Modelling the impact of benthic filter-feeders on the composition and biomass of river plankton

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) Hydroecologie Appliquee, 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.