Harmful algal bloom impacts on foodweb structure in western Lake Erie

Human-driven environmental change has caused harmful algal blooms to increase in aquatic ecosystems worldwide, yet our understanding of their impacts on foodwebs and the fisheries that they support remains incomplete. This information gap is especially conspicuous in Lake Erie (USA-Canada), where harmful algal blooms have been increasing owing to nonpoint source nutrient runoff and climate change. From a summer 2019 field investigation designed to test hypotheses concerning the effects of harmful algal blooms on western Lake Erie's foodweb, we confirmed that nanoplankton and netplankton primary production rates, cyanobacteria biomass, and microcystin, chlorophyll a and turbidity concentrations were higher inside of the blooms than outside, with Secchi disk transparency being ∼3-fold lower inside of these blooms. Harmful algal bloom sites also had higher levels of total and soluble reactive phosphorus, dissolved organic carbon, and silica than non-harmful algal bloom sites, with nitrate and nitrate showing the opposite pattern. Counter to expectations, crustacean zooplankton biomass was higher inside than outside the blooms due to more calanoid copepod and non-daphnid cladoceran biomass inside the harmful algal blooms; Daphnia spp., bosminid, cyclopoid copepod, and predatory cladoceran biomass did not differ between areas. Dreissenid mussel veligers were almost 10-fold lower inside than outside the blooms. Finally, fish community structure was near-identical between harmful algal bloom and non-bloom areas, except for Shiner (Notropis) species, which were more abundant inside. Collectively, our findings indicate that, despite their negative effects on water quality (e.g. toxin production, reduced water clarity), western Lake Erie harmful algal blooms are biologically productive and may not be a trophic dead end as is commonly believed. In discussing these findings, we identify knowledge gaps that can help fishery management agencies better understand how these blooms affect foodwebs and the fisheries that they support.