Carbon budgets of the microbial food web in estuarine enclosures

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Abstract
During 9 to 25 June 1987, carbon budgets were established for estuarine enclosures manipulated by additions of nutrients and suspension-feeding bivalves. An intensive sampling program and a detailed examination of autotrophic and heterotrophic microorganisms enabled construction of carbon budgets of the microbial food web and comparison of flow rates through a number of microbial components. Phytoplankton biomass and production covaried, and, as expected, lowest values were recorded in enclosures with added mussels, and highest values in enclosures with added nutrients. Bacteria and heterotrophic nanoflagellates peaked a few days after maxima in phytoplankton biomass and production. In enclosures with added mussels, biomasses were lower for bacteria and microzooplankton, and mesozooplankton, but slightly higher for heterotrophic nanoflagellates. Bacteria, flagellates, and microzooplankton, mostly ciliates, dominated heterotrophic processes, whereas larger mesozooplankton ingestion did not exceed 5% of phytoplankton primary production. Microzooplankton and flagellate clearances were higher in enclosures with added nutrients, whereas no such changes were found in the macrozooplankton, probably because the duration of the experiments did not allow full development of the macrozooplankton. The added mussels dominated heterotrophic consumption and controlled organisms > 20 .mu.m. Exclusion of mussels induced a primary dominance of microzooplankton followed by a subsequent increase of mesozooplankton. Additions of nutrients and filtration by suspension-feeding bivales caused qualitative and quantitative changes at all levels in the microbial food web. These changes were measured from a large number of microbial components and allowed balances of the carbon budgets to be made as well as identification of factors controlling the structure and function of the pelagic carbon cycle.