“Dead-Zones” and Coastal : Case- Study of Chesapeake Bay

W. M. Kemp University of Maryland CES Horn Point Laboratory Cambridge, MD

Presentation to COSEE Trends Orientation at UMCES HPL 4 August 2009 Outline

• Background Concepts of Eutrophication and “Dead-Zones”

• Introduce Features of Chesapeake Bay

• Describe Patterns in Bay

• Explain Factors Regulating Hypoxia : Physical—Flow, Stratification, Mixing Biological—Nutrient Loading

• Ecological Responses to Hypoxia

• Concluding Comments

• Epilogue: “Ecosystem Feedbacks” and Restoration of Eutrophic Coastal Systems Eutrophication Effects on Coastal Ecosystems

Oligotrophic Eutrophic

“Stratification” “” Global Scale of Eutrophication

Harmful Algal Blooms Loss Hypoxia/Anoxia Background Information • Chesapeake Bay Key Bay Features

•Large ratio of watershed to estuarine area (= 14:1)

• Deep, narrow channel is seasonally stratified

• Broad shallows flank channel (mean z = 6.5m)

• Most of Bay volume is in the mainstem

• Most of its surface area in tributaries & embayments

• Relatively long water residence time (~ 6 mo) (1850 – 2000 to developed farm (thru1850) from forestto • shed population growth inwater- • Land-use shift Exponential Watershed Changes: Land-Use&Population )

% of Total Land Population x106 100 20 40 60 80 12 16 0 4 8 b) Land-Use a) HumanPopulation 6010 7010 8010 902000 1950 1900 1850 1800 1750 1700 1650 Forest Year Agriculture Developed Susquehanna River Flow is Large (Flood Flow At Conowingo Dam) Watershed Changes & Variations: Flow & Fertilizer

c) Susquehanna Flow Variable wet -1

s 1600 Tropical Storm Agnes • Large variations in 3 Wet river flow (~4X); wet and 1200 Drought dry decades but no long-term trends 800 Mean Flow m 400

40 d) Nitrogen F ertilizer Use •Fertilizer use in basin 6 has been increasing x10 30 since 1950, tripling -1 20 since 1960

kg N y 10

0 1950 1960 1970 1980 1990 2000

Year Hypoxia Patterns in Chesapeake Bay Summer Hypoxic “Dead-Zone”: 1950 - 2003

) 3 14 DO < 1 mg L-1

m

9 1998 2003 • Clear increasing trend 0 12 Wet Year

1 1993 in volume of severely ( Dry Year

e 10 1989 hypoxic (O2 < 1 mg/L) m

u

l

from 1950-2003 o 8 V Tropical

O Storm Agnes

D 6

• Within long-term trend, w

o

L 4

hypoxia is greater in high r flow years (wet = green e 1992 m 2 dot) compared to low m 2001

u flow years (dry = red dot) S 0

0

0 0 0

0

0

8

5 6 7

9

0

9

9 9 9

9

0

1

1 1 1

1

2 Years

( Hagy et al 2004) Location of Bay Hypoxic Zone

mg/l 7

6

5

4

0 3

20 2

40 1 Depth (m) 7/20/1998 0 60 0 50 100 150 200 250 km from Susquehanna River

(Hagy 2002) Seasonal Cycle of Algal ,

Temperature and Bottom Dissolved O2

• Hypoxia confined to summer (June-September) • Hypoxia coincides with peak temperature and productivity Spatial Distribution of Bay Hypoxia: 1959 vs. 1995 (low flow)

• Longitudinal sections of summer dissolved for two years with similar (low flow) freshwater inputs

• No anoxic conditions in 1959 but large anoxic (dead) zone in summer of 1995

• Upper oxic layer was much deeper in 1959 (10-12 m) compared to 1995 (5-10 m) Factors Regulating Hypoxia • Physical Factors • Ecological Factors Stratification Control of Hypoxia:

Position & Intensity of Low O2 Water

Susq. R. MD/VA Atlantic 0 5 10 Pycnocline 15 20 Anoxic 25 Depth (m) 30 35 40 DO declines along landward bottom-layer flow … Vertical Exchange between Upper & Lower Layers

