Food-Web Responses to Species Invasion by a Predatory Invertebrate: Bythotrephes in Lake Michigan

Several developments in the offshore plankton community accompanied the invasion of Bythotrephes cederstroemi Schoedler (Crustacea: Cladocera: Cercopagidae) into Lake Michigan. A native predatory cladoceran, Leptodora kindti, became significantly reduced in a abundance and biomass in the presence of Bythotrephes. The offshore Daphnia assemblage, which had consisted of three species before the arrival of Bythotrephes, was reduced to dominance hy only D. galeata mendotae. Abundances of Daphnia species exhibited reciprocal relationships to Bythotrephes abundance in both space and time. The surviving Daphnia populations offshore exhibited altered daytime vertical distributions which reduced spatial overlap with the invading predator. Model calculations indicate that energetic requirements by Bythotrephes equaled or exceeded replacement production by Daphnia during midsummer of the first years of species invasion. Decreases in midsummer Daphnia biomass did not produce significant increases in midsummer algal biomass, measured as particulate chlorophyll a. Instead, physical mixing depth and epilimnetic temperature, a correlate of the intensity of density stratification, exhibited the strongest statistical relationship to Chl a over the years studied.