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96 OCEANOGRAPHY IN THE NEXT DECADE 



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 activ- 

 ity. However, the comparative magnitude of the ocean and terres- 

 trial sinks of carbon is in dispute (Tans et al., 1990), owing primarily 

 to lack of knowledge about air-sea gas exchange rates, the vari- 

 ability of carbon dioxide saturation of surface waters, and the 

 effects of food webs on the production, reoxidation, sedimenta- 

 tion, 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 wa- 

 ters above continental shelves and between shelf and oceanic wa- 

 ters) 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 eu- 

 photic 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 biologi- 

 cal 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 oc- 

 curred in response to a period of cooling that decreased the abun- 

 dance of certain zooplankton species during the time of the year 

 when young herring require zooplankton as food (Gushing, 1982). 

 The impact of El Nino 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- 



