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Browse History: DO - hypoxia (2012)
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Dissolved oxygen is critical to the survival of Chesapeake Bay's aquatic life. The amount of dissolved oxygen needed before aquatic organisms are stressed, or even die, varies from species to species.


Indicator Details

Indicator Icon DO - hypoxia

Forecast

July 2012 hypoxic volume forecast

Given the average January-May 2012 total nitrogen load of 161,802 kg per day from the Susquehanna River, July 2012 hypoxic (Dissolved Oxygen ≤2.0 mg·L‑1) volume forecast is 6.4 km3, with 95% confidence interval that the hypoxic volume will be between 3.3 and 8.2 km3. This is slightly below average for the time period (1985-2011). Given the warm winter and spring in 2012, and the recent rainfall, this may underestimate the July hypoxic volume.

July hypoxia forecast provided by Don Scavia and Mary Anne Evans, University of Michigan. For more information, please visit their website.


Data


July 2012 Hypoxia Forecast

The average volume of hypoxic water (Dissolved Oxygen ≤2.0 mg·L‑1) in Chesapeake Bay for July 2012 is predicted to be 6.4 cubic kilometers, with 95% confidence that the hypoxic volume will be between 3.3 and 8.2 cubic kilometers. Compared to past years (1985 to 2011), this July is expected to have the 10th smallest hypoxic volume. This forecast is based on a model that was developed to assess the impacts of changes in nitrogen loads on Chesapeake Bay hypoxia (Scavia et al 2006). Considering the warm winter and spring in 2012, as well as recent rains, this may underestimate the July hypoxic volume.

Time series of July hypoxia and 2012 forecast

July forecast provided by Don Scavia and Mary Anne Evans (University of Michigan). For more information, please visit their website.

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 1986-2011, and the model is recalibrated for this application to the most recent three years of data (currently 2009-2011). Calibrating to recent years for annual forecasts allows the predictions to track physical and ecological changes in the Bay. The summer hypoxic volume forecast was generated using the following relationship. For more information, visit http://snre.umich.edu/scavia/hypoxia-forecasts/

Model used to forecast July hypoxia

The solid black curve is the forecasting result (mean value); dashed black curves are forecasting confidence intervals (2.4 and 97.5% vlaues). The open blue circles are the observed values for 1985-2007, the closed black circles are observed values before 1985, and the closed blue circles are observed values for 2009-2011 (the years used for model calibration). The vertical line represents the 2012 January to May total nitrogen load of 161,802 kg day-1 and the horizontal lines show the forecast hypoxic volume (mean and confidence intervals) associated with this load.


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.

Chesapeake Bay Organisms

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.


organism DO requirements figure

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