Oysters eat silt, in addition to the phytoplankton that drift in the currents. As they eat, they also ingest some of the bacteria and organic compounds contained in sewer overflow, which Jay said runs untreated into the Charles from houses and streets during heavy rains.
So the Massachusetts Oyster Project dropped 150,000 oysters into the Charles River Basin around Constitution Marina this past weekend.
Oysters may be able to offset point pollution sources such as partially treated sewage coming out from Combined Sewage Overflows (CSOs). The number, flow, and impact of these has been reduced dramatically by the work of the MWRA and the cleanup of Boston Harbor. But it will be difficult to totally eliminate CSOs entirely.