456 



118 OCEANOGRAPHY IN THE NEXT DECADE 



such as those of sediments, that are of scientific interest. Estua- 

 rine and cross-shelf exchange is also of interest from a societal 

 standpoint, for example, in determining the fate of riverine inputs 

 of excess nutrients or pollutants. Thus it seems likely that estua- 

 rine and cross-shelf exchanges will be a central focus of future 

 efforts in coastal ocean science. 



Carbon Cycles 



An important and controversial question in oceanography is, 

 What is the role of the coastal ocean in global cycles of carbon, 

 oxygen, nitrogen, and other significant elements? The coastal 

 ocean occupies approximately 20 percent of total ocean area, yet 

 accounts for approximately 50 percent of ocean primary produc- 

 tion and approximately 50 percent of global ocean nitrate assimi- 

 lation by phytoplankton (Walsh, 1991). Describing the mecha- 

 nisms controlling cycling rates of essential elements bars taken on 

 new urgency because of the recently recognized potential for hu- 

 man alteration of global chemical cycles. Biological processes 

 mediate the cycling of many elements and control the fate of 

 numerous materials that enter the ocean. Constructing accurate 

 models of biological controls and predicting their effect on the 

 fate and transformation of dissolved substances and particles in 

 the ocean are severely limited by our lack of understanding of the 

 structure and function of marine ecosystems and their responses 

 to physical and chemical processes. Elucidating these mecha- 

 nisms is critical to understanding the coastal ocean because of its 

 generally high productivity (and thus its processing capability), its 

 substantial biological variability in space and time, and its role as 

 a conduit between the continents and the deep ocean basins. 



A major uncertainty in models of global change, including 

 climate change, is the role of biological processes in mediating 

 and controlling geochemical cycling of important elements. Most 

 scientists agree that biological processes play a key role in the 

 ocean carbon cycle and the cycle of nitrogen, oxygen, and related 

 elements. However, the possible role of marine plants as a sink 

 for carbon dioxide from human activities is highly controversial, 

 and no generally acceptable model has been proposed to explain 

 how the transfer of carbon from the ocean surface to the seafloor 

 (the biological pump) should be working significantly faster now 

 than before the Industrial Revolution. This is an important issue 

 to be considered during the next decade. Understanding ocean 

 margin food webs is of particular interest because they can be 

 altered by eutrophication and other human activities. 



