Patterns and drivers of seasonal succession and vertical distribution in the rotifer community of Lake Ontario in 2018

Rotifers are among the most abundant zooplankters in lakes yet are often overlooked, and limited information is available on their seasonal and spatial distribution within the Laurentian Great Lakes. Herein, we present data on the seasonal succession of the Lake Ontario rotifer community, with samples collected from April to October 2018 as part of the bi-national inter-agency Cooperative Science and Monitoring Initiative. All sites had epilimnetic samples, but several sites included meta- and hypolimnetic samples. Prior to and during stratification, rotifers were most abundant in the epilimnion and differences in community composition with depth were minor. Mean epilimnetic rotifer density peaked in August (average 330 L⁻¹) when surface water temperature was highest. Seasonal succession included a spring dominated by Synchaeta to the co-dominance of Conochilus and Keratella in early summer and the eventual dominance of Keratella during late summer through fall. Model selection found temperature and Bythotrephes to be key factors influencing overall rotifer abundance as well as genera-specific abundances. The best models for specific genera varied and included Secchi depth, chlorophyll, Cercopagis, Diacyclops, and Daphnia. Rotifers reached their highest proportion of total zooplankton epilimnetic biomass in early summer at 11 %. Analysis of seasonal patterns in 2018 suggest rotifer data collected by the Great Lakes biological monitoring program in April and August represents spring and summer through fall communities but not the June community which was different from other seasons.