Changes in phytoplankton community structure during the zebra mussel (Dreissena polymorpha) invasion of the Hudson River (New

Abstract We analyzed differences in the cell density and taxonomic composition of phytoplankton assemblages in the tidal, freshwater portion of the Hudson River to assess the impact of the recent zebra mussel invasion. In order to assess this change, phytoplankton genera were identified and counted during two representative periods, 1987-88 (before zebra mussel invasion) and 1993-94 (after invasion) and major groups in 199S and 1996. Average cell density of phytoplankton declined-17-fold from 25.6 x 106 cells H before invasion to 1.5 x 106 cells I"1 after invasion. Dominance of the phytoplankton (by cell density, assessed on an annual scale) shifted from cyanobacteria before the invasion to diatoms during the invasion; cyanobacterial density decreased 778-fold and diatoms decreased by a factor of only 2.5. Samples from 1995 and 1996, counted only to class, confirm the patterns from the more detailed counts in 1993 and 1994: a major decline in cell numbers and a dramatic decline in cyanobacteria relative to diatoms. The taxonomic composition of assemblages based on relative abundances of both genera and divisions showed significant shifts from colonial and unicellular cyanobacteria to large, colonial or benthic diatoms. Also, the large decline in cyanobac-teria and some diatom genera, which had been abundant in late summer, dampened the seasonal variation in the taxonomic composition of phytoplankton following the zebra mussel invasion