Impact of invasive Asian clams, Corbicula spp., on a large river ecosystem

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. Corbicula density was estimated at between 50 and 900 m^-2, depending on the site. Assuming 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 bivalve on the river plankton and water quality. 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. 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.