CHANGES IN PHYTOPLANKTON COMMUNITY STRUCTURE DURING THE ZEBRA MUSSEL (DREISSENA-POLYMORPHA) INVASION OF THE HUDSON RIVER (NEW-YORK)

We analyzed differences in the cell density and taxonomic composition of phytoplankton assemblages in the tidal, freshwater portion of the H udson River to assess the impact of the recent zebra mussel invasion. In order to assess this change, phytoplankton genera were identified a nd counted during two representative periods, 1987-88 (before zebra mu ssel invasion) and 1993-94 (after invasion) and major groups in 1995 a nd 1996. Average cell density of phytoplankton declined similar to 17- fold from 25.6 X 10(6) cells l(-1) before invasion to 1.5 X 10(6) cell s l(-1) after invasion. Dominance of the phytoplankton (by cell densit y, assessed on an annual scale) shifted from cyanobacteria before the invasion to diatoms during the invasion; cyanobacterial density decrea sed 778-fold and diatoms decreased by a factor of only 2.5. Samples fr om 1995 and 1996, counted only to class, confirm the patterns from the more detailed counts in 1993 and 1994: a major decline in cell number s and a dramatic decline in cyanobacteria relative to diatoms. The tax onomic composition of assemblages based on relative abundances of both genera and divisions showed significant shifts from colonial and unic ellular cyanobacteria to large, colonial or benthic diatoms. Also, the large decline in cyanobacteria and some diatom genera, which had been abundant in late summer, dampened the seasonal variation in the taxon omic composition of phytoplankton following the zebra mussel invasion.