Trends in Lake Erie phytoplankton biomass and community structure during a 20-year period of rapid environmental change

Abstract

Since the 1990s, Lake Erie has experienced resurgent eutrophication due in part to climate change-driven increases in precipitation, which have combined with increasingly intensive agricultural practices in the region to produce excessive nutrient runoff into the lake. Harmful blooms of the cyanobacterium Microcystis aeruginosa (“Microcystis”) in Lake Erie’s western and central basins (WB and CB, respectively) have been a highly visible consequence of this eutrophication, however few studies have characterized intra- or interannual trends in less abundant, though likely more edible, phytoplankton taxa over the last 25 years. Here, we used the 20-year Lake Erie Plankton Abundance Study (LEPAS) dataset to quantify intra- and interannual trends in the dynamics of six major phytoplankton groups in the WB and CB during 1995–2015. Cyanobacteria biomass in the WB increased >1000-fold during this period, while biomass of all other major taxa groups increased between 10- and 100-fold. Early summer (June–July) and spring (May) communities saw more modest directional change in the biomass of both edible and less-edible taxa as well as community structure. Around 2008, the CB also began to experience Microcystis blooms concurrent with those in the WB, with similar, though less dramatic consequences for phytoplankton community structure and edible biomass. The biomass of several phytoplankton groups exhibited intra-annual oscillations with a ∼5-year period. The mechanisms underlying changes in the phytoplankton community structure and their consequences for higher trophic levels are not well understood, however increases in edible phytoplankton may be sustaining long-term upward trends in many zooplankton taxa.