Aquatic Life Criteria for Dissolved Oxygen - (Saltwater) Cape Cod to Cape Hatteras

Fact Sheet; October 2000

The U.S. Environmental Protection Agency (EPA) is recommending ambient water quality criteria for dissolved oxygen that will protect coastal and estuarine animals in the Virginian Province (Cape Cod, MA to Cape Hatteras, NC). The criteria combine features of traditional water quality criteria with a new biological framework that integrates exposure to low dissolved oxygen over time rather than averaging dissolved oxygen exposure conditions into one single value. The criteria also establish protection limits for different life stages (i.e., larvae versus juveniles and adults).

Background

EPA is recommending ambient water quality criteria for dissolved oxygen (saltwater) to protect coastal and estuarine animals in the Virginian Province (Cape Cod, MA to Cape Hatteras, NC) from low dissolved oxygen levels (DO). EPA has not issued saltwater DO criteria before because information on the adverse effects of low DO on aquatic organisms was insufficient. The new DO criteria result from a 10-year research effort. The water quality criteria represent EPA's best estimates, based on the data available, of DO concentrations necessary to protect aquatic life and uses associated with aquatic life. States, territories, and authorized tribes may use these water quality criteria as guidance in setting water quality standards for coastal and estuarine waters as required by Section 304(a) of the Clean Water Act.

Why is EPA publishing dissolved oxygen criteria?

EPA is publishing the saltwater DO criteria to protect organisms and their uses from the adverse effects of low DO conditions. The Agency developed these criteria because hypoxia (low dissolved oxygen) is a significant problem for coastal waters that receive a lot of runoff thatcontain nutrients (for example, nitrogen and phosphorous and other oxygen-demanding biological wastes). Excessive nutrients in aquatic systems stimulate algal growth, which in turn uses up the oxygen needed to maintain healthy fish and shellfish populations.

EPA's Environmental Monitoring and Assessment Program (EMAP) for the estuaries in the Virginian Province (defined as Cape Cod to Cape Hatteras) has shown that 25% of the area is exposed to some dissolved oxygen concentrations of less than 5 mg/L. Long periods of DO below 5 mg/L can harm larval life stages for many fish and shellfish species. Field data collected through EMAP has shown a correlation between many of the biologically degraded benthic areas and low dissolved oxygen in the lower water column. These observations serve to emphasize the fact that hypoxia is a serious concern for the Virginian Province and other coastal locations in the United States. Hypoxia is regulated primarily by controlling (typically a reduction) nutrients (largely nitrogen) put into a water body. The DO criteria identify waters with DO problems and can form the basis for necessary reductions in nutrient levels.

Saltwater DO criteria for coastal and estuarine waters will help states, territories, and authorized tribes develop and adopt DO water quality standards. These standards, in turn, may provide a basis for:

• appropriate total maximum daily loads (TMDLs) for nutrient control,
• numerical discharge limits in permits established under the National Pollutant Discharge Elimination System (NPDES),
• appropriate nonpoint source runoff controls,
• needed wetlands protection, and
• other water resources management efforts.

Environmental planners can also use the approach to evaluate conditions under different management scenarios and make better decisions. The general public will benefit because the DO criteria will help EPA, the states, and authorized tribes achieve the Clean Water Act goal of "fishable and swimmable" waters.

What are the criteria limits?

Dissolved oxygen criteria apply to both continuous and cyclic low DO conditions. If the DO conditions are always above the chronic criterion for growth (4.8 mg/L), the aquatic life at that location should not be harmed. If the DO conditions at a site are below the juvenile/adult survival criterion (2.3 mg/L), there is not enough DO to protect aquatic life. When persistent DO conditions are between these two values, further evaluation of duration and intensity of low DO is needed to determine whether the level of oxygen can support a healthy aquatic life community (see Figure 1).

The approach for episodic or cyclic low DO conditions requires that the DO be directly measured over a daily cycle or that the daily DO cycle be estimated. The cyclic pattern is then compared to allowable levels to determine suitability of DO conditions for the juvenile/adult survival, larval growth, and larval recruitment endpoints.

Where do these criteria apply?

These water quality criteria recommendations apply to coastal waters (defined as within three miles from shore under section 502(8) of the CWA) of the Virginian Province (Cape Cod, MA to Cape Hatteras, NC) of the Atlantic coast of the United States. Under the CWA, states, territories, and tribes must adopt water quality criteria to protect designated uses. EPA has promulgated regulations to implement this requirement (see 40 CFR 131). This criteria document does not substitute for those provisions or regulations, nor is it a regulation. Risk managers can apply the criteria to other coastal waters if they can scientifically determine that their location-specific biological, physical, and water quality conditions are comparable to those of the Virginian Province.

What are the Endangered or Threatened Species Policy Recommendations?

It may be appropriate to derive site-specific DO criteria when a threatened or endangered species is at a site, and when data indicates that it is sensitive at concentrations above the recommended criteria.

What are the future activities related to these criteria?

EPA will publish an addendum to this criteria document to address implementation issues in greater detail. Topics may include:

• accuracy of monitoring data,
• identification of biological effects,
• importance of spacial extent of low DO,
• application to differing salinities, and
• consideration of threatened and endangered species.

Environmental managers should consider all of these issues when they adopt and implement DO water quality standards. The planned addendum may use real world examples to illustrate these implementation issues. It will also discuss applying this guidance to marine waters outside the Virginian Province.

How do I get a copy of the criteria document?

You can get a copy of the complete document, Aquatic Life Water Quality Criteria for Dissolved Oxygen (Saltwater): Cape Cod to Cape Hatteras (EPA-822-R-00-012) by calling EPA's National Service Center for Environmental Publications (NSCEP) at 1-800-490-9198.

For more information about the development of the criteria, contact:

USEPA
Office of Science and Technology
Health and Ecological Criteria Division
Mail Code 4304T
1200 Pennsylvania Avenue, NW Washington, DC 20460
(202) 566-1100