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98 OCEANOGRAPHY IN THE NEXT DECADE 



In the latter case, vertical flux is small, mainly because of the 

 number of steps in the cyanobacteria-based food webs, which lead 

 to more recycling. In either case, the importance of the down- 

 ward fluxes to the biota is that food resources are no longer avail- 

 able to the community from which they exit but seem to fuel 

 successively deeper communities. 



Most particles in the ocean are small and sink slowly. Par- 

 ticles that account for most of the transfer of material to the 

 seafloor are the rarer, large particles that have both high mass and 

 high sinking rates. They include the fecal pellets produced by 

 large zooplankton, large aggregates of detritus and plant debris 

 (marine snow), and living organisms. Zooplankton can increase 

 vertical flux by repackaging and concentrating organic matter from 

 small, slowly sinking phytoplankton and microorganisms into fe- 

 cal pellets and mucous feeding structures that sink much faster 

 than individual particles. Sinking flux varies by an order of mag- 

 nitude among food webs. Food webs dominated by large zoo- 

 plankton consumers may export a much greater percentage of con- 

 sumed primary production than food webs in which phytoplankton 

 are initially consumed by smaller protozoans and zooplankton, 

 owing to the relative sizes and sinking rates of fecal pellets. Epi- 

 sodic zooplankton swarms could dominate the long-term average 

 export of organic matter from surface ocean communities, but 

 such swarms are often missed by short-term studies. 



The activities of marine animals in breaking apart and con- 

 suming large aggregates on their way to the seafloor may also be 

 significant, and as yet poorly quantified, factors in controlling 

 particle flux. Many of these particles are consumed by animals as 

 they sink and are converted into smaller fecal pellets, new animal 

 growth, respired carbon dioxide, and dissolved organic matter. 



Dissolved Organic Material 



The measurement of dissolved organic material (DOM) is also 

 of great interest to biological oceanographers and is an area of 

 overlap between the disciplines of biological and chemical ocean- 

 ography. The size, average age, and biological availability of the 

 DOM pool are controversial, but the pool could be significant in 

 global fluxes of carbon, nitrogen, and other biologically important 

 elements. Furthermore, a major unresolved question is the degree 

 to which DOM provides nutrition for the ubiquitous microbial 

 community, which may use organic carbon at 10 to 40 percent of 

 the rate at which phytoplankton use it. 



