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Managing Wastewater in Coastal Urban Areas
observed in some Swedish coastal waters as a result of increased incidences of hypoxia (Baden et al. 1990).
Anoxia and hypoxia are major and growing problems in many estuaries and coastal seas. Over the past few decades, the volume of anoxic bottom waters has been increasing in the Chesapeake Bay (Officer et al. 1984, D'Elia 1987), the Baltic Sea (Larsson et al. 1985), and the Black Sea (Lein and Ivanov 1992). The apex of the New York Bight (an area of some 1,250 km2) becomes hypoxic every year, and a large region of the Bight became anoxic in 1976 (Mearns et al. 1982). Hypoxic events appear to be becoming more common in waters such as the Long Island Sound (EPA 1990, Parker and O'Reilly 1991), the North Sea (Rosenberg 1985), and the Kattegat (the waters between Denmark and Sweden; Baden et al. 1990), although historical data on oxygen concentrations in coastal waters are often poor.
Anoxia and hypoxia result from oxygen consumption exceeding oxygen supply. Oxygen is consumed by the respiration of organisms, including animals, plants, and the decomposing activity of microorganisms. Oxygen is supplied to waters through the process of photosynthesis and through diffusion and surface entrapment from the atmosphere. Organic matter released in sewage effluent can contribute to anoxia and hypoxia by creating biochemical oxygen demand (BOD), the oxygen consumed during the microbial decomposition of this organic matter and chemical oxygen demand (COD), the oxygen consumed through the oxidation of ammonium and other inorganic reduced compounds. For example, altered structure, reduced diversity, and elevated biomass of benthic animal communities—related in part to high organic solids inputs—once characterized up to 95 square kilometers of the coastal shelf around major outfalls in San Diego, Orange, and Los Angeles Counties in southern California (Mearns and Word 1982). However, BOD inputs to most estuaries and coastal seas are well controlled and represent at most a localized problem. Of more concern in most estuaries and coastal marine ecosystems is the oxygen consumption that results from the decomposition of the excess phytoplankton production characteristic of eutrophication (Officer et al. 1984, Larsson et al. 1985, Jensen et al. 1990, Rydberg et al. 1990, EPA 1990, Parker and O'Reilly 1991, Lein and Ivanov 1992). Photosynthesis by phytoplankton produces oxygen, but much of the photosynthesis in eutrophic waters occurs near the surface, and oxygen readily escapes to the atmosphere. The majority of the phytoplankton material is decomposed deeper in the water column, consuming oxygen there. In contrast to the rather localized effects of BOD inputs, nutrient inputs can lead to eutrophication and anoxia or hypoxia far from the original source of the nutrient. In some cases, improved sewage treatment may aggravate this situation by resulting in more distant transport of nitrogen (Chesterikoff et al. 1992). Some evidence points to increasing hypoxia in the western basin of the Long Island Sound as being a result of