Seasonal and historical food web relationships between the water flea (Daphnia mendotae), the spiny water flea (Bythotrephes longimanus), Limnocalanus macrurus, and Diaptomus spp. in Lake Michigan

Abstract

Limnocalanus macrurus, a large hypolimnetic calanoid copepod, has experienced increased proportional presence in Lake Michigan in recent years. This increase has come at the expense of decreased biomass of many cladoceran species, such as Daphnia mendotae. Introduction of the spiny water flea, Bythotrephes longimanus, is thought to have played a large role in this dominance reversal. The increase in proportional biomass of Limnocalanus is attributed to an increase in overall species abundance, and not an increase in individual body mass of these organisms. What remained unknown was where Limnocalanus was positioned in the historical planktonic food web, and how its position changed seasonally. We applied nitrogen isotope spectrometry to historical zooplankton samples from Lake Michigan. We found that Bythotrephes was positioned about 1.5 trophic levels higher than Daphnia, and that Limnocalanus was about one–half trophic level above Bythotrephes and thus about two levels above Daphnia during the summer. It is unlikely that adult Limnocalanus encounter Bythotrephes during summer months because of vertical segregation during thermal stratification. Bythotrephes became isotopically heavier and Limnocalanus became isotopically lighter from May to August. Limnocalanus either shifts its diet seasonally to one consisting of more phytoplankton in summer, or its prey (e.g., Diaptomus spp.) shifts 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 like Bythotrephes and quagga mussels may have altered the Great Lakes zooplankton communities and bioenergetic relationships within the lakes.