO 3 *rf tH o co O'* CO CM 00 rH rH CO o CM CM rH CO < CM CTi CO 00 CO 00 JJ jj j3 Jj cd cd cd a o G u • « • • ft • • • rH Td •H )33 3 o 0) do 0) cd •3 H >v a & u *3 - a> r-* r^. o o n- •H rH rH rH o\ rH CO vO rH vO n n in P-l CM CM CM JJ JJ Ua) i-H CM o <1“ o n* PM ■*3- CO n CM o0 o a> O rH *H vO 00 o o p" r*-. 0o 00 o JJ + + CM 3 CO G C0 a § a O co rH CO u rH P C5 co U P P 3 3 P P O ft 3 Q ft 3 bO a 3 3 P 3 3 3 CO 3 P O 3 a § 3 bt) M O & 3 T3 H lP iH CO i-t ft M| W | M H Ml H M M O 3 3 P 0 ft 3 bO co •P ft 3 3 3 3 < * ■k ■k He -k ■k ■k He * He He * He * * He He JJ P 3 3 3 3 O O O 3 O 3 U P 3 P 3 r-\ u T3 1 P 1 — 3 He ■k < < ■k * •k ■k •H He He * * j j JJ 3 3 3 3 JU CJ j v 13 •H T3 ip j j 3 y ft ft O 3 * •H * P jj 3 P 3 3 3 3 *0H Q> 3 rt O M ■k * * k• k•k ■k •k -k PU •k ■k -k * £ 3 CO (X) ft

w *—s 3) Ip ■H p 3 3 3 k•k ■k ■k ■k •k ■k ■k He 3) k- ■k -k •k ■* ft 33 * He p p 3 3 3 ft O 3 3 3 O 3 O 3 3 y O P 3 0 0 <^-N 33 *H 3) O y O 0 ft rH •P 3 ft ■k •k He He He He He He a P• 3 3 3 •P •P £ £ £ £ £ £ t 3 < K ■# d < -K C He He a << 3 a a < a B; C * < < Jsaaaaaa* a 3 3 3 3 3 P P 3 ft 3 3 3 0 0 U * •k ■k ■k J3 3) * H3 •H 3 P 3 y u 3

k< «i . >. 3 O y 3 3 ft

He H3 •H a a

/*% rH rH P 3 bO 3 3 He He He He He He He •k H3 * ■k ft •H He * * He ■K P p 3 O 3 3 3 3 3 ft 3 O P 0 3 y O O 3 ✓ rH rH P - bO 3 3 3 3 3 V

significance: * =p<0.10, **=p<0.05, ***=p<0.01 31 NA not applicable, All criteria for the particular situation did not have density >1 m ^ or a relative abundance >0.1%. 32

Vertical Distribution

Acartia tonsa. Densities in the upper and lower strata for the

adults and copepodids are presented in Figures 6 and 7, respectively.

There is substantial diurnal variation, particularly for the adults,

during the initial stratified period (period I) with a day-night

difference of approximately two orders of magnitude in the upper water

densities. The densities during the unstratified and restratified

periods (periods II and III) indicate a relatively homogeneous

distribution with substantially reduced diurnal migration patterns.

Densities in the upper stratum exceed those in the lower stratum in all

but two samples for the adults and all but three for the late

copepodids during period III. When presented as a percentage of the

total sample (Figures 8 and 9), A. tonsa adults comprised a

consistently greater percentage of both strata for period I. This strong dominance of A. tonsa adults in the upper stratum at night

(Figure 8) further indicates strong diurnal migration during period I.

Periods II and III reveal a reduced dominance of the adults, similar densities for the adults and copepodids in each stratum and no noticeable diurnal pattern of dominance of the samples. The late copepodids (Figure 9) for the same period constitute a substantially smaller percentage of the total sample.

Pseudodiaptomus coronatus. The densities for the adults and copepodids during each sampling period are presented in Figure 10 and

11, respectively. The densities for the adults in the lower stratum exceed those in the upper stratum except during a few night 33

Figure 6. Density of Acartia tonsa adults in the upper and lower strata for each sampling period. The numerals indicate the sampling period. L«l#/*J) L0C(#/H3) L0t(f/H3] o a 3 ■4- s 1 ■ i ■ TIME TIME TIME iue 6 Figure CK**0 CES-O EBT) T B (E i a rn _ LOWER LOWER _ UFRER . UPPER LOWER LOWER URRER 34

Figure 7. Density of Acartia tonsa late copepodids in the upper and lower strata for each sampling period. The numerals indicate the sampling period. LOClf/if) LOG(#/li#) L O C ( | / U f ) o a a a a A a 1 i m n V B C E M I T TIME TIME o iue 7 Figure 0 * a c c CESTO ia I I I LOWKN — UPPER - UPPER . LOWER LOWER 35

Figure 8. Acartia tonsa adults, as a percentage of the total sample, in the upper and lower strata for each sampling period. The numerals indicate the sampling period. SAMPLE Z SAMPLE Z SAMPLE Z i 1 o o oo so so ao ao ■ < so so -- • so 4-0 ■ 1 i a m TIME i e tim iue 8 Figure : CESTO ( t s e ) LOWER 36

Figure 9. Acartia tonsa late copepodids, as a percentage of the total sample, in the upper and lower strata for each sampling period. The numerals indicate the sampling period. sample x n m i x sample x 1OO 1 o o ao •4-0 SO ao ao 0 3 ao o o o 1 a i a TIME IE CE=»T> TIME IE MT) CM TIME iue 9 Figure CEST) i a i a 13 n p p u n w o t n p p u n w a L UIPFKN lqwhi 37

Figure 10. Density of Pseudodiaptomus coronatus adults in the upper and lower strata for each sampling period. The numerals indicate the sampling period. -4-

LOWIR

2

1

O i a TIME ( E S I )

a

1 2 i m 12 TIME (EST)

11

LOWER URRER 3

2

1

O 1 2 1 2 O 1 2 TIME

Figure 11. Density of Pseudodiaptomus coronatus copepodids in the upper and lower strata for each sampling period. The numerals indicate the sampling period. L06l#/»3) L06(#/H3) L0S(#/1I3J O 2 A 1 o 2 -4- O 2 3 1 1 3 1 3 1 3 i • 1 • CE3IT5 E M I T TIMES TIME iue 11 Figure O (E3T)

collections. The pattern in the first two sampling periods indicated a

diurnal tendency for P. coronatus adults to actively migrate into the

upper stratum. There is no migration pattern evident during period III

despite the increased abundance during the entire third sampling

period. The copepodids, being less active migrators, do not

demonstrate diurnal patterns of densities in either water strata

(Figure 11).

