Univariate Step-trend and Multivariate Assessments of the Apparent Effects of P Loading Reductions and Zebra Mussels on the Phytoplankton of the Bay of Quinte, Lake Ontario

The purpose of this work was to identify the apparent effects on the phytoplankton of the Bay of Quinte (northeastern Lake Ontario) of two environmental alterations—a point-source phosphorus loading reduction in the late 1970s and the establishment of invading zebra mussels, Dreissena spp., about 1995. After the P loading reduction, there were significant declines in biovolume of total phytoplankton (−51%), Chlorophyceae (−66%), Dinophyceae (−58%), Bacillariophyceae (−56%), Cryptophyceae (−52%), and Cyanophyceae (−26%). The cyanophyte decline was attributed mainly to non-nitrogen-fixing species. Two dominant diatom genera, Aulacoseira (Melosira) and Stephanodiscus were 53 and 78% lower, respectively. In contrast, after Dreissena spp. establishment, total phytoplankton, Bacillariophyceae, Chrysophyceae, and Cryptophyceae biovolumes did not change significantly; however, total Chlorophyceae, Cyanophyceae, and Dinophyceae showed declines of 42, 51, and 55%, respectively. There were major declines in the diatoms Stephanodiscus (−82%), Synedra (−84%), and Tabellaria (−98%) after the arrival of zebra mussels. Average biovolume of the cyanophyte Microcystis (a potential “bloom”-forming toxin producer) increased 13-fold after the establishment of zebra mussels. Multivariate analyses (ANOSIM tests of within-vs between-group differences), based on phytoplankton community structures defined by the quantitative contributions from 20 phytoplankton taxa, revealed statistically significant differences among communities representing pre- and post P control and pre-and post Dreissena conditions. The shifts in phytoplankton community composition between the identified time periods were attributed to several different taxa, whose relative contributions to the inter-community dissimilarities differed between the upper and lower bay stations.