Growth of the harmful benthic cyanobacterium Microseira wollei is driven by legacy sedimentary phosphorous

Models for cyanobacterial harmful algae blooms (cHABs) in fresh waters are usually predicated on the rela-tionship between cyanobacterial ecology and dissolved nutrients, particularly phosphorous. Here we show legacy sediment-associated phosphorous as the primary driver of a benthic cHAB, not phosphorous in the water column. Biogeographical surveys by teams of citizen science volunteers working with the University of South Carolina identified over 200 distinct mats of Microseira wollei in Lake Wateree, SC based on toxin characterization. In sum these were estimated to affect approximately 175 km of the lake's shoreline. This growth occurred under water quality conditions that were near or below the regulatory total maximum daily load for phosphorous and ni-trogen. A series of established predictive models for cyanobacterial biomass growth were applied retroactively to match the measured growth with measured water quality parameters. The only component of the system that successfully predicted microbial biomass was sedimentary phosphorous. Concentrations of the Lyngbya wollei toxins (LWTs) 1, 4, 5, and 6 were determined at multiple sites over an 18-month period and a toxin inventory for the lake was calculated. Toxin profiles between sites differed at the 95% level of confidence, establishing each site as a unique mat. An empirical model of toxin production potential based on sedimentary phosphorous was developed.