250

cm d-1 • Vertical exchange is minimal in mid-Bay 200 from May-August 180

140 150 • Corresponds to 100 location and duration of hypoxia. 60 100 40 • How does it vary inter-annually? 20

Distance from Atlantic (km) Distance from 50

0 123456789101112 Month (Hagy 2002) River Flow, Stratification, & Mixing (1986-’98)

Stratification • Stratification strongly 12 correlated with river flow. -2 s

2 10

rad • Vertical exchange relation

-3 8 to flow is weaker 10 (buoyancy vs. mixing). 28 Vertical Exchange 24

-1 • High flow also delivers 20 1996 more nutrients.

cm d 16 12 1995 8 1000 1500 2000 2500 3000 Winter-Spring River Flow (m3 s-1)

(Hagy 2002) Increasing Nitrogen in Susquehanna River: Seasonal and Long-Term Trends

• Long-term Nov increases in nitrate levels & changes in seasonality seen Sep over five decades

• Highest nitrate Jul levels (yellow, red) occur in cold months May

• Nitrate trends are Mar closely related to total Nitrogen Jan • N-loads to Bay doubled from 1945 1945 1955 1965 1975 1985 1995 to 1970 (Hagy et al 2004) Hypoxia Response to N Loading (1950-2001): Unexpected Shifts in Ecosystem Processes

1950-1969 1980-1989 • Hypoxia increases with 1970-1979 1990-2001 N loading. 12 ) ) 3 3 m m 10 9 9 ) ) • Equivalent N load since 1 1 - - 8 1980 generates more hypoxia than in past. 6

4 (DO<1 mg l (DO<1 mg l • Is system less able to 2 assimilate N-load? Hypoxic Volume (10 Hypoxic Hypoxic Volume (10 Hypoxic 0 10 15 20 25 30 35 • No clear explanation for ‘regime shift’. - 6 Winter-Spring NO3 Loading (10 kg)

(Hagy et al. 2004) Ecological Consequences of Hypoxia & Dead-Zones Summary of Faunal Responses to Hypoxia in Mississippi River Plume

(Rabalais et al. 2001) Hypoxia Degrades Habitat for Benthic Fauna in Chesapeake Bay

• Comparing ) 70 -2 worldwide (#1-14), benthic animal 60 1 2 Upper abundance tends to be Bay 50 proportional to algal 3 food produced in water 40

4 • Upper and lower Bay 30 generally follow this 6 5 20 10 9 Lower trend, but hypoxic mid 11 8 Bay Severe 7 Bay has lower animal 10 12 hypoxia than expected 13 Mid 14 Bay

Macrobenthic Biomass (g AFDW0 m 0 100 200 300 400 500 600 700 • Loss of bottom habitat -2 -1 causes loss of important Annual Production (gC m y ) and invertebrate animals (Hagy 2002, Herman et al. 1999) Degraded Bottom Habitats Cause Loss of Benthic Fauna in Hypoxic Regions of Bay

• With increasing nutrient Sediment Profile Photos enrichment and organic production, depth of sediment oxidized zone declines Conceptual Model • Fauna shift from diverse large deep-burrowing forms to few small surface-dwellers

•Benthic macrofaunal abundance declines markedly

• Model derived in part from work of by Don Rhoads in LIS Benthic Community Change

Disturbance Gradient (Nilsson and Rosenberg 2000) Degraded Bottom Habitats Alter Fish Community Structure and Harvest

• Steady decrease in the proportion of fisheries harvest from bottom- dwelling animals • General degradation of bottom habitats in shallow (loss of SAV) and deep (hypoxia) waters • Similar trends are being reported in other systems worldwide • Possible loss of trophic efficiency (fish harvest per unit photosynthesis)

(Houde in Kemp et al 2005) Concluding Comments 1

• Coastal eutrophication is a global scale phenomenon

• Many features of Chesapeake Bay make it susceptible to seasonal development and expansion of hypoxic “dead zones”

• Seasonal deep-water hypoxia is generally regulated by stratification and enhanced nutrient loading

• A dramatic upward shift in the size and intensity of Bay hypoxia occurred in the early 1980s