Neomysis americana (Figure 12) and Leucon americanus (Figure 13)

confirm the widely documented distribution patterns represented by

consistently higher densities in the lower stratum, an increased

density in the upper water at night and higher densities for combined

strata in night samples. The observed diurnal patterns indicated no

correlation with tidal stage or current.

Sagitta tenuis (Figure 14) demonstrates a consistently higher

density in the lower waters and provides preliminary evidence of

diurnal migrations trends during the initial stratified and well-mixed

regimes (periods I and II). No pattern of diurnal migration was

evident in period III. Analysis of length frequencies of night

collections (Figure 15) indicates a greater percentage of the larger

specimens occur in the upper water column while the smaller

chaetognaths appear to be restricted to the lower stratum during each

stratified period. In the well-mixed environment, the upper and lower

populations are more uniform with respect to sizes. Due to the smaller number of night samples when compared to day samples and probable net avoidance during the day, particularly in the upper stratum, no

statistical tests were performed on the length frequency data. Despite 40

Figure 12. Density of Neomysis amerlcana in the upper and lower strata for each sampling period. The numerals indicate the sampling period. o o 1 St TIME

_t

o 1 • 1 st TIME (CSTJ

L.OWKR

a

o o T I M E C E S T 3 Figure 12 Figure 13. Density of Leucon americanus in the upper and lower strata for each sampling period. The numerals indicate the sampling period. _ LOWER . UI-PKN a

a

o 1 3 i m t a TIMB CEST)

1 3 1 3 TIME

O 1 3 TIME C » T ) Figure 13 Figure 14. Density of Sagitta tenuis in the upper and lower strata for each sampling period. The numerals indicate the sampling period. S

_ LOWKN ■4- . u r ^ * n

3

2

1

O TIME CESS TO

I I

LOWBN

_ UPPER

3

13 1 ■ TIME CES-TO

3 I I

_ LOWER

. UPPER

St

1

O 1 a 1 • o 1 a T I M E ( E S T J

Figure 14 43

Figure 15. Length frequency as a percentage of the total stratum for night collections of Sagitta tenuis. The numerals indicate sampling period. PERCENT PERCENT PERCENT 90 ­ o l -t I St EGH M 5 MM C LENGTH EGH M 3 MM C LENGTH EGH M 5 MM C LENGTH • 7 • B 4 iue 15 Figure 1 O ■ ■ LOWKR^ H III ll I N K W O . I UPPBN H K P P U UOWVN U,-,

44

these restrictions, Figure 15 implies the halocline or thermocline may

alter the vertical distribution of _S. tenuis.

During period I, Anchoa spp. exhibited increased densities in

both strata during low light conditions (Figure 16). During the well-mixed and restratified conditions, less pronounced diurnal variations and an uncertain pattern of depth distributions were observed. Identification of vertical migration tendencies based on upper and lower strata densities are not justified due to likely net avoidance of all size intervals during daylight sampling, increased net avoidance of larger specimens at all times and the greatly reduced densities during period III. However, preliminary analysis of the length frequencies for the night samples indicated the mode and mean length in the upper stratum exceeded those for the lower stratum in each sampling period.

Paired samples were examined for the percentage of the total water column densities which occurred in the upper stratum.

Distributions for A. tonsa (adults and copepodids), P. coronatus adults, 1J. americanus and j>. tenuis are illustrated (Figures 17-21, respectively). Each of these numerically dominant taxa demonstrated obvious patterns during the first sampling period with distinct increases in percentage composition in the night samples. Percentage composition during the well-mixed and restratified periods do not indicate consistent and identifiable diurnal patterns. The percentage of 11. americana and j>. tenuis populations remained higher in the lower water stratum at all times during period I despite a tendency for increased proportions in the upper stratum during night sampling 45

Figure 16. Density of Anchoa spp. in the upper and lower strata for each sampling period. The numerals indicate the sampling period. LOWER UFI*ER I I I a 1 i a i oa o O Figure Figure 16 TIME

(t«/»|l01 (tl/DlOl ((1/1)901 46

Figure 17. Percentage of Acartia tonsa adults in the upper stratum during each sampling period. The numerals indicate the sampling period. PERCENT IN UPPER STRATUM 100 IE (EST) TIME iue 17 Figure 0 6 12 PERIOD 47

Figure 18. Percentage of Acartia tonsa late copepodids in the upper stratum during each sampling period. The numerals indicate the sampling period. PERCENT IN UPPER STRATUM 100 20 40 0 6 80 " 12 18 IE (EST) TIME iue 18 Figure 0 6 12 48

Figure 19. Percentage of Pseudodiaptomus coronatus adults in the upper stratum during each sampling period. The numerals indicate the sampling period. 100 percent IN upper s m m PERIOD 49

Figure 20. Percentage of Neomysis americana in the upper stratum during each sampling period. The numerals indicate the sampling period. PERCENT IN UPPER STRATUM 100 20 40 60 80 0 18 IE (EST) TIME iue 20 Figure 0 6 12 Figure 21. Percentage of Sagltta tenuis in the upper stratum during each sampling period. The numerals indicate the sampling period. PERCENT IN UPPER STRATUM 100 0 2 -- 18 IE (EST) TIME iue 21 Figure 0 6 12 PERIOD 51

(Figures 20 and 21). However, A. tonsa adults and copepodids, and P.

coronatus (all stages) (Figures 17-19) clearly illustrate vertical migrations during sampling period I. These patterns were not repeated in sampling periods II (unstratified) or III (restratified).This reflects the reduced vertical migration patterns during periods II and

III (Figures 6-16).

