Nearshore and embayment zooplankton community structure in Lake Michigan, implications for invasion by bigheaded carps

Zooplankton communities in Lake Michigan's nearshore and embayment regions are important food sources for planktivorous and larval fish. Geographically extensive surveys of nearshore zooplankton abundance and composition are crucial for understanding spatial patterns in community structure and Lake Michigan's food web. Over 400 zooplankton samples collected in 2013 and 2014 from 25 locations in Illinois, Michigan, and Wisconsin harbors, open waters, and drowned river mouth lakes of Lake Michigan were used to identify spatial and temporal patterns in zooplankton assemblages. Special focus was placed on the accurate collection of small-bodied taxa such as rotifers, copepod nauplii, and dreissenid veligers to explore the contribution of smaller taxa to total zooplankton biomass. Multivariate analysis identified zooplankton biomass and community structure in Green Bay, Wisconsin and Michigan drowned river mouth lakes as distinct from Illinois and Wisconsin harbor and nearshore areas. Our zooplankton biomass estimates were then applied to an existing bioenergetics model for bigheaded carps (Hypophthalmichthys nobilis and H. molitrix) to quantify their predicted growth, as a proxy for successful establishment in Lake Michigan. Locations where bigheaded carps were predicted to grow were consistent with patterns in zooplankton community structure and were restricted to a handful of sites in Green Bay and Michigan drowned river mouths. These results emphasize the importance of embayment zooplankton communities and small-bodied zooplankton as energy sources for both native and non-native fishes in an increasingly oligotrophic Lake Michigan.