Predatory demands of Bythotrephes and Leptodora in Saginaw Bay, Lake Huron

Predatory invertebrates can be a major factor in structuring zooplankton communities. We collected zooplankton and the predatory cladocerans Bythotrephes longimanus and Leptodora kindtii at 4 stations in inner Saginaw Bay, Lake Huron during May-October 2009 and April-November 2010. Production of zooplankton prey was estimated using temperature and biomass based empirical models. Population consumption rates of Bythotrephes and Leptodora were estimated using a combination of conversion efficiency, bioenergetics and experimental based models. Bythotrephes biomass peaked in July of 2009 and 2010, and in 2010, the biomass rebounded in the fall after a late summer decline. Leptodora biomass was generally lower than that of Bythotrephes except in June 2009 and August 2010. Zooplankton biomass, production and composition showed strong seasonal patterns, with biomass more than tripling between May and June each year, largely due to an increase in biomass of Daphnia in June. By contrast, total zooplankton biomass declined by over 70% between June and July owing to declines in Daphnia biomass. Population consumption by predatory cladocerans was a large percentage or even exceeded prey production during July and August in Saginaw Bay regardless of which consumption models were used. This short term (< 2 months) but pronounced predation capacity of predatory cladocerans in Saginaw Bay was mainly due to consumption by Bythotrephes rather than Leptodora. Bythotrephes likely play an important role in food web function and should be accounted for when evaluating the flow of energy within the Laurentian Great Lakes systems.