Indicator Details

Forecast

The average volume of hypoxic water (Dissolved Oxygen ≤2 mg·L^‑1) in Chesapeake Bay this July is predicted to be 5.1 cubic kilometers, with 95% confidence that the hypoxic volume will be between 2.7 and 7.4 cubic kilometers. Compared to past years (1985 to 2008), this July is expected to have the 3^rd lowest 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).

Hypoxia forecast courtesy of Don Scavia.

Data

Methodology

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.

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