Nonparametric tests to detect significant differences between the densities in day-night and upper-lower strata were performed

(Mann-Whitney, 2 tailed). In all but one instance (A. tonsa adults, period I, night) there was no significant difference between upper night and lower night samples for the six most dominant taxa (Table

12), Additionally, there were no significant differences between strata for either light condition for A., tonsa (all stages). The most dominant taxa showed no significant difference between the day and night samples for either the lower or upper stratum during periods II and III (Table 13). These results further indicate a more vertically homogeneous distribution during the well-mixed and restratified periods.

The means and standard errors of the means, as presented in the tables, confirm the highly variable nature of zooplankton populations.

This between-sample variation, when incorporated with a relatively small sample size, may adversely affect rigorous parametric statistical procedures. When these data are subjected to community analysis procedures or examined for diurnal trends and patterns of altered species dominance, the conclusions presented by this study are supported. 52

Table 12. Comparison of densities for dominant zooplankton species between upper and lower strata during the day and night. D = day, N = night. (Mann-Whitney, 2-tailed)

Sampling Period

Species I IX n:

Acartia tonsa (adult) D a a AA N *

Acartia tonsa (late copepodid) D ** AA N

Acartia tonsa (early copepodid) D a a AA N

Pseudodiaptomus coronatus (adult) D a a ** N

Pseudodiaptomus coronatus (copepodid) D a a A* N

Neomysis americana D a a A* AA N barnacle cyprid D a a AA AA N A unld. mollusca D a a AA N fish eggs D A N unid. polychaetes D ** AA N ** A

Leucon americanus D AA AA N

Sagitta tenuis D ** AA N significance: * = < 0.10, ** = < 0.05 53

Table 13. Comparison of densities for dominant zooplankton species between day and night for the upper and lower sampling strata. U = upper, L = lower, (Mann-Whitney, 2-tailed)

Sampling Period

Species I II III

Acartia tonsa (adult) U ** L **

Acartia tonsa (late copepodid) U ** L

Acartia tonsa (early copepodid) U L *

Pseudodiaptomus coronatus (adult) U ** L

Pseudodiaptomus coronatus (copepodid) 0 L

Neomysis americana U ** ** L

barnacle cyprid U L

unid. mollusca U ** ** L *

fish eggs U ** L **

unid. polychaetes U ** L

Leucon americanus U ** ** L *

Sagitta tenuis U * L * significance: * = < 0.10, ** = < 0.05 DISCUSSION

Community Structure

The zooplankton communities described in this study showed

consistent organizational patterns with light being the predominant

regulating factor governing distributions and densities. In all three

sampling periods, the most distinctive characteristics were the upper

day and lower day assemblages generally reflecting the high densities

of Acartia tonsa, Pseudodiaptomus coronatus and Neomysis americana at

depth during the day and comparatively low densities for all species in

the upper day samples. Based on the cluster analyses, distinct assemblages were identified during each sampling period with the greatest between-group separation in the pre-mixed period and apparently more obscured relationships during periods II and III.

During a "normal" stratified period (period I), the zooplankton communities exhibited a distinct separation based solely on relative densities of the depth distribution of the taxa despite the predominance of A. tonsa adults in all samples. These densities vary diurnally and provide ample evidence of distinct migrations of the dominant species. The overall community structure, as defined by

54 55

species proportions, is essentially identical for any depth strata or light condition.

The data from the well-mixed environment indicate a more homogeneous structure with the relative proportions being unaffected by depth or light conditions. The rank dominance of the major taxa are

Identical to the data collected prior to tidal mixing; however, the densities and proportions of A. tonsa copepodid and adults were approximately equal. The distinguishing feature of the well-mixed period is the relative homogeneity of the densities. The absence of stratification during this period cannot completely explain the uniform mixture. Unlike sampling period I, there is substantially less differentiation based on depth or light. Vertical distribution plots confirm the apparent absence of distinctive migration patterns or trends of preferred depths for the dominant taxa, particularly A. tonsa, P. coronatus and 1J, americana.

The post-mixing, restratified period (III), documents anomolous proportions and densities for the dominant taxa. The most notable feature is the dominance of P. coronatus during certain collections and evidence that the upper water stratum was the preferred depth for iV. tonsa during all light conditions. High densities have been previously reported for P. coronatus. Burrell (1972) reported one instance, near the mouth of the Chesapeake Bay, where the density of P. coronatus exceeded that of A. tonsa. Maurer et al. (1978), in a survey of the

Delaware Bay, reported that P. coronatus dominated in three samples from summer collections. In both studies, speculations to account for 56

these unusual events were not proposed. The times of unusually high P.

coronatus densities were not related to tidal currents.

Additionally, data for sampling period III indicate an overall

homogeneity of taxa and densities. The densities and relative

abundances are more similar to the well-mixed condition than the

initial stratified period. I expected the structure in the

restratified period to closely resemble the results from the initial

stratified sampling period. This speculation was not supported by the

data.

Considerable research using field enclosures has been conducted

in recent years (Oviatt, 1981; Grice and Reeve, 1982) and unnaturally

high predation rates of larvaceans, chaetognaths, ctenophores and

salmon juveniles on crustacean zooplankton have been reported.

Extrapolations of these results to natural aquatic systems should be

considered theoretical since the enclosures often have unnatural ratios of predators to prey, lack of suitable refuge for strong migrators, different mixing schedules than those occuring in natural systems, and the absence of zooplankton advection. Despite their limitations, they provide the background necessary to evaluate natural situations related to higher trophic level interactions.

Grice and Reeve (1982), during a field enclosure experiment, documented a temporary twofold increase in calanoid copepod biomass and a slight increase in cyclopoid copepod biomass in unmixed conditions when compared to similar well-mixed experiments. Eventually, ctenophores reduced the corresponding copepods to virtual extinction 57

and interpretations of water stability effects on secondary production 3 were not possible. Oviatt (1981), using 13 m enclosures, reported a

three-fold higher density and increased diversity of copepods in

unmixed enclosures when compared to enclosures which simulate "natural"

mixing. The data presented in this study detail a reduction in copepod

densities, and presumably biomass, between the initial stratified and

mixed conditions, although no increase was noted with the

restratification of the water column. The densities reported, in all

sampling periods, for A. tonsa and £. coronatus are within observed

limits for the lower Chesapeake Bay (Jacobs, 1978; Grant and Olney,

1983).

