Causes and Consequences of Cladoceran Dynamics in Lake Michigan: Implications of Species Invasion by Bythotrephes

When Bythotrephes cederstroemi (Crustacea: Cladocera) invaded Lake Michigan, large changes occurred to the indigenous zooplankton community of the lake. Abundances of three Daphnia species declined precipitously in 1987 on a lake-wide scale. Subsequently, offshore Lake Michigan became dominated by the single species, D. galeata mendotae, that has been able to persist in the presence of the new predator. Although planktivory by alewife or other fish has been proposed as the cause for these changes, species changes and size frequency distributions of the surviving Daphnia contradict that hypothesis. The Daphnia community changed in a way consistent with removal of the smallest individuals present, an attribute of invertebrate planktivory, and not with the known predation patterns of Alosa or Coregonus. Moreover, the increased relative abundance of very large specimens of D. pulicaria (> 2.5 mm body length) during 1987 and 1988 suggests that planktivory by fish was relaxed. Observational data support the conclusion that Bythotrephes, rather than fish, changed the plankton community of Lake Michigan.