Trophic relationships between planktonic microorganisms in the river Meuse(Belgium): a carbon budget

During the period of plankton development (April to October 1996), the trop hic relationships among phytoplankton, metazooplankton, protozooplankton an d bacterioplankton were quantitatively studied at one location in the Belgi an part of the river Meuse. Biomass fluctuations of phytoplankton, metazoop lankton, bacteria and protozoa were monitored fortnightly. On the basis of in situ measurements of the fluxes of primary production, metazooplankton g razing, organic matter consumption by bacteria and grazing of bacteria by p rotozoa, the carbon fluxes between the different compartments of the first trophic levels were estimated on eleven sampling dates. Net primary product ion was measured from incubations with C-14 bicarbonate and grazing of alga e by metazooplankton was determined by in situ incubations with labelled al gae in a grazing chamber. The algal biomass ingested was corrected for the edible phytoplankton and the carbon assimilated by metazooplankton was calc ulated taking into account zooplankton assimilation yield estimated from ex perimental data and from literature values. Bacterial production was evalua ted by H-3-thymidine and H-3-leucine incorporation rates, and bacterial car bon demand was calculated taking into account a growth yield of 30%. Measur ements of mortality and grazing of bacteria showed that grazing by protozoo plankton was the major loss process indicating the main role of this compar tment in the control of bacterial biomass. In the studied stretch of the ri ver Meuse, autochthonous production exceeded, in most situations, the alloc hthonous inputs to the river. On average over the studied period, carbon co nsumption by metazooplankton and bacteria balanced organic carbon inputs fr om primary production and external loading. The large part of primary produ ction used by bacteria and indirectly by protozooplankton points to the pot ential importance of the microbial food web in this river system.