Mullin et al. (1985) examined the zooplankton and phytoplankton

in the upper 50 m of the California current before and after a storm.

They found increased abundances of several zooplankton taxa, particularly copepod nauplii, following the storm. This study, with the increased proportion and Increased densities of A. tonsa late copepodids during the well-mixed and restratified periods, concur with

Mullin et al. (1985).

Numerous studies confirm the dominance of _A. tonsa in Middle

Atlantic estuaries during the summer months and the higher densities of

tonsa and the subdominant species, £. coronatus, N. amerlcana,

Sagitta tenuis and Anchoa mitchilli, in night collections. Grant and

Olney (1983) used oblique sampling and were able to partially distinquish day and night communities in the lower Chesapeake Bay.

Youngbluth (1980), using oblique tows, reported abundances for 58

copepods, particularily tonsa, were significantly higher at night.

Other studies primarily designed for examining seasonal or annual

trends (Williams et al., 1968; Thayer et al., 1974) may have misinterpreted the species associations and overall abundances due to

inadequate sampling of all depths. Burrell (1972) alluded to the fact

that different copepod communities exist in near-surface and near-bottom strata; however, he did report consistently higher densities in the near-bottom samples for day samples. Unfortunately, the majority of these studies incorporated data derived from integrated surface to bottom samples, which subsequently prohibit any interpretations of depth related issues or relationships to hydrographic features.

Fulton (1984a) extensively describes different copepod community composition in Beaufort, North Carolina estuaries. He provides substantial data for depth discrete distributions and multi-species associations. However, the estuaries were shallow and well mixed and direct comparisons with this study could be misleading.

In general, this study shows a consistency and persistence in the rank dominance and relative proportions of the species assemblages or communities. The effects of destratification and restratification alter the contribution of the subdominant species and partially obscure the identification of the vertical components of the community structure. However, in all cases, the assemblages are consistent with documented communities for Middle Atlantic estuaries in the summer and 59

this study provides evidence for the existence of distinct vertical

communities which are primarily regulated by light.

Vertical distributions

The extensive literature on vertical migration behavior and

mechanisms has been reviewed by Banse (1964), Pearre (1979) and

Longhurst (1981). The majority of the individual vertical migration

studies reviewed by these authors have been performed in coastal and

oceanic systems. The greater depths and short-term predictability

permit resolution of the particular depth strata of interest, and more

detailed sampling in discontinuity layers, chlorophyll and production maximum zones in an environment where the short-term physically dominated fluctuations are minimized.

The principal hypothesis underlying most estuarine community structures has been that the observed patterns are strongly related to

temperature-salinity profiles, but temperature is reported to exert the strongest influence on vertical distributions of zooplankton (Sameoto,

1984); however, Fulton (1984a) documents that light mediates vertical distribution in well-mixed environments. Initial laboratory experiments investigating the reaction of zooplankton to thermal and salinity gradients provided valuable data (Hardy and Brainbridge, 1951;

Lance, 1962; Harder, 1968). Recent advances in laboratory techniques

(Latz and Forward, 1977) have permitted the determination of mesozooplankton threshold gradients for salinity or temperature. 60

Rhithropanopeus harrisii will exhibit phototaxis reversal when

encountering a salinity gradient of 1.1 - 2.0 °/oo depending on the

stage of the larvae, but a recovery to normal phototaxic response will

occur in 5 rain (Latz and Forward, 1977). McConnaughey and Sulkin

(1984) determined stage I Callinectes sapidus zoeae could penetrate

sharp thermoclines and Sulkin and Van Heukelem (1982) showed C. sapidus

could penetrate haloclines of 2.5 - 10.0 °/oo. Field collections

later documented the ability of _C. sapidus to transverse temperature

and salinity gradients similar to those encountered in this study

(Proven2ano et al., 1983).

Substantial field data exist to support the claims that the dominant estuarine zooplankton are not limited to a particular depth stratum by thermoclines or haloclines. Rupp (1969), Bosch and Taylor

(1970, 1972), Sandifer (1975), Grant and Olney (1983), Fulton

(1984a,b), and Stearns and Forward (1984a,b) demonstrate active nocturnal migration, even if it was inferred from higher night densities in oblique tows, for A. tonsa, K[. araericana, P. coronatus, j3. tenuis, L. amerlcanus and the majority of decapod larvae.

Turner and Dagg (1983) describe vertical migration patterns between stratified conditions near Long Island Sound and well-mixed conditions over Georges Bank. Centropages typicus and Paracalanus parvus occurred above the thermocline in stratified conditions, but were evenly distributed in well-mixed environments. Paracalanus sp. was confined to the bottom stratum in this study as well as in data reported from Beaufort N.C. (Fulton, 1984a). The difference may be due to ontogenetic variations or different species may be involved. 61

Fulton (1984a,b) and Stearns and Forward (1984a,b) documented

diurnal vertical migration patterns for A. tonsa in a shallow

well-mixed estuary. This study confirms similar diurnal migration

patterns in sampling period I for tonsa. Densities were

significantly different between upper and lower strata during day

sampling (Table 12) and distinct diurnal differences were observed in

the upper stratum (Table 13).

The results from periods II and III do not support the findings

from the initial stratified period. The densities for A. tonsa do not

indicate any discernable vertical migration patterns during the

well-mixed and restratified periods. Furthermore, there were no

significant differences between light levels or strata for A. tonsa

during period II, The significant differences between strata during

period III resulted from the avoidance of the lower stratum by A.

tonsa.

