| Summary: | Detailed biological measurements (phyto- and bacterioplankton biomass and activity and counting of two classes of protozooplankton) were carried out in the marginal ice zone of the northwestern Weddell Sea during sea ice retreat 1988 (EPOS expedition, Leg 2). These measurements clearly showed enhanced phyto-, bacterio- and protozooplankton production in the marginal ice zone, as compared to ajacent open sea and permanently ice-covered areas. The combined analysis of available physical, chemical and biological observations indicated that the initiation of the phytoplankton bloom -dominated by nanoplanktonic species -was determined by physical processes operating in the marginal zone at the time of ice melting. The additional effects of grazing pressure by protozoa and deep mixing appeared responsible for a rather moderate phytoplankton biomass (4 µg Chl α 1 -1 ) with a relatively narrow geographical extent (100-150 km). The role of trace metals, in particular iron, was minor. On the basis of these data, as well as of physical measurements related to the hydrodynamical stability of the water column, a coupled hydrodynamical-biological model describing the microbial network developing at the receding ice-edge of the circumpolar marginal ice zone of the Southern Ocean has been established. This model takes into account the various physical and biological controls exerted on phytoplankton development, and allows calculation of carbon and nitrogen circulation through the lower trophic levels of the pelagic ecosystem. Carbon budget calculation thus reveals the quantitative importance of heterotrophic microorganisms in the fate of primary production: 88% of net primary production is assimilated by the microbial loop composed of bacteria, bactivorous and herbivorous protozoa. These latter, ingesting as high as 61% of the primary production play a key role, both by linking krill and other mesozooplankton to microorganisms, and by regenerating ammonium. Total net microbial food web secondary production contributes 66% of ...
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