Ecological Indicators for the Nation by National Research Council

Author:National Research Council
Language: eng
Format: epub
Tags: Environment and Environmental Studies : Ecology and Ecosystems
Publisher: NATIONAL ACADEMY PRESS
Published: 2000-04-11T00:00:00+00:00

Indicators of Ecological Capital: Abiotic Raw Materials

Nutrient Runoff

Water quality and ecological conditions in U.S. coastal waters have been subjects of growing national concern for more than two decades. Several major types of pollutants—oxygen-demanding organic matter, microbial pathogens, heavy metals, synthetic organic compounds that bioaccumulate to potentially toxic levels, and excessive levels of nutrients—all have taken their toll on these ecosystems. Massive efforts sparked by the Clean Water Act to clean up municipal and industrial sewage effluents have yielded substantial reductions in point-source pollution and improved water quality near such sources. Efforts to control potentially toxic heavy metals and synthetic organic chemicals have resulted in major declines in loadings of these substances to coastal waters, but the legacy of many years of inadequate controls is still seen in the polluted sediments and high body burdens of these substances in the marine organisms of many coastal areas. Among the major potential pollutants affecting coastal environments, nonpoint sources of nutrients—N and P, in particular—have received relatively little regulatory attention.

The results of excessive nutrient loadings are seen in reduced water clarity; nuisance algal blooms, including species with toxic forms like Pfiesteria piscicida and Gymnodinium spp. (red tide); and hypoxic (low oxygen) bottom waters. Outbreaks of toxic and other algae have been correlated with nutrient enrichments and appear to be increasing in estuarine and coastal waters (Burkholder 1998). Hypoxia, generally defined as persistent oxygen concentrations of less than 2 mg/L, affects areas of Long Island Sound, Chesapeake Bay, the near-shore Gulf of Mexico near the mouth of the Mississippi (e.g., Rabalais et al. 1996), and many other coastal areas around the world. In the Gulf of Mexico, the affected area has grown from about 9,000 km2 in 1985 to approximately 18,000 km2 at present. A large increase in the affected area occurred in 1993, apparently in response to the large spring flood, which brought a corresponding increase in nutrients into the Gulf from the Mississippi River. Current evidence links hypoxia primarily to increased inputs of nitrogen forms (mainly nitrate) to coastal waters, inputs that stimulate algal growth. Nitrogen generally limits plant growth in coastal and ocean waters, whereas phosphorus is usually the limiting nutrient in fresh waters. Although growing algae produce oxygen in surface waters, decomposition of dead algae in bottom waters consumes oxygen, leading to loss of habitat for fish and other forms of aquatic life.

Human alterations of the biogeochemical cycles of major nutrient elements have reached global proportions. For example, human contributions to the cycling of nitrogen forms equal the contributions from all natural processes (Ayres et al. 1994). Although nearly all human additions to nutrient cycles occur in terrestrial ecosystems, these systems are leaky, which is why nutrient loadings to coastal zones are elevated substantially above background levels continent-wide. Quantitative data on these increases are sparse, however, and insufficient to document temporal trends in nutrient losses from the continental United States to coastal and marine systems. Evidence is also lacking on the geographic extent of impacts of elevated nutrient loading on the

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