This study does not report any species being restricted to the upper stratum or exhibiting reverse diel vertical migration patterns

(Ohman et al., 1983). The smaller J5. tenuis may have been restricted

to the lower stratum by the stratification (Figure 15). No data are known for tenuis which could explain this distribution. The absence of diurnal patterns during the well-mixed and restratified conditions may be related to reduced predation pressures, predator avoidance in the lower stratum (Ohman et al., 1983), reduced light levels, sampling before the taxa could reestablish 'normal' behavior, reactions to unfavorable environmental parameters or advectlon of higher salinity water during restratification. 62

Despite the extensive literature, there remains a controversy as to the adaptive advantage of the migrations or the stimuli which may initiate or maintain the observed distributions. Numerous theories are concerned with ontogenetic, developmental or seasonal data while others have been proposed for specific or system-specific situations and may not be generally applicable. Pertinent theories which may be considered in this situation include those related to reduced horizontal transport (Peterson et al., 1979), differential dispersal for enhanced reproductive success (see Sulkin, 1984, for a review) avoidance of visually feeding predators (Zaret and Suffern, 1976), avoidance of non-visually feeding predators (Ohman et al., 1983), and reactions to regions of favorable food concentrations (Pearre, 1973;

Longhurst and Herman, 1981).

A response to either a chlorophyll maximum layer or a production maximum zone (Longhurst and Herman, 1981) for feeding related considerations is difficult to support in this study. Haas £t^ al.

(1981a) reported the concentration of total chlorophyll ji averaged over all depths and for all collection times was not significantly different for all three sampling periods. Although no overall difference was detected, sampling periods I and II were characterized by the euphotic zone (1-4 m) fluorescent values always exceeding those for the deep water (>10 ra). During the restratified period, the deep water fluorescent values at night exceeded those in the euphotic zone. This was attributed to the diurnal vertical migration of the fluorescence maximum (Haas et al., 1981a). Phytoplankton cell counts indicate that 63

sampling periods I and II were dominated by small flagellates while

Period III was characterized by a reduced dominance of small flagellates and by the increased dominance of the diatom Skeletonema costatum.

Additionally, assimilation values (ug C hr-1 per ug Chi a) for the surface waters during the stratified periods (periods I and III) were 18 and 16, respectively. Values from the lower waters (>10 m) during the stratified periods and the euphotic zone and raidwater levels during the destratified period were all between 8 and 10.

Small scale variations (less than 1 m) in total and size-fractioned chlorophyll a^ data and cell counts are unavailable since values are reported for the euphotic zone (1-4 m), midwater and deep water(>10 m) intervals. Also, zooplankton samples were integrated for each of the upper and lower levels. Based on the data presented by

Haas et al. (1981a) and the highly variable vertical distributions of the zooplankton, casual explanations relating differential migration patterns in response to overall chlorophyll, fluorescence or production variations are not justified. These data do not preclude the existence of behavioral responses to varying phytoplankton dynamics, but the data are limited in their ability to resolve small scale spatial and temporal fluctuations. Although these relationships are well documented for marine environments (Legendre and Demers, 1984), it is uncertain if similar trends can be detected in shallow estuarine systems.

Predation has been suggested to be a major factor controlling zooplankton communities (Dodson, 1974; Zaret, 1980). Visually feeding 64

vertebrates tend to prey on larger and more visible prey (Brooks and

Dodson, 1965; Zaret and Kerfoot, 1975) while the majority of invertebrate predators respond to tactile cues (Feigenbaum and Reeve,

1977), smaller sizes (Yen, 1982) or feed continuously when suitable prey are available as in the case of Mnemiopsis leidyi (Kremer, 1976).

The predator encountered during this study with the greatest potential impact on zooplankton communities was Mnemiopsis leidyi. The

-3 mean density for each sampling period was, <1 m with a maximum -3 density of 5 m , which is substantially less than maximum values _3 recorded in the Chesapeake Bay during the summer (>25 ra ; G. Grant, unpub. data). The M. leidyi data did not indicate any diurnal migrations or patterns related to tidal currents during any sampling period. There was a slight decrease in occurrences and overall densities in Period HI; however, no pattern can be interpreted from the M. leidyi data or the corresponding zooplankton data, particularly

Acartia tonsa.

Other macroinvertebrate predators, all of which are known predators of A. tonsa, include Chrysaora quinquecirrha (Feigenbaum and

Kelly, 1984), Sagitta tenuis (Canino and Grant, 1985) and Neomysis americana (Fulton, 1982). Chrysaora quinquecirrha was randomly distributed temporally and spatially with no apparent diurnal or depth related patterns. These species are not reported to exert a major effect on zooplankton populations. Canino and Grant (1985) and

(Fulton, 1982) report that S. tenuis and _N. americana may consume 5 % of the secondary production per day, with J5, tenuis having its highest 65

predation rates at night. These data indicate that, despite the anomolous distribution of A, tonsa and Pseudodiaptomus coronatus in period III, there are no observed patterns suggesting the predators respond to alterations in prey densities.

The apparent inability of the mesozooplankton communities to

'reestablish' following mixing was unexpected. This study partially supports the conclusions of enclosure experiments by documenting substantial increases of at least one taxa, in this case increased proportions of A. tonsa copepodids, during the well mixed sampling period. However, no overall density increases were observed following the termination of mixing.

Projections for the preceding month indicated a homogeneous water column should have been observed during 24-27 July. The initial sampling period in this study was conducted approximately 12 d after the termination of the July tidal mixing period. The vertical and temporal components of the initial 'established' community have been described. Sampling period III was conducted approximately 6-7 d after the cessation of the August mixing. The absence of the expected community structure (resembling period I) during period III may be attributed to inadequate time for the taxa to 'reestablish'. This is not proposed as a likely explanation since the time following mixing approximates the generation time of A. tonsa (Heinle, 1969a) and this

6-7 d time lag exceeds the 3-4 d duration of the well-mixed water column.

Fulton (1984a) detailed diurnal migration for A. tonsa and P. coronatus in the Beaufort, North Carolina estuarine system. Although 66

data for light penetration were not provided, no unusual patterns

during day sampling were noted, Stearns and Forward (1984a,b) examined

the response of A. tonsa to variations in light intensity and quality.

