Summer declines of potamoplankton have been reported from several large rivers, including the Meuse and Moselle, two European eutrophic rivers. Longitudinal declines of producer biomass may result from changes in physical conditions along the river course but may also be a consequence of biotic interactions. In this context, zooplankton grazing on the planktonic algae was monitored throughout the growing season for three years (1994-96), at one site, 537 km from the source, in the Belgian section of the R. Meuse, and on some occasion, along a stretch of 185 km, in the R. Moselle. Both rivers are heavily regulated for navigation but some natural stretches have been maintained in the R. Moselle, so that the river flows alternately through deep, slow-flowing reaches, and wild, shallow, sometimes meandering reaches.
In both rivers, phytoplankton dynamics are shown to be driven firstly by physical factors (discharge, temperature, light). Algal dynamics follow the general pattern of « succession » in rivers. High net production rates in spring have been pointed out. As in other rivers, algal production is limited by light. In the R. Moselle, decreasing net production resulted from increased channel depth, providing some evidence that river morphology may be, to some extent, responsible for longitudinal declines of phytoplankton.
In situ grazing was measured using the Haney method and 14C-labelled Chlorella-like algae as a tracer in the 6.5 liter grazing chamber. Total zooplankton community filtration rates recorded in the R. Meuse ranged between 1 and 113 % of the water volume filtered per day and maxima were observed during the summer period. Every year, a drastic algal decline was observed at the beginning of the summer and may be related to high densities of a rotifer-dominated zooplankton community. A rapid biomass recovery was then observed, along with a shift of the algal community toward larger units. In contrast, phytoplankton biomass remained comparatively low in the R. Moselle throughout the low-flow period, as did zooplankton numbers. The proportion of crustaceans was rather higher than in the R. Meuse, and they dominated at times, in biomass as well as in numbers. Nevertheless, measured in situ grazing rates (1-27 %) could not entirely account for the low summer algal biomass, even if zooplankton grazing sometimes represented a significant carbon loss for phytoplankton, when and where net algal production was low.
We suggest that planktonic grazers may at times control the composition of the phytoplankton community of eutrophic large rivers. However, such biotic interactions can take place only when physical constraints are reduced, i.e. when discharge is low, and when increased transfer time, high temperature and availability of edible algae allow high zooplankton biomass. The zooplankton assemblage, dominated by r-strategist rotifers, is inherently unstable and may fluctuate, namely as a consequence of meteorological and hydrological fluctuations. Hence, the grazing pressure on planktonic algae is highly variable. However, another cause of zooplankton variability may be predation, for instance by benthic filter-feeders. Whatever the uncertainties remaining about zooplankton grazing in the R. Moselle, it appears that the long-lasting summer decline of the plankton in this river requires another explanation. The high densities of benthic filter-feeders, which are able to ingest particles over a wide size spectrum, might explain the summer depression of both phytoplankton and zooplankton.
Two complementary approaches of phytoplankton community changes and trophic relationships among the river plankton have been investigated: carbon fluxes between different compartments in the R. Meuse; and changes in size and areal surface to volume ratio of the algal units, in an attempt to relate them to adaptive characteristics of algae in the particular river environment.
Modeling of the phytoplankton in the R. Meuse has been performed to confirm our conclusions on the impact of zooplankton grazing on potamoplankton from in situ observations.