grams globally making this process the largest biotic sink of the global carbon cycle (Moore and Bolin, 1986).

One of the major uncertainties in the models of the global carbon cycle is the role of marine organisms in the ocean carbon budget. During the spring bloom in the North Atlantic, the air-sea carbon dioxide flux is strongly controlled by biological activity. However, the comparative magnitude of the ocean and terrestrial sinks of carbon is in dispute (Tans et al., 1990), owing primarily to lack of knowledge about air-sea gas exchange rates, the variability of carbon dioxide saturation of surface waters, and the effects of food webs on the production, reoxidation, sedimentation, and burial of carbon.

The rate at which dissolved or particulate matter passes through the horizontal plane at any particular depth in the ocean is called vertical flux, whereas lateral flux refers to flux through a vertical plane. In the ocean, the vertical flux of organic material (as well as the lateral flux of organic material between estuaries and waters above continental shelves and between shelf and oceanic waters) and its burial rate in ocean sediments are not simple linear functions of primary production. The structures of marine food webs (the number and type of organisms at various feeding levels and the feeding relationships among the organisms) in the euphotic zone, in mid waters, deep waters, and at the seafloor are key variables affecting vertical and lateral fluxes of biologically important elements.

As indicated above, marine food webs affect global biogeochemical cycles, and marine populations, in turn, are affected by changes in global climate and human-induced changes in ocean environments. Some of the best examples of climate effects on marine organisms come from European fisheries, for which long time series exist for fish catch and abundance in relation to key physical and biological variables. An extraordinary event occurred during the 1960s in the North Sea, where the abundance of codlike fish exploded as the herring population declined. This major change probably occurred in response to a period of cooling that decreased the abundance of certain zooplankton species during the time of the year when young herring require zooplankton as food (Cushing, 1982). The impact of El Niño on South American anchoveta populations is another well-known example.

Human activities also affect marine populations, particularly in estuarine and coastal waters, although anthropogenic effects are difficult to distinguish from highly variable natural cycles. Of particular concern are the long-term effects of nutrient enrich-

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