• Similar hypoxic ‘regime shifts’ have been reported elsewhere

• Hypoxic dead zones result in reduced abundance, diversity and production of benthic invertebrates and . Epilogue: ‘Ecosystem Feedbacks’ and Restoration of Eutrophic Coastal Ecosystems Bottom-Water Hypoxia Enhances Recycling of Benthic Nutrients

Benthic DIP-Recycling • Benthic nutrient (PO4 & NH4) recycling sustains algal production and hypoxia thru summer (Boynton in Kemp et al. 2005) • Hypoxia causes higher rates nutrient recycling rates DIN-Recycling “Efficiency” •Thus, hypoxia promotes more algal growth per nutrient input to the Bay • For N & P recycling, same effect of low O2 but different (Cornwell in Kemp et al. 2005) mechanisms Seagrass (SAV) Decline: Loss of Particle & Nutrient Trapping

• Dramatic decline in SAV between 1962 & 1982 throughout the Bay • Many factors contributed to decline, but increased nutrients was primary factor

0.3 ResponseNutrient to NutrientResponse Loads

0.3 Plant ) Epi 1999

-2 Phyto 0.2

0.2

0.1 Plants (kg m (kg Plants

0.1

0.0 Control Low High Excess Nutrients Inhibit SAV Survival But Healthy Beds are Nutrient Sinks

• Historical Bay SAV 100 Atmosphere beds were capable of ‘removing’ ~45% of current N Loading 80 • Primary pathways of N removal would be 60 Diffuse trapping particulate N Sources & direct assimilation • Calculation only 40 considers mainstem Particulate Nitrogen upper (MD) Bay Trapping • N removal rates 20 Denitri- Percent of Total N-Sources Point fication would be larger if Sources Plant whole Bay were Uptake considered 0 N-Sources N-Sinks (SAV Beds) ( Kemp et al 2005) Oyster Decline: Loss of Particle Filtration

•Decline in oyster abundance has 700 Days to Filter Maryland's 700 Portion of Chesapeake Bay caused loss of ) 40 600 6 nutrient filtration 500 400 capacity Maryland 300 30 228 •Oyster declines due Time (days) 200 100 primarily to over- 3.6 0 fishing and disease 20 1880 1988 2003

•Historic oyster (Newell 1988) populations were able to filter Bay water 10 volume in days Virginia WetMeatWeight(kgx10

•Current oyster 0 populations filter Bay 1880 1900 1920 1940 1960 1980 2000 water in months-years Time (years) •Oyster restoration would help mitigate (Kemp et al 2005) eutrophication effects Oyster Filtration Effects on Bottom Hypoxia

Days to Filter Maryland's700 •Oyster restoration to meet Portion of Chesapeake Bay management mandate 700 (10x), and to estimated pre- 600 500 colonial conditions (100x) 400 300 228 Time (days) • Dramatic declines in 200 phytoplankton with 100 3.6 restoration throughout Bay 0 1880 1988 2003 •Small improvements in Dissolved O bottom O with oyster Phytoplankton 2 2 Chlorophyll-a restoration (~ effects of reduced nutrient loading) • Restoration improves water clarity (& SAV cover)

•10x restoration ~ 50% mid- Large Small Whole mid- Large Small Whole effect of 100x restoration Bay Trib Trib Bay Bay Trib Trib Bay

(Cerco and Noel 2007) Tidal Marshes Expanding with Soil Erosion, But Contracting with Sea-Level Rise

• Tidal marshes are important features of Bay watershed • Marsh area expanded since colonial times due to increased soil erosion from watershed • Marshes have served as buffers filtering nutrient inputs from watershed • Marsh area is declining due to sea level rise and reduced soil erosion

Pre-Colonial (hypothetical) 1903 1949 Tidal Marshes Serve as Nutrient Filters at Watershed- Margins

• Tidal marshes have enormous capacity to filter sediments & nutrients Brackish (High diversity) • Nitrogen removal capacity measured in experimental Saline marsh ecosystems (Low diversity) • 80% of N-inputs from land and estuary removed in three year-old marshes • Similar effects on N-loading for diverse (brackish) and Inputs) (% Nitrogen Removal mono-specific (salt) marshes • Marsh restoration would help re-establish lost Time (years) filtration capacity Ecosystem Feedbacks affect Bay Response to Nutrient Management

