RETENTION VERSUS EXPORT FOOD-CHAINS - PROCESSES CONTROLLING SINKING LOSS FROM MARINE PELAGIC SYSTEMS

The role of export and retention food chains for pelagic-benthic coupl ing is considered by evaluating different food chain scenarios and pro cesses such as aggregation, grazing and zooplankton-mediated fluxes. T he consequences of grazing of primary production by different zooplank ton for the vertical export of particulate organic matter from the eup hotic zone are discussed. Reference is made to existing data and algor ithms regarding primary production and vertical export of carbon from the euphotic zone, both on annual and daily time scales. Examples rega rding the role of nutrient addition, removal of pelagic carnivores and zooplankton grazing for vertical flux are presented. It is speculated how variable grazing impact of micro- and mesozooplankton, as well as herbivorous, omnivorous and carnivorous feeding strategies of mesozoo plankton could compete with aggregation during phytoplankton blooms an d influence export fluxes. It is concluded that the transport of parti culate organic matter to depth not only depends on bottom-up regulatio n as determined by physical forcing, but also on the structure and fun ction of the prevailing planktonic food web. Scenarios are presented w hich indicate that top-down regulation plays a pivotal role for the re gulation of vertical flux. This conclusion may have crucial consequenc es for future biogeochemical programmes investigating pelagic-benthic coupling in the ocean. The endeavours of many research programmes are dominated by lines of thought where straightforward biogeochemistry an d bottom-up regulation is the focus. Phyto- and zooplankton as well as process-oriented research activities have to be the focal point of fu ture research if the current comprehension of export from and retentio n in the upper layers is going to make distinct progress.