Trends in Lake Erie zooplankton biomass and community structure during a 25-year period of rapid environmental change

Abstract

Zooplankton play a key role in aquatic ecosystems, providing potential top-down control on phytoplankton and linking primary production to higher trophic levels. Thus, knowledge of zooplankton dynamics is fundamental to any assessment of the impacts of human driven rapid environmental change on lakes. Lake Erie has undergone eutrophication since the 1990s, resulting in summer harmful cyanobacterial blooms, dominated by Microcystis aeruginosa, since approximately 2003 in the western basin (WB) and 2008 in the central basin (CB). The effect of eutrophication on zooplankton in Lake Erie is unclear; few studies have characterized trends in zooplankton biomass and community structure during this period of rapid change. We used the Lake Erie Plankton Abundance Study (LEPAS) zooplankton dataset to analyze interannual trends in the dynamics of eight major zooplankton groups in the WB and CB during 1995–2020. In both basins, we found directional change in zooplankton biomass and community structure. These directional trends in zooplankton biomass overlaid approximate five-year cycles in nearly all taxa, potentially linked to the El Niño Southern Oscillation. Eutrophication was associated with an increase in the summer biomass of total zooplankton, calanoids, and large cladocerans but surprisingly, not cyclopoids, rotifers or small cladocerans. The surprisingly positive or neutral effect of eutrophication and M. aeruginosa on zooplankton biomass may be due to a combination of bottom-up (e.g. concurrent increases in more edible algae) and top-down (e.g. changes in planktivory) forces.