glaciers, the volume of fresh water that enters polar waters (e.g., the Gulf of Alaska) is likely to increase substantially. The input of fresh water can be critical to coastal currents, as seen by the effects of the Mississippi and St. Lawrence rivers. Changes in precipitation (temporal and spatial) and the amount of ice melt could shift the direction and change the magnitude of coastal currents. Such physical changes will affect fish populations by affecting transport of eggs and larvae.

The effects of El Niño events on eastern boundary current ecosystems in the Pacific Ocean could serve as a model of the possible effects of global warming (McGowan, 1990) in terms of decreased primary and secondary production. In addition, an increased temperature differential between land and ocean could enhance coastal winds and hence wind-induced transport of surface water away from the shore, reducing the reproductive success of species that spawn offshore but rely on coastal habitats later in their life cycles. Stronger winds would also increase turbulence in the surface mixed layer, dispersing patches of planktonic food, and thereby making the food less available for fish.

The third example involves the impacts of a changing sea level. If sea level rises at a rate of 1 to 3 millimeters per year over the coming 50 to 100 years, profound impacts on nearshore habitats would result. In areas with broad, flat coastal plains, the width of the inner continental shelf may be expected to increase greatly. This change would wipe out many coastal habitats. In addition, distribution of the wave energy over a wider continental shelf may substantially modify the transport of planktonic organisms to shore, affecting the success of larval recruitment and the transition of organisms from larval to juvenile stages.

Other Anthropogenic Influences

Other human-induced environmental changes also affect marine populations, although they vary regionally and their extent is disputed. For example, McGowan (1990) reported no detectable change of pelagic species or of ecosystem structure in the California Current ecosystem despite extensive harvesting (fishing) of top predators and vastly increased inputs of pollutants. In contrast, the Baltic Sea ecosystem has changed significantly in the past 50 years in response to eutrophication (Kullenberg, 1986).

The incidence of unusual, and sometimes harmful, phytoplankton. blooms is increasing in coastal waters around the world. The evidence is particularly compelling in European and Japanese wa-

The National Academies of Sciences, Engineering, and Medicine
500 Fifth St. N.W. | Washington, D.C. 20001

Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement