Factors affecting the vertical distribution of the zooplankton assemblage in Lake Michigan: The role of the invasive predator Bythotrephes longimanus

We analyze six years of survey data in Lake Michigan, which spanned large ranges in the abundance of the invasive predatory cladoceran Bythotrephes, to quantify the effect of this predator on the daytime vertical distribution of the nine most common species and life stages of Lake Michigan zooplankton. We found that Bythotrephes abundance and hypolimnion depth explained almost 50% of the variation in the vertical distribution of many zooplankton. Bythotrephes abundance was associated with significant and large (approximately 5-11 m) depth increases in cladocerans Daphnia and Bosmina, adult and copepodite stages of cyclopoid copepods, and adult diaptomid copepods Leptodiaptomus minutus, and Leptodiaptomus ashlandi; but did not significantly affect the depth of copepod nauplii, diaptomid copepodites, and adult Leptodiaptomus sicilis. Whereas other environmental factors, such as light attenuation coefficient, epilimnion and hypolimnion temperature, and sampling date significantly influenced the depth of various species and life stages, the inclusion of such environmental factors into linear models did not significantly lower the predicted influence of Bythotrephes. These results suggest that Bythotrephes abundance has a significant and large influence on the vertical distribution of a large component of the zooplankton assemblage in Lake Michigan. We argue that this pattern is driven by a Bythotrephes-induced anti-predator response in zooplankton prey. Such effects could lead to widespread growth costs to the zooplankton assemblage due to the colder water temperatures experienced at greater depths, which could in turn affect the rapidly changing Lake Michigan food web.