Abstnct-^The biomass and production of the phytoplanlcton in a relatively unpoUuted leach of the River Meuse (Belgium) were foUowed toough two yean (1983 and 1984). Chlorophyll a varied from 0.2 to about 120 mg m"', and production ranged between 0.05 and 5.78gCm~'d~'. The mean photosynthetic quotient (PQ) was 1.25. The parameters of the lightphotosynthesis relationship (P^ and 1^) were calculated and related to the variations of température and light in the waier column. A simple mode! allowed calculations of the annual production, which was estimated to be 494gCm~'yr"' in 1983 and 547gCm"'yr"' in 1984. Finally, a simple mode! is developed, which explains the relationship between phytoptânkton devei opment and discharge: this mode! shows how the effect of discharge can be described by a "dUution rate" of the plankton growing in the river water.

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1. Over the past few decades, Asiatic clams (Corbicula spp.) have spread spectacularly in several large European rivers. In the River Meuse, a transnational lowland river, a substantial chlorophyll a decline has been recorded since the mid-2000s, which seems to be related to the invasion by these exotic bivalves. This study aimed at verifying this hypothesis, using data on clam density from field surveys, water quality monitoring data and a simulation model. 2. Corbicula density was estimated at between 50 and 900 m À2 , depending on the site. Assuming a maximum filtration rate per clam body mass of 0.086 m 3 g C À1 day À1 at 20°C, derived from the literature, we ran simulations with a non-stationary model to estimate the impact of the bivalve on the river plankton and water quality. 3. In the stretches where the invasive clams were most abundant, we estimated a 70% loss of phytoplankton biomass, due to their filtration, and a 61% decline in annual primary production compared with a situation without clams. Model simulations also showed that zooplankton may have suffered as much as a 75% loss of biomass. 4. The simulations also point to substantial effects of Corbicula on the river oxygen budget and on nutrient cycling. We suggest that, in the heavily regulated sectors of the river, the loss of planktonic production due to these invasive filter-feeders negatively affects other suspension feeders and alters ecosystem processes and productivity.

SUMMARY 1. The POTAMON model [Everbecq E. et al. (2001) Water Research, 35, 901] has been used to simulate the effect of benthic bivalves (mainly Dreissena polymorpha) on the phytoplankton and zooplankton in a lowland Western European river (the Moselle). Here we use a modified version of the POTAMON model with five categories of phytoplankton (Stephanodiscus, Cyclotella‐like, large diatoms, Skeletonema and non‐siliceous algae) to model filter‐feeding effects of benthic bivalves in the Moselle. Zooplankton has been represented in the model by two categories, Brachionus‐like and Keratella‐like rotifers. 2. According to density estimates from field surveys (Bachmann V. et al. (1995) Hydroécologie Appliquée, 7, 185, Bachmann V. & Usseglio‐Polatera P. (1999) Hydrobiologia, 410, 39), zebra mussel density varied among river stretches, and increased through the year to a maximum in summer. Dreissena filtration rates from the literature were used, and mussels have been assumed to feed on different phytoplankton categories (but less on large and filamentous diatoms) as well as on rotifers. 3. The simulations suggest a significant impact of benthic filter‐feeders on potamoplankton and water quality in those stretches where the mussels are abundant, their impact being maximal in summer. Consequently, different plankton groups were not affected to the same extent, depending on their period of development and on indirect effects, such as predation by mussels on herbivorous zooplankton. 4. A daily carbon balance for a typical summer shows the effect of benthic filter‐feeders on planktonic and benthic processes: the flux of organic matter to the bottom is greatly enhanced at high mussel density; conversely, production and breakdown of organic carbon in the water column are reduced. Mussel removal would drive the carbon balance of the river toward autotrophy only in the downstream stretches.

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