Coupling between primary production and pelagic consumption in temperate ocean margin pelagic ecosystems

Three fates potentially consume primary production occurring on ocean margins: portions can be oxidized within the water column, portions can sediment to shelf/slope depots, and portions can be exported to the interior ocean. Zooplankton mediate all three of these processes and thus can alter the pathway and residence time of particulate organic carbon. As part of both US DOE- and NSF-sponsored studies on the Cape Hatteras and South Atlantic Bight (SAB) shelves, the role of microzooplankton in these processes was determined by measuring phytoplankton production and its consumption by microzooplankton. Grazing and growth rates were measured during 46 dilution incubation experiments using chlorophyll a (chl a) as a proxy for phytoplankton (prey) biomass. Chl a production and grazing were determined for the < 200 gm phytoplankton community and also the < 8 gm size class. Primary production at Cape Hatteras was determined using (HCO3-)-C-14 incubations during two Lagrangian drifter studies lasting several days in March and July 1996. From similar measurements during cross-shelf transects over larger spatial scales, primary production was also calculated for the Hatteras study area using a wavelength-resolved bio-optical model. Primary production during the Lagrangian studies was generally 0.5-1.0gC/m(2)/d in March and 0.5-2.0 gC/m(2)/d in July. Modeled estimates of primary production for the larger Hatteras study region in March and July averaged 1.8 gC/m(2)/d . Typically, < 8 mum cells contributed one-half or more of primary production. Positive linear regressions described relationships between phytoplankton production measured as changes in chl a and its grazing by microzooplankton. In the dilution experiments conducted throughout the SAB and Hatteras shelves, microzooplankton grazed 65% of <200 mum chl a production, and 81% of < 8 mum chl a production. These relationships were temperature-dependent: losses of chl a production in both size fractions to microzooplankton herbivory increased with increasing temperature. Higher grazing rates were found in the < 8 mum compared to the < 200 mum size class. Model regressions were used to estimate the impact of microzooplankton grazing on (HCO3-)-C-14-derived estimates of primary production in Cape Hatteras shelf waters. Integrated water column grazing removed 40% and 58% of <200 mum and <8 mum primary production, respectively, during the Lagrangian experiment in March, and 61 % and 74% in July. Averaged over larger spatial scales using a bio-optical model, microzooplankton ingested 42% and 61 % of primary production in March and July, respectively, with an overall mean of 52%. These data generally support the notion that, contrary to traditional paradigms about shelf ecosystems, small autotrophs contributed significantly to production, and that this carbon was actively incorporated into the microbial food web. (C) 2002 Elsevier Science Ltd. All rights reserved.