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

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“Dead-Zones” and Coastal Eutrophication: Case- Study of Chesapeake Bay “Dead-Zones” and Coastal Eutrophication: 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 Hypoxia 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” “Dead Zone” Global Scale of Eutrophication Harmful Algal Blooms Seagrass 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) Watershed Changes: Land-Use & Population 16 6 a) Human Population 12 • Exponential growth in water- 8 shed population Population x10 4 •Land-use shift 100 b) Land-Use from forest to 80 farm (thru 1850) 60 Forest to developed (1850 – 2000) 40 Agriculture % of Total Land 20 Developed 0 1650 1700 1750 1800 1850 1900 1950 2000 Year Susquehanna River Flow is Large (Flood Flow At Conowingo Dam) Watershed Changes & Variations: Flow & Fertilizer c) Susquehanna Flow Variable wet -1 1600 s Tropical Storm Agnes • Large variations in 3 Wet 1200 river flow (~4X); wet and 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 30 x10 since 1950, tripling -1 20 since 1960 kg N y 10 0 1950 1960 1970 1980 1990 2000 Year Hypoxia Patterns in Chesapeake Bay • from 1950-2003 flow years( dot flow years( hypoxia isgreater inhigh hypoxic (O in volumeofseverely • Within long-term trend, Clear increasingtrend ) comparedto low Summer Hypoxic “Dead-Zone”: 1950- 2003 2 dry =reddot wet =green < 1mg/L) ) Summer Low DO Volume (109 m3) 10 12 14 0 2 4 6 8 1950 DO < 1 mg L Wet Year Dry Year 1960 Storm Agnes Tropical Tropical -1 1970 Years 1980 1989 ( Hagy et al 2004) 1993 1990 1992 1998 2001 2000 2003 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 Productivity, 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 oxygen 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 location and duration 100 of hypoxia. 100 60 40 • How does it vary inter-annually? 20 Distance from Atlantic (km) 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 Volume (10 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 estuaries ) 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 biomass 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 Phytoplankton Production (gC m y ) fish 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 demersal fish. 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 0.2 Phyto 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 700 700 Days to Filter Maryland's •Oyster restoration to meet Portion of Chesapeake Bay management mandate 600 (10x), and to estimated pre- 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 (~
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