The Physical Modulation of Seasonal Hypoxia in Chesapeake Bay

1. The Physical Modulation of Seasonal Hypoxia in Chesapeake Bay Malcolm Scully Center for Coastal Physical Oceanography Old Dominion University Center for Coastal Physical Oceanography CCPO Seminar: Monday, March 28, 2011 Outline: Background and Motivation Role of Physical Forcing Simplified Modeling Approach Sensitivity Studies Physical Mechanisms and Oxygen Budget Historical Observations of Hypoxia Conclusions

2. Map of Mean Dissolved Oxygen -- Summer 2005 • Low DO has significant impact on a wide array of biological and ecological processes. • Large regions of Chesapeake Bay are impacted by hypoxia/anoxia. • Over $ 3.5 billion was spent on nutrient controls in Chesapeake Bay between 1985-1996 (Butt & Brown, 2000) • Assessing success/failure of reductions in nutrient loading requires understanding of the physical processes that contribute to the inter-annual variability. From Chesapeake Bay Program newsletter: http://ian.umces.edu/pdfs/do_letter.pdf

3. Conceptual Model for Hypoxia in Chesapeake Bay From Chesapeake Bay Program newsletter: http://ian.umces.edu/pdfs/do_letter.pdf Physical forcing is thought to play an important role in extent and severity of hypoxia: 1) River Discharge; 2) Temperature; 3) Wind forcing

4. Seasonal and Inter-Annual Variability in Hypoxic Volume (from CBP data 1984-2009) Maximum observed Minimum observed Data compiled from Murphy et al. (submitted)

5. Regional Ocean Modeling System (ROMS) • Model forcing • Realistic tidal and sub-tidal elevation at ocean boundary • Realistic surface fluxes from NCEP (heating and winds) • Observed river discharge for all tributaries. • Temperature and salinity at ocean boundary from World Ocean Atlas. • Very simple oxygen model ChesROMS Model Grid

6. Depth-dependent Respiration Formulation • Oxygen Model • Oxygen is introduced as an additional model tracer. • Oxygen consumption (respiration) is constant in time, with depth-dependent vertical distribution. • No oxygen consumption outside of estuarine portion of model • No oxygen production. • Open boundaries = saturation • Surface flux using wind speed dependent piston velocity following Marino and Howarth, 1993. • No negative oxygen concentration and no super-saturation. Surface Oxygen Flux using Piston Velocity: Model assumes biology is constant so that the role of physical processes can be isolated! From Marino and Howarth, Estuaries, 1993

7. Comparison with Bottom DO at Chesapeake Bay Program Stations

8. Comparison with Chesapeake Bay Program Data Bottom Dissolved Oxygen Concentration (mg/L) July 19-21, 2004 August 9-11, 2004

9. Simple model captures seasonal cycle of hypoxia as well as a more complicated bio-geo-chemical model.

10. In addition to seasonal cycle, model captures some of the inter-annual variability 707 km3days 485 km3days 476 km3days Model predicts roughly 50% more hypoxia in 2004 than in 2005, solely due to physical variability.

11. Variability of Physical Forcing What is relative importance of different physical forcings in controlling seasonal cycle of hypoxia?

12. Sensitivity to River Discharge

13. Sensitivity to Temperature

14. Sensitivity to Wind

15. Differences between 2004 and 2005 are almost entirely due to wind forcing

16. Sensitivity to Summer Wind Magnitude Average Monthly Wind Speed from Model at PNAS Wind speed during May-August was increased/decreased by 15%

17. Changes in average summer wind speed of %15 result in roughly 2-fold change in hypoxic volume.

18. Sensitivity to Summer Wind Direction Modeled summer wind direction Positive 90° Base Summer Winds Negative 90° 180°

19. Sensitivity to Summer Wind Direction Along axis winds result in less total hypoxic volume

20. The Physical Modulation of Seasonal Hypoxia in Chesapeake Bay Malcolm Scully Center for Coastal Physical Oceanography Old Dominion University Center for Coastal Physical Oceanography CCPO Seminar: Monday, March 28, 2011 Outline: Background and Motivation Role of Physical Forcing Simplified Modeling Approach Sensitivity Studies Physical Mechanisms and Oxygen Budget Historical Observations of Hypoxia Conclusions

21. Mechanisms for Oxygen “Ventilation” Lateral Advection Lateral Advection Direct Vertical Mixing Lateral Advection Along-Channel Advection detrainment

22. Oxygen Budget Calculations July 2004 average bottom Oxygen Advection: Rate of change Turbulent mixing horizontal lateral Respiration Fixed Volume for Budget calculations Integrate all terms over entire volume

23. Monthly Averaged Sub-Pycnocline Oxygen Budget

24. x Response of Chesapeake Bay to Wind Forcing is Strongly Impacted by Rotation wind stress wind stress x Ekman Ekman North Wind South Wind wind stress wind stress Ekman Ekman West Wind East Wind

27. For most of the deep areas of the Bay, the gradient Richardson almost never drops below 0.25 in pycnocline (year round!). Richardson Number for CPB Station 4.3

28. Sensitivity to Summer Wind Direction Along axis winds result in less total hypoxic volume

29. The Physical Modulation of Seasonal Hypoxia in Chesapeake Bay Malcolm Scully Center for Coastal Physical Oceanography Old Dominion University Center for Coastal Physical Oceanography CCPO Seminar: Monday, March 28, 2011 Outline: Background and Motivation Role of Physical Forcing Simplified Modeling Approach Sensitivity Studies Physical Mechanisms and Oxygen Budget Historical Observations of Hypoxia Conclusions

30. Historical Observations of Hypoxia in Chesapeake Bay Data from Jim Hagy Observed hypoxic volume (< 1 mg/L) Residual Regression Model Multiple regression based on estimated nitrogen loading explains relatively small amount of observed variance. Residuals to fit suggest hypoxic volume is increasing, despite recent reductions in estimated nitrogen loading. One interpretation is that Bay is less able to assimilate nutrient inputs because of ecosystem degradation.

31. Correlation of Historic Data (Hagy et al.) with Wind Direction Wind data from Patuxent Naval Air Station (1950--2007) * values in red denote significance at 95% confidence interval

32. Multiple Regression based on Nitrogen Loading and Duration of Westerly Winds Observed hypoxic volume (< 1 mg/L) Residual Regression Model When you account for changes in wind direction, residual slope is no longer significant.

33. Have the Winds over Chesapeake Bay Changed in recent Decades?

34. Conclusions A relatively simple model with no biological variability can reasonably account for the seasonal cycle of hypoxia in Chesapeake Bay. Wind speed and direction are the two most important physical variables controlling hypoxia in the Bay. Model results are largely insensitive to variations in river discharge. The model suggests that the dominant balance controlling hypoxia is between respiration and advective processes not vertical mixing. During winter months ventilation is dominated by longitudinal advection. During the summer months ventilation is greater by lateral advection. Because of the width of Chesapeake Bay, the rotational response to wind forcing is greater for along-channel winds than for across-channel winds. Winds from the north enhance the residual circulation, increasing the longitudinal flux of oxygen into the hypoxic zone. However, winds from then north are not common during the summer months and subtle shifts between south and west winds may play a significant role in the observed inter-annual variability.