Vertical movements could be initiated with changes of 9-90 % in the light intensity; however, the vertical displacement would cease when

the new light level remained constant.

Haas (1981a) reported 1 % light levels during the sampling periods to vary between 3.1 and 4.1 m. Red water, composed primarily of Cocchlodinium heterolobatum, was observed during period XXX but the red water distribution was inconsistent and often not directly associated wih the zooplankton sampling area. Considering what may be generally consistent light levels during each sampling period, the rapidly fluctuating species distributions during period III could not be solely attributed to short term variations in light penetration.

The restratification of the water column in the lower York River results from the intrusion of deep higher salinity water (Haas et^ al.,

1981b; Hayward et al., 1982). The restratification process is completed within approximately 4 d. This net upriver transport of deep higher salinity water is widely documented and an essential component of numerous studies related to the reproductive success and estuarine retention mechanisms of estuarine organisms. Tidal advection and intrusion of deep waters have been proposed as primary mechanisms governing the community structure of zooplankton in coastal

(Paffenhofer, 1980; Tremblay and Roff, 1983) and estuarine (Gagnon and

Lacroix, 1982) environments. 67

Grant and Olney (1983) sampled the lower Chesapeake Bay during

August 23-25, 1978 (between periods II and III). Although their data were derived from surface to bottom oblique samples, their data could not support any of the anomalous patterns documented for periods II and

III. There Is no evidence the higher densities of Pseudodiaptomus coronatus were advected into the York River. The higher densities of

Acartia tonsa in the upper day samples during period III can not be directly attributed to advective processes since the water column was restratified and vertical distributions similar to period I should have been observed.

The advection of higher salinity water is an important physical parameter which must be considered when examining zooplankton community structures. The specific effects of this physical process on the estuarine zooplankton communities in this study are unknown. It is proposed that the effects of the intrusion of higher salinity water are secondary to those of low oxygenated waters.

Limnologists have long incorporated low oxygen concentrations into their models describing the behavior, physiology and vertical migration patterns of limnetic organisms, particularly crustaceans.

The predictable nature of the extensive and long-lasting anoxic waters permits species-specific studies (see Meyers, 1980, for a general review).

Recent data on oxygen levels and species distributions in marine environments substantiate the theories that lowered oxygen levels may ellicit marked responses for selected species, with a particularly strong influence on their downward migration capabilities. Smith 68

et al. (1981) reported Oncaea sp., Oithona sp. and Paracalanus parvus

remained above the layer where dissolved oxygen was less than 0.5 ml

1 *, both during the day and night in Peruvian shelf waters. Reduced migrations were noted for Oncaea sp. and j?. parvus. Judkins (1980)

examined the same three species, as well as Centropages sp., Calanus

sp. and Eucalanus sp., in the Peruvian shelf water and reported that only Eucalanus sp. migrated into areas where oxygen was <0.1 ml 1 *.

These two authors concluded that dissolved oxygen was the single most important factor regulating copepod vertical distributions in the

Peruvian waters. These conclusions were further supported by Boyd et al. (1980) who observed similar trends but suggested that the most important variable was food, at least during the duration of the particular study. Their evidence indicated that Eucalanus sp. could withstand 12 h in anoxic waters.

Oxygen concentration less than 18 ug-at 1 * (0.2 ml 1 *) resulted in nearly undetectible levels of zooplankton in the Eastern

Tropical Pacific (Longhurst, 1967). Batfish profiles have shown that

Calanus sp. and Eucalanus sp. do migrate into anoxic regions, but only during the day (Herman, 1984). Studies on respiratory parameters

(Devol, 1981), indicate a complete absence of zooplankton respiration in waters with oxygen concentrations less than 5 ug-at 0 1 *.

The most detailed examination of zooplankon responses was conducted by Childress (1975). The results indicate that although some species are found in low oxygen waters, they had developed a unique ability to exist anaerobically at levels as low as 0.13 ml 1 *. 69

These same species demonstrated lower specific respiration rates when

measured at 1 atm. and derived their energy from settling fecal

material and decaying particles within the water mass. Despite

short-term tolerances, the only organisms that could permanently adapt

are parasites, diurnal migrators that can have periodic access to

well-oxygenated waters and resting stages of species which undergo

ontogenetic migrations. This evolution of unique aerobic abilities

results from adaptation to the long-term stability of low oxygen

regions of oceanic environments.

Limited data are available for tolerances of Chesapeake Bay

species to reduced oxygen levels. Podon polyphemoides apparently

avoids waters with levels less than 0.2 ml 1 ^ (Bosch and Taylor,

1970). Saksena and Joseph (1972) reported TL50 (median tolerance

limits at 24 h) of 2.5, 1.33, and 1.0 rag 1 * for the newly hatched

larvae of Chasmodes bosquianus, Gobiosoma bosci and Gobiesox strumosus, respectively. They also calculated a "critical dissolved oxygen level", a level they claim would kill well in excess of 50% before the next maximum tidal current, of 1.0, 0.5, and 0.6 mg 1 *, respectively.

These previous studies have demonstrated the strong influence of reduced oxygen concentrations on the vertical distribution of zooplankton. The potential long-term effect of low oxygen waters in nearshore and estuarine environments is currently under investigation.

Taft et al. (1980) and Seliger et al. (1985) have suggested that anoxic conditions may exist for several months in the Chesapeake Bay. The 70

values observed during this study (Table 14) are not as low as the

majority of those presented for the individual studies described;

however, the lowest oxygen values during the third sampling period

approach the levels at which an adverse response has been noted.

Hydrographic data were collected within 1-2 m of the bottom, but

values adjacent to the bottom sediments are unknown. Sediment oxygen

demand during this study (Phoel et al., 1981) could have resulted in

decreased oxygen concentrations near the sediments. This could explain

the observed distributions of Pseudodiaptomus coronatus. Jacobs (1961)

and Jacoby and Youngbluth (1983) report P. coronatus as a demersal

zooplankter which may be an occasional component of the plankton.

