Note that the data published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution. For the most recent data, purchase the 2008 Report from Island Press.

The Indicator

The percentage of brackish water exposed to a range of oxygen concentrations for at least 1 month will be reported as anoxic (no oxygen), hypoxic (>0 and <2 parts per million [ppm]), low (2–4 ppm), or sufficient (>4 ppm). Low oxygen levels for a brief period may do little to disrupt the marine ecosystem; however, when those levels persist, significant effects on the local ecosystem can be expected. The percentage of brackish waters that are hypoxic for at least 1 month will be reported by region. Note that bottom waters are the first to become hypoxic or anoxic because less light is available for the oxygen-producing algae to grow, excess organic matter generally sinks and is decomposed in bottom waters, and little exchange with the atmosphere occurs—a process that can introduce oxygen to surface waters.

Dissolved oxygen is an important habitat parameter for both aerobic and anaerobic organisms. In addition to directly affecting the distribution, abundance, and diversity of aerobic organisms (including commercial and sport fish), oxygen depletion (the development of hypoxia and anoxia) in bottom waters alters biogeochemical reactions involving biologically important elements such as carbon, nitrogen, phosphorus, sulfur, and iron.

The distribution of dissolved oxygen is an integrative measure of the dynamic balance between processes that produce, consume, transport, and exchange dissolved oxygen. Plants (dominated by microscopic algae called phytoplankton) generally produce more oxygen than they consume. The amount of dissolved oxygen in the water at any given time and place reflects the balance between this production and several ways that oxygen is lost from a given location: consumption by respiration, loss from surface waters to the atmosphere because of equilibrium processes (e.g., cold water can hold more oxygen than warmer water), and movement of water masses between regions of the world’s ocean.

Anthropogenic nutrient loading is considered by many to be the primary cause of increasing trends in the duration and extent of hypoxia and anoxia. Influxes of nutrients stimulate phytoplankton production which can lead to more bacterial decomposition and rapid increases in biological and chemical oxygen demand when this production is not consumed by zooplankton, fish, or shellfish. Consequently, the occurrence of hypoxic and anoxic bottom waters is an important indicator of coastal eutrophication and of the effects of human activities on biogeochemical cycles. Long-term observations of the extent of oxygen depletion in brackish waters will reflect changes in land cover and land-use patterns in coastal watersheds, including the efficacy of efforts to control nutrient loading to coastal ecosystems (see the National Oceanographic and Atmospheric Administration study, Hypoxia in the Gulf of Mexico, at www.nos.noaa.gov/products/pubs_hypox.html).

The Data Gap

Dissolved oxygen should be measured with a precision of ±0.5 ppm. Measurements should be frequent enough (several times a month) to capture seasonal variability on the spatial scales appropriate to estuaries and coastal areas. In addition, the required data cannot be easily accessed because they reside in a variety of databases that are maintained on an ecosystem-by-ecosystem basis by county, state, and federal agencies and institutions.

Observations with sufficient resolution in time and space to calculate the extent of bottom water hypoxia with known certainty exist for some regions (e.g., the northern Gulf of Mexico, Gulf of Maine, Middle Atlantic Bight, South Atlantic Bight) and for many estuaries and bays (e.g., Long Island Sound, the lower Hudson River estuary, Chesapeake Bay, San Francisco Bay, Puget Sound). Although some of these data are available from the National Oceanographic Data Center (www.nodc.noaa.gov/), the National Estuary Program (http://www.epa.gov/owow/estuaries/), and the National Estuarine Research Reserve System (www.ocrm.nos.noaa.gov/nerr/), much of the required data resides with state agencies and other federal programs; and sufficient data on both dissolved oxygen and salinity distributions are not available to calculate this index on regional to national scales.