Effects of the zebra mussel (Dreissena polymorpha) invasion on the macrobenthos of the freshwater tidal Hudson River

Abstract

To assess the effect of the zebra mussel (Dreissena polymorpha) invasion on benthic animal communities, we monitored the macrozoobenthos at eight sites in the freshwater tidal Hudson River in 1990-1995. Zebra mussels were absent or scarce in the Hudson River before September 1992, but abundant (mean 17 000/m2) on hard substrata in 1993-1995 and responsible for large declines in phytoplankton biomass. All of our monitoring stations had soft sediments, and so had low local densities of zebra mussels (mean 78/m2). Nevertheless, we observed marked changes in the macrozoobenthos at these stations. Sphaeriid clams declined by 67% between 1990-1992 and 1993-1995, but no other group of macrobenthos showed a simple change in density between 1990-1992 and 1993-1995 across all eight monitoring sites. Instead, most taxa showed a strong interaction between zebra mussel impacts and water depth. At deep-water sites, macrozoobenthic density declined by 33% between 1990-1992 and 1993-1995, while at shallow-water sites, density rose by 25%. We suggest that these changes were probably caused by reduced sedimentation of edible particles at deep-water sites combined with increased biomass of macrophytes and attached algae at shallow-water sites.