Mesozooplankton grazing patterns and preferences during a cyanobacterial harmful algal bloom (cHAB) in a large eutrophic lake

Mesozooplankton (200–2,000 µm) are an important link between primary producers and higher-level consumers and can influence phytoplankton biomass and community structure via grazing. Yet, during cyanobacteria harmful algal blooms (cHABs) we have a poor understanding of which food resources mesozooplankton use and their grazing pressure on phytoplankton. To investigate this, we conducted two gradient-grazer assays to measure mesozooplankton community grazing rates in western Lake Erie during the 2021 cHAB season. We measured mesozooplankton grazing on various food sources, including: total phytoplankton, phytoplankton major taxa (Cyanobacteria, Cryptophyta/Bacillariophyta, Chlorophyta), and autotrophic and heterotrophic picoplankton and nanoplankton. We used these data to estimate mesozooplankton feeding selectivity, the importance of each food source toward mesozooplankton carbon intake, as well as the percent standing stock and growth consumed by mesozooplankton. Mesozooplankton selected for autotrophic and heterotrophic nanoplankton, were neutrally selective for Cryptophyta/Bacillariophyta, and avoided Cyanobacteria and the total phytoplankton community. Mesozooplankton carbon intake was dominated by autotrophic nanoplankton and Cyanobacteria, indicating these food sources may be important in supporting zooplankton production during these cHAB events, although our understanding of the incorporation of Cyanobacteria carbon into mesozooplankton production is limited. While mesozooplankton grazed a small portion of total phytoplankton (1–12 % d^-1) and Cyanobacteria (5–13 % d^-1) standing stock biomass (<200 µm), they exerted a relatively stronger control on the growth of total phytoplankton (4–38 % d^-1) and Cyanobacteria (64–106 % d^-1), primarily due to the low measured growth rates of these food items during the study. Our results demonstrate that mesozooplankton graze on Cyanobacteria during cHAB events, and under the right conditions influence cHAB dynamics through direct consumption of Cyanobacteria and indirect effects on Cyanobacteria grazers and competitors.