Tracking continued change in offshore zooplankton in the three basins of Lake Erie, 1997-2022

Abstract

In contrast with the deeper Great Lakes, Lake Erie has become more eutrophic with intense summer cyanobacterial blooms. It has also been an active site for new species introductions.  It is important to evaluate how these changes could be impacting the zooplankton community in Lake Erie.  Changes at this intermediate trophic level could affect both the grazing control of zooplankton and the production of fish.  Long-term Great Lakes National Program Office (GLNPO) zooplankton monitoring employs April and August sampling of several routine offshore sites in each of the three basins.  In 2019, Cooperative Science and Monitoring Initiative (CSMI) added seasonal collections between April and August and into September to track the seasonal development of the zooplankton community in each basin. We use these data to characterize the unique zooplankton community of each of the three basins seasonally in 2019 and over time from 1998 to 2022.  Unexpectedly, calanoid copepods are becoming a larger component of the zooplankton community over time despite the general association of calanoids with lakes of lower algal productivity.  Summer levels of small bosminid grazers and Daphnia retrocurva have steadily decreased in the central basin, likely due to predation by the invasive predatory cladoceran Bythotrephes longimanus since its introduction in 1985.  Although overall zooplankton biomass has been maintained, the species composition is shifting toward larger species, consistent with the impact of this nonnative invertebrate predator.  Although fish visual predation selects larger individuals, the loss of the smaller cladocerans could exert a competitive effect on fish.