Historical trophic position of Limnocalanus macrurus in Lake Michigan

In Lake Michigan, oligotrophication and invasive species proliferation have led to dramatic changes in the planktonic food web. Limnocalanus macrurus is a native, glacial relict predatory copepod whose trophic role within the zooplankton community may help us better understand the dynamics behind those changes. We applied nitrogen isotope spectrometry to zooplankton from Lake Michigan to quantify the yearly and seasonal position of Limnocalanus and other taxa in the historical Lake Michigan planktonic food web. We found that Limnocalanus was positioned about one-half trophic level above Bythotrephes and about two levels above Daphnia during the summer in 1989, 1993, 1995 and 1997. It was unlikely that adult Limnocalanus encountered Bythotrephes during summer months because of vertical segregation during thermal stratification. Instead, Limnocalanus probably had greater access to copepod prey such as Diaptomus and Epischura. Limnocalanus became isotopically lighter seasonally in relation to Daphnia and either shifted its diet to one consisting of more phytoplankton, or its prey (e.g., Diaptomus) shifted to a more phytoplankton food base, thus indirectly resulting in decreased trophic status of Limnocalanus. This study serves as a historical, foundational basis for zooplankton food web relations in Lake Michigan that complements similar investigation in Lake Huron. Comparing the historical to the recent zooplankton food web may now elucidate how invasive species such as Bythotrephes and quagga mussels have altered zooplankton communities and bioenergetic relationships within the Great Lakes.