the upper water column indefinitely (i.e. through recycling) or be removed to deeper waters 

 and sediments. Carbon in zooplanktonic biomass will tend to move through the food web and, 

 ultimately, be recycled as respiratory DIC. Large detrital particles such as the fecal material of 

 copepods and salps may sink rapidly to deep waters and underlying sediments. Smaller 

 detrital particles may remain in the upper water column via breakdown and subsequent 

 utilization by bacteria or direct ingestion by protozoan zooplanktonic grazers. Alternatively, 

 these smaller particulates, as well as COC, may be scavenged by sinking organic materials 

 such as 'marine snow'. This division will depend on the relative abundance and activities of 

 bacteria, protozooplanktonic grazers, and marine snow particles and their producers. The fate 

 of DOC is less clear. If bacterial uptake is sufficiently slow, much of it may move via 

 diffusion and advection into the deep ocean, where it may ultimately enter a detrital food web 

 largely isolated from the food web of near-surface waters. Much of the carbon entering the 

 deep-sea may be deposited as either fecal material or animal carcasses on the sea floor. 



Previous research has shown that organisms in the 0.5 - 20 |im size range are responsible 

 for the bulk of both carbon fixation and respiration in shelf systems. However very little is 

 known, either qualitatively or quantitatively, of the fate of these cells in pelagic food webs. It 

 is generally believed that these nanoplankton are ingested primarily by protozoan zooplankton. 

 The relative importance of microbial food webs vs metazoan food webs in marine systems 

 determines to a large extent the proportion of fixed carbon which wUl be respired or converted 

 to non-sinking dissolved or colloidal organic matter, compared to the fixed carbon (as fecal 

 pellets or decaying phytoplankton debris) which wiU either sink out from the euphotic zone or 

 be transported elsewhere on the shelf. At present, the question of what proportion of the 

 carbon contained in <20 fxm-sized cells is recycled, channeled to metazoan food webs, or 

 transported from the pelagial, is an open question. The issue is complex, since various 

 categories of pico- and nanoplanktonic organisms could have dramatically different fates. 



The abundant occurrence of both protozoan and metazoan zooplankton in shelf waters 

 implies a close coupling between production and consumption in the water column. 

 Approximate rates of ingestion of pelagic tunicates calculated from in situ growth rates, can 

 range from <100% to >600% of their body carbon per day, depending on the type and 

 concentration of food. That explains their potential of controlling phytoplankton as well as 

 protozoa. When a large proportion of primary production is consumed by quickly responding 

 zooplankton, less material should be available for sedimentation or export. Yet, there is limited 

 evidence of deposits of organic carbon in slope sediments, and phytoplankton blooms 

 themselves are offered as evidence of trophic decoupling, presumably between 

 metazooplankton and larger size classes of phytoplankton. These conflicting hypotheses 

 underscore our inadequate understanding of the relative importance of smaU and large 

 phytoplankton in ocean margins, the roles of protozoan and metazoan grazers in regulating 

 their dynamics, the temporal and spatial heterogeneity of these processes, and their relationship 

 to particle export. 



F. Production of DOC 



Dissolved organic carbon is a potentially important reservoir of organic carbon that has 

 been nearly ignored in budgets of carbon flux and storage in marine ecosystems. Like POC, 

 DOC is not a homogenous pool of organic matter, but a heterogenous pool of materials with 

 variable reactivity. Included in the pool of DOC on continental margins are small molecular 



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