Does planktonic community structure determine downward particulate organiccarbon flux in different oceanic provinces?

A previous study based on data from a NE Atlantic site provided evidence, u sing an existing foodweb/vertical-flux modelling approach, that the size-st ructure of the phytoplankton community - rather than primary production - c an be the dominant control on downward particulate organic carbon (POC) flu x. In order to assess whether taking community structure into account can a lso provide more reliable estimates of downward POC flux in other oceanic p rovinces, epipelagic observations (mainly size-fractionated primary product ion, biomass, community structure data and heterotrophic bacterial producti on) and POC flux data from deep-moored sediment traps were collated from th e six different ocean regions for which suitable data are available. At eac h sire the epipelagic data were used in conjunction with two standard versi ons of the foodweb/vertical-flux model (one permits direct sinking of large ungrazed algae out of surface waters, the other does not) and published em pirical depth/POC-flux algorithms to predict the POC flux to the deep ocean . Predictions were also made using published primary-production/POC-flux al gorithms, and the two sets of predictions were compared to the deep-ocean P OC flux measurements. While the version of the foodweb/vertical-flux model permitting the direct sinking of ungrazed algae provided the most reliable predictor of POC flux for five of the six sites, no conventional algorithm provided comparable predictions for more than two sites. The reliability of these predictions is discussed in the context of recent modelling studies that explore the extent to which Row fields in the water column overlying d eep-moored sediment traps confound attempts to relate particle flux measure ments to observations of surface-water processes. The present study suggest s that the sinking of ungrazed large cells, probably diatoms, may be the ke y determinant of the magnitude of the downward POC flux in a variety of oce an regions. Planned ocean-observing programmes may provide sufficient epipe lagic data to allow this approach to be used to improve the accuracy of bas in-scale estimates of downward POC flux and hence reduce the uncertainty of the magnitude of this flux within the oceanic carbon budget. (C) 1999 Else vier Science Ltd. All rights reserved.