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.

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Qualitative and quantitative aspects of the phytoplankton of the River Meuse were studied during 1992, at a point 537 km from the source. The phytoplankton was dominated by diatoms and green algae. The Stephanodiscus hantzschii-group was especially prominent. Other important taxa were Cyclotella meneghiniana, small Cyclotella and Thalassiosira, Aulacoseira ambigua and Nitzschia acicularis. Cell abundances varied from less than 1000 units mll to about 25 000 -30 000 units ml-during the blooms. The Stephanodiscus hantzchii-group constituted almost entirely the first spring bloom. During the summer period, small Thalassiosiraceae developed markedly and large Thalassiosira weissflogii appeared. During this period, green algae dominated diatoms as expressed in cell abundances. The main Chlorococcales were Scenedesmus quadricauda, Scenedesmus div. sp., Dictyosphaerium ehrenhergianum and Pediastrum duplex. Dinophyceae contributed a significant biomass during the summer period. Total biomass varied between 100 and 3 650 ttg Cl ' . As previously observed (Descy, 1987), the factors regulating the phytoplankton growth were clearly physical variables: discharge, temperature and irradiance. However, in the summer period, low abundances might indicate a regulation by biotic factors. The impact of grazing by zooplankton is discussed, on the basis of observations of zooplankton development in the River Meuse and on the basis of simulation by a mathematical model. A comparison is carried out with recent data of phytoplankton in large European rivers.