Phytoplankton assemblage structure, drivers, and thresholds with a focus on harmful algal bloom ecology in the Lake Okeechobee system, Florida, USA

Untangling the complexities of harmful algal bloom (HAB) dynamics is an ongoing effort that requires a fundamental understanding of spatiotemporal phytoplankton patterns and the environmental filters through which assemblages are structured. To this aim, monthly field surveys were conducted from 2019 to 2021 at 21 sites in Lake Okeechobee, Florida – a large, shallow, eutrophic, and heavily managed lake with coastal connectivity that experiences intense and recurrent HABs. Phytoplankton assemblages were strongly spatially structured forming 7 distinct lake zones with significant dissimilarity in composition and total abundance. While successional patterns were not apparent across seasons or wet/dry periods, total phytoplankton abundance was significantly greater towards the end of the wet season. Distance-based linear models using 16 abiotic variables were used to identify significant explanatory variables of spatial and temporal patterns. The spatial model explained 93 % of the variability suggesting deterministic processes largely control spatial patterns. The temporal model explained only 48 % of the temporal variability suggesting stochasticity in lake-wide shifts in assemblages over time. However, the strong spatial structuring of assemblages may preclude lake-wide succession patterns. Total algal abundance metrics were inversely related to nitrate, orthophosphate, and total alkalinity, the strongest explanatory variables of assemblage patterns, suggesting a lag between peak resources and peak abundance as phytoplankton cycle “boom-to-bust” phases. Consistent with this inverse relationship, Threshold Indicator Taxa Analysis returned almost exclusively negative responder indicator taxa for all three explanatory variable gradients. The assemblage-level threshold defined the gradient boundary between boom- and bust-associated indicator taxa. These data contribute novel information about HABs ecology pertinent to management strategies.