Indicator Details

Forecast

Given average Jan-May 2010 total nitrogen load of 164,624 kg per day from the Susquehanna River, July 2010 hypoxia volume forecast is 5.7 km³, with 95% confidence interval that the hypoxic volume will be between 3.4 and 7.8 km³. This is below average for recent years and the 6^th lowest in the post-1985 period.

July hypoxia forecast provided by Don Scavia and Mary Anne Evans, University of Michigan.

The average volume of hypoxic water (Dissolved Oxygen ≤ 2 mg/l) in the Chesapeake Bay is predicted to be 3.4 km³ for July-August, 2010, with 95% confidence interval that the hypoxic volume will be between 2.0 and 4.9 km³.

In order to predict hypoxic volume in the current year, the summer hypoxic volumes from July to August were used as a dependent variable, and Susquehanna River discharge (data from the U.S. Geological Survey), mid-Bay chlorophyll a concentration (data from the Chesapeake Bay Program), and the cross-bay (east-west) wind speed (data from the Patuxent River Naval Air Station) were used as independent variables.

July-August hypoxia forecast courtesy of Younjoo Lee, Chesapeake Bay Laboratory, UMCES

Data

Methodology

July 2010 Hypoxia Forecast

The hypoxic forecast model predicts oxygen concentration downstream from point sources of organic matter loads using two mass balance equations for oxygen-consuming organic matter, in oxygen equivalents (i.e., BOD), and dissolved oxygen deficit. This approach to modeling coastal and estuarine hypoxia has also been used successfully for Gulf of Mexico hypoxia (Scavia et al. 2003, 2004). The original model was calibrated and tested against 1950-2003 nitrogen load and hypoxic volume estimates assembled by Hagy (2002). The Chesapeake Bay Program provided load and hypoxic volume updates for 1986-2008, and even though the new estimates varied little from the original ones; the model was recalibrated for this application to the new 1986-2008 estimates. The summer hypoxic volume forecast was generated using the following relationship. For more information, visit http://www.snre.umich.edu/scavia/hypoxia-forecasts/

July and August 2010 Hypoxia Forecast

Hypoxic volume is predicted using a multiple linear regression model. A similar approach to predicting coastal hypoxia has been applied to Long Island Sound, New York (Lee and Lwiza, 2007). The summer hypoxia (HYPOXIA) forecast is based on three variables measured during the winter/spring season: river discharge (RIVER), chlorophyll a concentration (CHLA), and cross-bay (east-west) wind speed (XWIND).

Susquehanna River discharge is used to represent the freshwater input into the Chesapeake Bay. January to May river discharge contributes both nutrients from the land but also buoyancy effects on estuarine dynamics. The former enhances the production of organic matter in the water column and the latter influences the water column stratification that prevents replenishing oxygen from surface waters.




 The concentration of spring chlorophyll a can be used as a proxy for spring algal biomass that is the fuel for oxygen consumption during the following summer. Therefore, it is important to include this term in the model since the amount of biomass produced in the water column can affect hypoxia through aerobic respiration in the water column and sediments. Mainstem (stations CB3.3 to CB5.3) chlorophyll a data was used.




We found that there was a strong statistical relationship between January to April cross-bay (east-west) wind speed and summer hypoxia. Although the mechanistic link between wind speed in spring and summer hypoxia is not clearly understood, the cross-bay wind significantly improves the output of the regression model. We believe that it may influence the transport and/or deposition of biomass via lateral circulation. For example, more biomass may be accumulated over shallow areas (rather than deep areas) due to stronger cross-bay wind, resulting in less hypoxia, and vice versa.

The multiple linear regression model expresses the value of a predicted (dependent) variable (HYPOXIA) as a linear function of three predictor (independent) variables (RIVER, CHLA, and XWIND) as shown the equation:

[HYPOXIA] = a + b₁·[RIVER] + b₂·[CHLA] + b₃·[XWIND] --- (1)

where a is regression constant and b₁, b₂, and b₃ are coefficients for the predictors. The current year hypoxic volume is predicted using all years of data from 1985 to 2007.

Background

All animal life in Chesapeake Bay, from the worms that inhabit its muddy bottom, to the fish and crabs found in its rivers, to the people that live on its land, need oxygen to survive. We breathe oxygen, which lets us extract energy from the food we eat. Our bodies use this energy to function. This process is essentially the same in all species with one major difference: worms, fish, and crabs use some form of gills instead of lungs to extract oxygen from the water. As water moves across the gills, dissolved oxygen is removed from the water and passed into the blood. As dissolved oxygen concentrations in water decrease, the animals that inhabit the Bay struggle to extract the oxygen they need to survive.



These organisms need dissolved oxygen to survive in Chesapeake Bay.

Chesapeake Bay scientists generally agree that dissolved oxygen concentrations of 5.0 mg·L^‑1 (milligrams of oxygen per liter of water) or greater will allow the Bay's aquatic creatures to thrive. However, the amount of dissolved oxygen needed before organisms become stressed varies from species to species. Although some are more tolerant of low dissolved oxygen than others, in some parts of the Bay dissolved oxygen can fall to the point where no animals can survive. When the levels drop below 2.0 mg·L^‑1, the water is hypoxic, and when it drops below 0.2 mg·L^‑1 the water is considered anoxic.

In an estuary such as Chesapeake Bay, there are several sources of dissolved oxygen. The most important is the atmosphere. At sea level, air contains about 21% oxygen, while the Bay's waters contain only a small fraction of a percent. This large difference between the amount of oxygen results in oxygen naturally dissolving into the water. This process is further enhanced by the wind, which mixes the surface of the water. Two other important sources of oxygen in the water are phytoplankton and aquatic grasses. Phytoplankton are single-celled algae and aquatic grasses are vascular plants; both produce oxygen during photosynthesis. Another source of dissolved oxygen in the Bay comes from water flowing into the estuary from streams, rivers, and the Atlantic Ocean.

See Methodology tab for factors that influence dissolved oxygen.

See Dissolved Oxygen newsletter for more information.

Additional Information

Relevant Web Sites

Don Scavia - Hypoxia Forecasts
Gulf of Mexico hypoxia