•Positive & negative feedbacks control paths of ecosystem change with Bay degradation • Among other mechanisms, N & P inputs affect hypoxia & light • Hypoxia leads to more nutrients, more algae, & more hypoxia • leads to less SAV causing more turbidity, less SAV • Oysters & marshes tend to reinforce these feedbacks

•Processes reverse w/ restoration, thus reinforcing trends

(Kemp et al. 2005) Concluding Comments 2

• “Dead-zones” are an expanding problem that is linked to coastal eutrophication at global scales

• Although some systems are more susceptible to hypoxia due to inherent physics, anthropogenic nutrient loading is a key driving factor

• Restoration of Bays and estuaries worldwide requires reduction in nutrient loading to coastal systems

• Diverse ecological feedback processes complicate Bay restoration

• Hypoxia stimulates more algal growth thru enhanced nutrient recycling

• Loss of SAV, tidal marshes and oyster beds causes reduced natural filtration of nutrients from coastal waters

• Ecological positive feedbacks reinforce both coastal ecosystem degradation and restoration

• Thresholds and delayed responses may be expected with loading,

Fishery Responses to Eutrophication

• Stages of Fish respond to nutrient enrichment

• First: Fishery production increases with nutrients ? (1) (2) (3)

• Second: Fishery does not respond to nutrients

• Third: Fishery production declines with nutrients

(Caddy 1993) Volume of Summer Hypoxic Water is Related to River flow and Nitrate Loading, with Regime Shift in Early 1980s

Hypoxia vs. River Flow • Volumes of summer hypoxic

(O2 < 1 mg/L) and anoxic (O2 < 0.5 mg/L) clearly related to winter-spring river flow

• Abrupt increase in slope of time trend from 1950-1980 (blue line) to 1980-2003 (magenta line). Currently, there is more hypoxia per unit NO3 Loading Hypoxia vs. NO3 Loading

• What factors have contributed to this abrupt regime shift leading to more hypoxia per loading? Positive feedback mechanisms at work?

(Hagy et al 2004, Kemp et al 2005) Dissolved O2 in Low-Flow Years

Susquehanna Atlantic River 0 10 20 mg/l 2.0 30 Major differences 40 1959 1.0 between conditions in 1950s and present are 50 0.7 -- increased intensity 0.5 10 -- seaward expansion. 20 0.2 30 0.0 40 50 1995

(Hagy 2002) Dissolved O2 in High-Flow Years

Susquehanna Atlantic River Ocean 0 10 mg/l 20 2.0 30 Major difference 40 1.0 between earlier years 1952 and present years is 50 0.7 -- seaward expansion 0 0.5 10 -- deeper hypoxic area 0.2 20 30 0.0 40 1998 50

(Hagy 2002) Oxygen Budget for Mid Bay

6.23 6.88 6.46 2.94 0.55 0.94 1.88

mg l-1 Average Flow (1990)

21% Vertical 34% 44% Diffusion 66% 56% 79% Net Horizontal Advection Chesapeake Bay System:

Watershed area = 116,000 km2

Water surface area = 11,500 km2 Susquehanna R. (55 % of flow) Land-Use in Watershed:

Agriculture 28 % Barren Developed Forest 58 % Water Concluding Thoughts • Restoration • Implications Salinity and O2 Seasonal and Vertical Distributions

(Kemp et al. 1992) Concluding Comments

• Coastal eutrophication is a global scale problem, and Chesapeake Bay is a system that is inherently susceptible to effects of nutrient enrichment • Eutrophication effects first evident 200 years ago, with intense hypoxia and dramatic SAV loss first occurring in the 1950s and 1960s • A dramatic upward shift in the hypoxic zone size occurred around 1980, with more hypoxia generated per nutrient loading now compared to past • Increased turbidity with eutrophication has caused large reductions in benthic primary production (algal & SAV) • Changes in abundance and community composition of demersal fish and benthic invertebrates have occurred in response to bottom habitat losses • Human-induced changes of oyster and marshes habitats further stimulate Bay ecosystem response to nutrient enrichment and nutrient abatement • Ecological positive feedbacks reinforce both Bay degradation response to nutrient enrichment, and Bay restoration response to nutrient reductions • Thresholds and delayed responses may be expected with reduced nutrient loading, but habitat restoration may tend stimulate recovery