Study on trophic relationships within river plankton: the microbial loop

Past studies carried out on the Meuse and other large rivers in western Europe, have emphasized the importance of trophic relationships within the plankton, and have led to a good understanding of the fate of phytoplankton-derived organic matter. A previous FRFC programme (Study on trophic relationships within plankton in a river, convention n° 2.4545.94) contributed significantly to our progress in that field. In particular, these studies pinpointed the importance of zooplankton grazing in the control of phytoplankton biomass, when environmental conditions are suitable for its development.
On the other hand, a strong coupling is generally observed between phytoplankton and bacterial production, because of the use of phytoplankton carbon (either through lysis or excretion, Servais 1989) by heterotrophic bacteria. Furthermore, a carbon budget calculated for the Meuse in 1996 made it possible to evaluate the fractions of algal carbon exploited respectively by zoo- and bacterioplankton (Servais et al., in press). Bacterial mortality seems largely determined by protozoan grazing. These play a regulatory role on bacterial biomass, and offer a pathway for the transfer of carbon from the dissolved fraction to higher trophic levels, through proto- and metazooplankton grazing (the so-called microbial loop). In lakes, the importance of this microbial loop seems, according to some authors (Gasol & Vaqué, 1993) dependent upon the predation exerted on given components of zooplankton: where they are locally abundant, bacteriophagous heterotrophic nanoflagellates, are maintained to low biomass levels.