review by Jones, 1992). In these regions the "megabenthos" (the largest bottom-dwelling animals and plants) have been entirely lost or significantly altered because of trawling and dredging activities.

By reducing the flow of water into estuaries, dams built throughout the twentieth century have significantly depressed the successful return of anadromous fish such as salmon and shad to their spawning grounds and have had many other effects on the local estuarine habitat by altering the natural salinity gradient (Skreslet, 1986). Although fewer dams are now being built, water diversion projects for agriculture and urban development are succeeding dams as a threat to estuarine biodiversity (Skreslet, 1986).

Invasions of Exotic Species

Biological invasions have become ubiquitous in virtually all habitats occupied or modified by human activities (OTA, 1993). Many estuarine and nearshore environments have been extensively invaded by exotic (nonindigenous) species, especially through the transport of larvae and spores in ballast water of ships, but also through introductions associated with mariculture. Box 7 focuses on biological invasions via ballast-water transport, and a specific example of ecosystem-level effects of a ballast-water invasion was given in Box 6.

The extent to which exotic invasions have affected the pelagic ocean, the coastal shelf, and tropical ecosystems such as coral reefs is largely unknown because of a lack of baseline information on the composition of these communities and a lack of studies focused specifically on invasions.

Global Climate Change

Atmospheric pollution is altering the exposure of the oceans to ultraviolet (UV) radiation and is increasing the concentration of gases that may lead to long-term climatic changes.

Compounds generated by human activities, including chlorofluorocarbons and brominated compounds rising into the stratosphere, destroy the ozone that shields the atmosphere from the sun's UV radiation. Increased UV-B (the biologically damaging UV) radiation penetrates many meters below the surface of the ocean (Fleishmann, 1989; R.C. Smith et al., 1992). UV-B exposure has increased under ozone "holes" in the Antarctic and elsewhere. In addition, recent satellite data indicate that volcanic activity (e.g., the 1991 eruption of Mt. Pinatubo) has reduced total air column ozone by as much as 10 percent (leading to increases in UV-B exposure of approximately 20 percent) in latitudes as low as Florida and the Bahamas (Gleason et al., 1993).

In turn, studies have confirmed that significant biological and ecological damage to phytoplankton and zooplankton (Hardy and Gucinski, 1989; Kramer, 1990; Behrenfeld et al., 1993a, 1993b), ichthyoplankton (Hunter et al., 1981),

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