Indicator Details

Forecast

Anoxic conditions (no dissolved oxygen) in the Bay's mainstem are predicted to be poor this summer, with the average anoxic volume forecast to be 1.7 ± 1.27 km³. Compared to the previous 23 summers, 2008 could have the 5^th largest anoxic volume if this prediction holds true. There is a large degree of uncertainty (±1.27 km³) in this summer's forecast due to a large range of possible predictions at the high nutrient index values.

Anoxia forecast courtesy of David Jasinksi.

Data

Methodology

There are many factors that determine the dissolved oxygen content of the tidal waters of Chesapeake Bay. Nutrient loading, water column stratification, wind and tidal mixing, and water temperatures are but a few of these factors. The two most important determining factors are water column stratification and nutrient loading.



Water column stratification is caused by density differences between the surface and deeper waters of the Bay. Cooler, saltier (more dense) water from the ocean flows underneath the warmer, fresher (less dense) water from the rivers that flow into the Bay. Between the lighter surface water and heavier deeper water is a boundary called the pycnocline. Oxygen consumed beneath the pycnocline cannot be replenished from above, and this leads to lower dissolved oxygen concentrations below the pycnocline. The pycnocline is typically strongest in spring and early summer when fresh water flows are usually at their highest.

Nutrient inputs to the Bay from the land are directly related to precipitation and therefore river flow. Nutrient loads from land-based sources (agriculture, urban runoff, etc.) are higher in the spring when river flows are typically at their highest. Nutrients that flow directly into the Bay from a pipe (sewage treatment plants, industry, etc.) are generally less sensitive to flow and are more consistent through the year. There is a direct relationship between the magnitude of these nutrient loads and the severity of low DO the Bay experiences. Nutrients – nitrogen and phosphorus – fuel the growth of the phytoplankton that make up the base of the Bay's food web. Unconsumed phytoplankton settle below the pycnocline and are decomposed by oxygen–consuming bacteria living in the mud on the bottom of the Bay. Since this is occurring below the pycnocline, this oxygen is not replenished from surface waters. This process occurs every year in Chesapeake Bay, fueled by spring flows that wash large amounts of nutrients into the Bay. Examination of the Chesapeake Bay Program's 20 year data set has shown that the severity of summertime low DO is directly related to the magnitude of spring nutrient loads.



The DO forecast was developed based on this relationship which is expressed as mean June to September anoxic volume (0.2 mg L^-1) versus nutrient loads to the northern Chesapeake Bay. Nutrient loads are the combined total nitrogen (TN) and total phosphorus (TP) loads from the Susquehanna River and point sources on the upper western shore, upper eastern shore and the Potomac River.

The first anoxic forecast released each year is based on the January to April nutrient loads. This forecast is updated towards the end of June to using January to May nutrient loads data which gives a more confident prediction of the summer anoxic volume.

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

Chesapeake Bay Environmental Observatory (CBEO)
COSEE Coastal Trends Dead Zone module
Chesapeake Bay Program