When oxygen is in water, it is in a dissolved form. Like animals on land, the Bay’s fish and shellfish need oxygen to survive.
The necessary amount of dissolved oxygen varies by species, season and location in the Bay:
- Generally, higher levels of oxygen are needed in shallow waters during the spring, which is when aquatic animals spawn.
- Slightly lower levels of oxygen are acceptable at other times of the year, particularly in deeper waters.
Learn more about dissolved oxygen.
The goal is for 100 percent of the tidal tributaries and the Chesapeake Bay to meet Clean Water Act standards for dissolved oxygen.
What is the long-term trend? (1987-2008)
Between 1987 and 2008, the dissolved oxygen score increased from 14.77 percent to 16.44 percent of goal achieved. During this time period, goal achievement has averaged 21.3 percent and has ranged from 9.82 percent to 36.13 percent.
What is the short-term trend? (Trend over the past 10 years)
Between 1999 and 2008, goal achievement decreased from 29.75 percent to 16.44 percent.
Change from previous year (2007 to 2008)
The dissolved oxygen score increased from 12.21 percent to 16.44 percent of goal achieved.
Water Quality Standards
State water quality standards have been adopted to reflect the dissolved oxygen needs of the Bay’s aquatic life. The standards vary with water depth, season and duration of exposure.
To meet state regulations, all data gathered within each tidal river and mainstem Bay segment must meet required dissolved oxygen concentrations, based on a combination of interpolation and cumulative frequency distribution (CFD) analyses. These analyses allow for some temporal and spatial exceedences of dissolved oxygen criteria. But if the segment has concentrations that exceed the permitted spatial and temporal allowances, the entire volume of water for that segment is considered out of attainment.
Dissolved Oxygen and Nutrients
The settling and subsequent decomposition of algae are largely responsible for the Bay’s low dissolved oxygen concentrations. Thus, dissolved oxygen concentrations are an important indicator of nutrient loadings to the Bay and the capacity of the Bay to support aquatic life.
Over time, large-scale reductions in the amount of nutrients flowing into the Bay will help improve low oxygen conditions.
Jeni Keisman at 800-968-7229 ext. 321
Chesapeake Bay Program Office