To explain summer declines in phytoplankton biomass in large rivers, we compared the effect of zooplankton grazing on the planktonic algae of two large European rivers, the Meuse and the Moselle. In situ grazing was measured during two years (1994 and 1995), using the Haney method. Total zooplankton community filtration rates recorded in the river Meuse ranged between 1 and 32% of the water volume filtered per day. A drastic algal decline was observed early July both years and may be explained by high densities of a rotifer-dominated zooplankton community (500-700 ind. l 1) with more than 75% of Brachionus calyciflorus. During the summer period in 1994, when grazing was over 20%, edible algal biomass was controlled by a diversified rotifer community (up to 2500 ind. l 1), while a non-edible algal assemblage developed. In contrast, phytoplankton biomass remained comparatively low in the Moselle throughout the low-flow period, as did zooplankton numbers during most of this time (fewer than 200 ind. l 1 during the summer period). The proportion of crustaceans in this zooplankton was rather higher than in the Meuse, and they dominated at times, in biomass as well as in numbers. Nevertheless, measured in situ grazing rates (1-15%) could not explain the low summer algal biomass, even if low filtration rates may at times represent a significant carbon loss for phytoplankton, when and where net algal production was low. As a conclusion, the role of phytoplankton-zooplankton interactions in controlling algal biomass in large rivers is discussed.

The aim of the workshop on rotifers in ecotoxicology was to stimulate discussions on new developments in the field. Discussions about the use of biomolecular tools indicate that gene expression analysis with rotifers should be available in the next few years. Such analyses will be a great asset as they enable ecotoxicologists to study molecular mechanisms of toxicity. Rotifers also appear as useful tools in the risk assessment of pharmaceuticals and their metabolites that find their way into aquatic ecosystems because their sensitivity to some of these substances is higher than that of cladocerans and algae. The nature and extent of the impact of potential endocrine disruptors on aquatic invertebrates is another poorly resolved issue for which rotifers are a promising tool. Indeed, rotifers seem to be particularly sensitive to androgenic and anti-antiandrogenic substances, whereas copepods and cladocerans are typically more affected by estrogens and juvenile hormone-like compounds. Besides their usefulness in these emerging fields of aquatic ecotoxicology, it was emphasized that research with rotifers on basic issues like, e.g., toxicant interference with predation, competition, or interspecific and interclonal variation in ecotoxicological tests is still needed.

The faecal indicator Escherichia coli plays a central role in water quality assessment and monitoring. It is therefore essential to understand its fate under various environmental constraints such as predation by bacterivorous zooplankton. Whereas most studies have examined how protozooplankton communities (heterotrophic nanoflagellates and ciliates) affect the fate of E. coli in water, the capacity of metazooplankton to control the faecal indicator remains poorly understood. In this study, we investigated how the common filter-feeding cladoceran, Daphnia pulex, affects the fate of E. coli under different experimental conditions. Daphnia ingested E. coli and increased its loss rates in water, but the latter rates decreased from 1.65 d-1 to 0.62 d-1 after a 1,000-fold reduction in E. coli initial concentrations, due to lower probability of encounter between Daphnia and E. coli. The combined use of culture and PMA qPCR (viability-qPCR) demonstrated that exposure to Daphnia did not result into the formation of viable but non-culturable E. coli cells. In lake water, a significant part of E. coli population loss was associated with matrix-related factors, most likely due to predation by other bacterivorous biota and/or bacterial competition. However, when exposing E. coli to a D. pulex gradient (from 0 to 65 ind.L-1), we observed an increasing impact of Daphnia on E. coli loss rates, which reached 0.47 d-1 in presence of 65 ind.L-1. Our results suggest that the filter-feeder can exert a non-negligible predation pressure on E. coli, especially during seasonal Daphnia population peaks. Similar trials using other Daphnia species as well as stressed E. coli cells will increase our knowledge on the capacity of this widespread zooplankter to control E. coli in freshwater resources. Based on our results, we strongly advocate the use of natural matrices to study these biotic interactions in order to avoid overestimation of Daphnia impact.

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