Impact of the invasive Asian clam, Corbicula spp., on a large river ecosytem

During the past recent decades, Asiatic clams (Corbicula spp.) have spread spectacularly in several large European rivers. In the River Meuse, a transnational lowland river, an important 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 that hypothesis, using clam density data from field surveys, water quality monitoring data, and a simulation model. Estimated Corbicula densities were between 50 and 900 individuals m-2, depending on the site. Using a maximum filtration rate per clam body mass of 0.086 m3 g C-1 day-1 at 20°C derived from the literature, we ran simulations with a non-stationary model to estimate the impact of the exotic bivalve on the river plankton and water quality. In the stretches where the invasive clams developed best, comparison with a clam-free scenario allowed estimating a 70 % phytoplankton biomass loss due to their filtration, leading to a 61 % loss of annual primary productivity. Model simulations also showed that zooplankton may have suffered as much as 75 % loss in terms of biomass. The simulations also point to substantial effects of the invasive Corbicula on the river oxygen budget and on nutrient cycling. We expect that, in the heavily regulated sectors of the river, the loss of planktonic production due to the invasive filter-feeders will negatively affect other suspension feeders and alter ecosystem function and productivity.