Spatial and seasonal patterns in the crustacean zooplankton community of the St. Marys River

Zooplankton communities of the Great Lakes have received significant attention in light of recent food web changes. No recent attempts have been made, however, to characterize the zooplankton community of the St. Marys River, the sole connecting channel between Lake Superior and Lake Huron. The purpose of this study was to provide new reference information for St. Marys River zooplankton communities and discuss the patterns observed in relation to recent changes in the Great Lakes. To address these objectives, sampling occurred at Lake Superior State University's Aquatic Research Laboratory from 2003 to 2007, and at 9 coastal wetlands in 2005. Compared to historical studies of St. Marys River riverine zooplankton, relative densities of calanoid copepods and daphnids have decreased while cyclopoid copepods and bosminids have increased. Coastal wetland communities continue to be diverse and exhibit similar community structure to previous studies, although Leptodora kindti, historically an abundant nearshore taxa, is now absent. Mechanisms driving differences in present community structure as compared to that noted in past studies of the St. Marys River are unknown but may be explained by the presence of the predatory, invasive zooplankton, Bythotrephes longimanus, local environmental variables, or changes upstream in Lake Superior. Temperature–zooplankton abundance patterns and nearshore community structure of Lake Superior are reflected in the St. Marys zooplankton community; thus, local environmental factors including changes in planktivory and proximity to upstream feeder communities likely play an important role in structuring St. Marys River zooplankton communities seasonally and spatially.