much reduced and in many ways the Great Lakes ecosystems are recovering. Mercury is no longer at levels that are dangerous in Lake St. Clair. Dichlorodiphenyltrichloroethane (DDT) was banned, as were polychlorinated biphenyls (PCBs), and by 1995 both had declined in all of the lakes to concentrations below which regulations require action (''action levels"). Reduced input of phosphorus was achieved from metropolitan and industrial point sources and the use of low-phosphate detergents. Phosphorus levels have declined to action levels and water quality has improved in the past 30 years. Nitrates and nitrites seem to be increasing, however, and will be more difficult to control. Eutrophication has resulted in increased algal blooms, especially in Lakes Erie and Ontario.

Perhaps the greatest problem in the Great Lakes today is the rate of arrival of undesirable exotic species, primarily through transportation in ships' ballast water, but also through accidental introductions caused by other human activities and some deliberate introductions. Among recent invaders of concern are the zebra mussel (Dreissena polymorpha), the European river ruffe (Acerina cernua), and a predaceous water flea (Bythrotrepes cederstroemi). According to Mills et al. (1993), 139 species of nonindigenous plants and animals have been introduced into the Great Lakes; at least 13 of those species have had significant ecological effects. Future invasions might be reduced by revised methods of handling ballast water (NRC 1996d), but these and other exotic species probably will spread throughout the Laurentian Great Lakes and beyond.

The Laurentian Great Lakes are a primary example of the dependence of fisheries on the entire land-water ecosystem and the direct and indirect impacts that humans have had on fisheries. Problems will continue to develop and persist. The region has a better chance of dealing with these issues and problems as they develop because of the coordinating roles played by the Great Lakes Fishery Commission and the International Joint Commission on Water Quality. However, Kerr and Ryder (1997) argued that the Great Lakes experience is an ominous portent for Canada's Atlantic Ocean fisheries, which will follow the same pathways and will be harder to remediate than Great Lakes fisheries.

San Francisco Bay

The San Francisco Bay estuary is the largest such body on the Pacific Coast of the United States. Including its delta, it encompasses more than 4,100 km2. Before 1950, the estuary contained 1,400 km2 of freshwater marshes and 800 km2 of salt marshes. The bay estuary drains 40 percent of California (Nichols et al. 1986). Runoff from the western slopes of the Sierra Nevada Mountains flows into the San Joaquin and Sacramento rivers and then into the San Francisco Bay and the Pacific Ocean. The estuary supports more than 120 species of fish and is critical habitat for migratory waterfowl (California Fish and Game Department 1998).

The estuary has undergone massive alterations through time in its physical

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