The effect of the Asiatic clam, Corbicula fluminea, on phytoplankton of the Potomac River, Maryland

In the summers of the 1960s and 1970s, phytoplankton in the fresh, tidal Potomac River showed a downstream gradient of low to high abundance. Phytoplankton abundance in a 6–8‐km segment of the river, in summer 1980 and 1981, was 40–60% below that upstream and downstream, forming a “sag.” In 1980, the highest densities of the Asiatic clam Corbicula fluminea were in the same reach as the phytoplankton sag. Filtration rates indicate that the volume of the sag reach could be pumped through the Corbicula population in 3–4 days. Corbicula removed 30% of the phytoplankton chlorophyll a from a river water sample in 2 h. Pheophytin a in surficial sediments correlated well with clam biomass as a consequence of phytoplankton excreted in pseudofeces. The hypotheses that peak discharges, zooplankton, toxic substances, and nutrient limitation induced the sag were not supported.