Migration into the water column may occur at night or during periods of maximum currents (Jacobs, 1961; Fulton, 1984a,b). The data from this

study do not support distributions related to tidal currents but do

support diurnal migration into the water column. The unexpectedly high densities during the day in the restratified period were not during

times of maximum currents but could be explained by upward migrations in response to temporary decreases in oxygen levels below the unknown threshold for this species. Fulton (1984a) documents Acartia tonsa as possessing a strong affinity for bottom waters. The overall low oxygen values in the lower stratum, particularly below 15 m, during period

III are proposed as the stimulus initiating the unexpected vertical distributions of A. tonsa. This could be substantiated by published values of oxygen tolerances for estuarine zooplankton, particularly tonsa. 71

Table 14. Selected oxygen values In ml 1 * for each of the sampling periods. .Lower set of values in parentheses represent (ug-at l“ )

Sampling Period

I IX III

Mean dissolved oxygen 1.8 4.6 0.9 concentration (10-19m) (160) (410) (80)

Lowest dissolved oxygen 1.0 : 18m 3.8 : 18m 0.4 : concentration and depth (89) (339) (36)

Lowest mean dissolved oxygen 1.3 4.2 0.6 concentration (10-19m) at (116) (375) (54) any sampling time during the period

Highest mean dissolved oxygen 2.7 4.8 1.3 concentration (10-19m) at (241) (428) (116) any time during the period 72

There is a need for detailed analysis of smaller scale

predator-prey relationships in estuaries. Acartia tonsa is considered

a facultative omnivore (Glasser, 1984), but the temporal and spatial

responses of A. tonsa and other estuarine mesozooplankton species to microzooplankton aggregations, if any, are currently unknown.

Instantaneous responses to prey aggregations are difficult to substantiate from field data, but this feeding behavior has been frequently cited as the only plausible mechanism which could account for observed species distributions.

This study demonstrates the necessity for ci priori considerations of the physical environment and the fundamental properties of the species or species groups being investigated. The examination of three sampling periods from different physical environments in the same location futher document the ephemeral and overall unpredictable nature of estuarine zooplankton communities. CONCLUSIONS

1. Sampling from above and below the pycnocline in a stratified water

column identified unique and distinguishable mesozooplankton

communities. Although few species were unique to either strata,

their separation was largely determined by density differences.

The lower stratum exhibited densities two orders of magnitude

higher than the upper stratum during day sampling. Readily

apparent diurnal migrations were observed for Acartia tonsa,

Pseudodiaptomus coronatus, Neomysis americana and Sagitta tenuis.

Night sampling documented greater densities in the upper water

stratum.

2. In a vertically homogeneous water column, zooplankton communities

were not easily identified. There were no apparent separations

based on time of day or water strata. In contrast to the pre-mixed

sampling period, densities of the dominant species were uniform

with respect to light and depth. Vertical migrations of A. tonsa

and P. coronatus were less dramatic due to the lack of a

pycnocline.

73 74

3. Communities identified during the initial sampling period were

anticipated to be evident during the restratified sampling period.

These species associations were not observed and the overall

associations resembled those from the well-mixed sampling period.

The upper day assemblage was the most distinct. Dominant species

exhibited higher densities in the upper stratum during all light

conditions. Vertical distributions revealed higher densities in

the upper stratum during all light conditions. Low dissolved

oxygen in the lower stratum was determined to be the primary factor

governing species associations and distributions during the

restratified period. Advectlon of higher salinity waters during

the restratification process may have partially affected the

observed communities. APPENDIX A -3 Mean Density (number m ) for All Taxa Collected in the Subsurface Samples

75 76

All Sampling Sampling Period Periods I II III

Acartia tonsa (adult) 7596 14642 4659 3369 Acartia tonsa (late copepodld) 4083 5144 3828 3266 Acartia tonsa (early copepodld) 507 860 227 424 Pseudodiaptomus coronatus (adults) 393 346 87 734 Pseudodiaptomus coronatus (copepodids) 343 301 238 485 unid. polychaetes 138 25 85 301 Neomysis americana 76 83 103 43 unid. mollusca 38 20 66 31 Sagltta tenuis 26 8 9 61 barnacle cyprid 18 29 15 10 barnacle nauplii 8 1 10 13 Paracalanus spp. 18 8 20 25 fish eggs 6 11 7 1 Leucon americanus 6 13 A 5 unid. cyclopoid copepod 1) 7 1 11 9 unid. phronid larvae 4 A 7 3 Upogebia affinis 3 2 2 4 Ogyrides limicola 3 6 1 1 Uca spp. 2 A 1 4 Harpacticidae 2 5 - - Labidocera aestiva 1 A 1 1 Pinnotheres ostreum 1 2 A 1 Anchoa spp. 1 2 2 A Palaemonetes spp. A A A A Sesarma cinereum * AAA Gobiidae A A A 1 Nemopsis bachei a A A 2 Panopeus herbstii AA 1 1 Hexapanopeus angustifrons AAAA Labidocera aestiva (copepodld) A AAA Neopanope texana sayi AA 2 A Pinnotheres maculatus AA A A Alteutha depressa A A A 1 Pagurus longicarpus A A A A Pinnixa chaetopterana AA AA Oithona sp. A 2 A A unid. cumacean AAA 1 Ampelisca vadorum A AAA unid. chaetognath AA A 1 Mnemiopsis leidyi A A A A Branchiostoma sp. A - AA Oxyurostylis smithi AAAA 77

All Sampling Sampling Period Periods I II III

Anchoa mitchilll AAA a unid. fish larvae AAA A Ampelisca abdita AA A a Corophium sp. AAA * Edotea triloba AAA A Libinia spp. AAA A Gobiosoma spp. AAA A Mysidopsis bigelowi AAA A unid. hydrozoa AAA A Pinnixa sayana AAA A Beroe ovata A - A A Calllanassa sp. A A - A A Euceramus praelongus A -- A Oncaea mediterranea A - - A unid. brachyuran zoeae AAA A Chrysaora quinquecirrha AAA A atherinid larvae A - A A Idotea baltica A - _ A Gammarus mucronatus AAA A Gammaridae AA A A Amplthoe sp. A - A A Ampithoe valida A A A A Aurelia aurita AAA A Membras martinica AA - A Argulus alosae AAA A Bougainvillea sp. A AA A Aegathoa oculata AAA A Ampelisca sp. AAA A Temora turbinata A A - Callinectes sapidus A - A A Centropages typicus A A - A Eucalanus pileatus AA - A Hyperiidae A A - Caprellidae AAA unid. immature isopod A A - Evadne tergestina AA - A Pinnixa cylindrlca A AA A unid. brachyuran megalop AAA A Sagitta enflata A A - A Parametopella cypris AAA A Squilla empusa AA — A Microgobius thalassinus AAA A 78

