A biological regime shift in the Bay of Quinte ecosystem (Lake Ontario) associated with the establishment of invasive dreissenid mussels

Thirty-two biological variables (taxonomic and/or functional groups) representing the four major communities, phytoplankton, zooplankton, benthos and fish, characterizing the upper Bay of Quinte (Lake Ontario) ecosystem, were assembled for the 27-year period, 1982–2008. Coincident regime shifts were detected in phytoplankton, benthos, and fish in 1995, which was just after invasive zebra mussels (Dreissena spp.) became established in the bay in 1993–1994. Two independent methods were used to detect these shifts: 1) principal components analysis followed by a Regime Shift Detector test for a change point in the running mean of the first principal component scores and (2) measurements of significant difference between pre- and post-Dreissena ecosystem structure based on measures of Bray–Curtis community similarity. Although a statistically significant shift was not detected in the zooplankton community by itself, zooplankton variables were instrumental in the overall ecosystem shift, determined for the combined four communities. All 32 variables were ranked for their individual contribution to the difference between the pre- and post-Dreissena ecosystem structures. The resolution of two distinct ecosystem structures, pre- and post-Dreissena, was greatly improved after employing a novel method of variable optimization that involved a selective and sequential removal of variables contributing least to the statistical difference between pre- and post-Dreissena ecosystem structures. The resultant 20-variable subset defined a 1995 ecosystem regime shift at very high level of statistical confidence (ANOSIM-R = 0.970).

Research highlights
► Invasive dreissenid mussels had become established in the Bay of Quinte by 1993-94.
► Fish, benthos, and plankton communities (32 variables) were monitored during 1982-2008.
► Ranking of variables for contribution to ecosystem change yielded an optimized set of 20 variables.
► The 20-variable subset defined a highly significant regime shift in 1995 (ANOSIM-R = 0.97).