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FUTURE DIRECTIONS IN OCEAN SCIENCES 97 



ment (eutrophication) resulting from altered land use, and waste 

 disposal as a reason for the low oxygen concentrations on conti- 

 nental shelves. An additional concern is the deliberate or acci- 

 dental transport of species from one ocean to another by shipping 

 or other activities, leading to outbreaks of the introduced species. 

 Documenting the causes and effects of changes in marine popula- 

 tions is difficult, but new techniques and approaches will make 

 this research possible in the future. 



Effects of the Food Web on Global Biogeochemical Cycles 



Phytoplankton, macroalgae, and sea grasses use energy derived 

 from sunlight to incorporate inorganic carbon, nitrogen, phospho- 

 rus, and other elements into organic molecules that are the build- 

 ing blocks of life and sources of energy for nonphotosynthetic 

 organisms that consume these plants. Some bacteria also synthe- 

 size organic matter using chemical energy (chem.osynthesis) rather 

 than sunlight as a primary energy source. In the ocean, most 

 organic carbon produced by photosynthesis and chemosynthesis is 

 ingested by zooplankton and higher animals, oxidized for energy, 

 and ultimately respired as carbon dioxide in the upper few hun- 

 dred meters of the water column. At the same time, nutrient 

 elements, such as nitrogen, phosphorus, and some trace metals, 

 are recycled and reused by phytoplankton. A variable fraction of 

 organic matter is not recycled in surface waters; instead, it settles 

 out of the upper ocean layers, thereby contributing to vertical 

 fluxes. Thus fluxes of carbon, nitrogen, oxygen, phosphorus, sul- 

 fur, and other biologically important elements are controlled by 

 food-web processes. A major research theme for the i990s will be 

 to describe the effects and possible controls that food-web struc- 

 ture and function have on fluxes from the euphotic zone to middepth 

 and deep waters, to the ocean sediments, and into the geological 

 record. 



Episodic Export of Material from the Surface 



The simplest description of the effects of marine food webs on 

 vertical flux involves only the size and species of phytoplankton 

 and whether the phytoplankton sink before being ingested. For 

 example, the spring diatom bloom in the North Atlantic is thought 

 to sink without significant predation, whereas where cyanobactena 

 are the dominant primary producers, sinking of organic material 

 from the upper ocean is largely mediated by food-web processes. 