All Sampling Sampling Period Periods I II III

Microprotopus raneyi A A A A Crangon septemspinosa AA A A Limulus polyphemus A A - Cymadusa compta A A - Centropages hamatus AA - Sphaeroma quadridentatum A A - A Ampithoe longlmana A - A A Trinectes maculatus A A A A Caligus chelifer A - A Gobisoma bosci A A A A Sagitta hispida A - - A Hypsoblennius hertzi A - A A Gammarus spp. AA A A Syngnathus fuscus A A A A Idunella sp. AA - Melita spp. AA A A Dipurena strangulata A A A A Lovenella gracilis AA A A unid. fish larvae A A A A Jassa falcata AA A Cynoscion regalis A A A A Argulus sp. AA A A Peprilus paru A A A A Obelia sp. AAA Gobiesox strumosus A A A Lucifer faxoni AA - A Mentlcirrhus americanus A A - A Eucalanus crassus A A - A Trichophoxus epistomus A A - A Penilia avirostris A A - A Rhithropanopeus harrisii A A - A Cunina octonaria AA - A

-3 less than 1 m absent

1) This represents the total of two juvenile forms of the Hemicyclops sp. / Saphlrella sp. assemblage (Lee, 1978). APPENDIX B —3 Densities (number m ) for All Species in the Neuston Samples

79 80

Sampling Period I II III

Day Night Day Nighl Day Night

Acartia tonsa 389 3887 83 1778 63 382 Pseudodiaptomus coronatus 17 283 4 67 A 572 Neomysis amerlcana 10 345 2 5 39 183 Uca spp. 17 51 A 6 - 43 fish eggs 13 35 5 66 1 12 Panopeus herbstii 2 26 - A - 15 Pinnotheres ostreum 6 23 - A - 71 Labidocera aestiva - 23 A 92 55 2 Neopanope texana sayi - 20 A 4 8 Upogebia affinis * 13 - 3 - 56 Palaemonetes sp. a 8 - 6 A 3 Gammarus mucronatus - 4 A 2 1 3 Ampithoe spp. - 4 4 1 7 11 Ogyrides limicola a 4 - 1 — 2 Sesarma cinereum A 4 - A Sagitta tenuis - 1 - A — 2 Membras martinica - 1 - 1 1 12 Harpacticidae - 2 A - AA Pagurus longicarpus *- 1 — A - 1 barnacle nauplii £ A 1 7 A 4 Chrysaora quinquecirrha A - - Idotea baltica J, Gammaridae AA - Libinia spp. — A — 6 A unid. mollusca — AA 3 A — Nemopsis bachei _ A — A Hexapanopeus angustlfrons — A — A barnacle cyprid — AA A A - Anchoa spp. - A - A .1. _ Mysldopsis bigelowi X 2 Pinnixa chaetopterana A - A - 4 Pinnixa sayana A - A Brachyuran megalop A . 3 A 8 A A Crangon septemspinosa - A - - Limulus polyphemus A A A Cymadusa compta - A J, Squilla empusa 7C atherinid larvae . AA AA unid. polychaetes — A 2 Pinnotheres maculatus A 4 Aegathoa oculata — A * * unid. phronid larvae A A unid. hydrozoa --- 3 81

Sampling Period II III

Day Night Day Night Day Night

Evadne tergestina -- - 5 - - unid. cyclopoid copepod * - - A - - Dipurena strangulata - - - 4 A A Mnemiopsis leidyi -- A 2 - - Beroe ovata - - A A - - Callinectes sapidus -- A -- - Melita sp. a - AA - - unid. brachyuran zoea * 1 A A A 11 Ampithoe valida * 3 3 A 2 2 Oxyurostylis smithi - - -- - A Goblldae * A AAA - Gobiosoma sp. - A - A - A Anchoa mitchilli - A - A - A unid. fish larvae * AAA - - Argulus alosae * A - AA A Corophium sp. A - A A A - Ampelisca sp. * - - — - - Temora turbinata a - - -- A Paracalanus sp. * -- -- - Alteutha depressa * - - - AA Leucon americanus * AAA A A Menidia menidia * - - - - - unid. Caprellldae - A A - A A Pinnixa cylindrica * A - - - - unid. cumacean a A - - -- Ampelisca vadorura a - - A - - unid. immature isopod * A A - - - Microgobius thalassinus ------Microprotopus raneyi - A - - A A Sphaeroma quadridentatum * A - - A A Ampithoe longlmana * A 2 A 3 A Trinectes maculatus - A -- - - Caligus chelifer - A - - - - Gobiosoma bosci - A - -- A Sagitta hispida - A - A - A Hemiramphus brasiliensls - A — — - - Hypsoblennius hertzi AA AA - - Syngnathus fuscus - A - A A A Gobisoma ginsburgi - A - —-- Strongylura marina - A -- -- Lovenella gracilis - - A A A - Euceramus praelongus -- A - - - Argulus sp. - - A AA A 82

Sampling Period _I II III

Day Night Day Night Day Night

Calllanassa sp. A - - * Oncaea mediterranea - - - * Mentlclrrhus amerlcanus - - - * Pontella meadil - A A

-3 * less than 1,0 m - absent LITERATURE CITED

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James Edward Price

Born in Passaic, New Jersey, 26 September 1949. Earned a Bachelor

of Arts Degree, Biology, Hope College, 1972. Received a Masters of

Science Degree, Biology, from Western Michigan University, 1977.

Currently employed by the National Marine Fisheries Service in

Washington, D.C.

92