The spatiotemporal distribution of potential saxitoxin-producing cyanobacteria in western Lake Erie

Cyanobacterial blooms in the western basin of Lake Erie have been well studied with a focus on planktonic Microcystis and the cyanotoxin microcystin, but recent research has shown that blooms are not entirely Microcystis. Previous studies have documented other taxa in blooms capable of producing other cyanotoxins. Furthermore, benthic cyanobacteria have historically been overlooked in Lake Erie. Saxitoxin is a cyanotoxin of emerging concern in freshwater, and the sxtA gene which encodes its production has been found in the Maumee River and central basin of Lake Erie. Collectively, these points indicated that saxitoxin-producing cyanobacteria may also occur in the western basin. We utilized three sources of data to determine the spatial and temporal distribution of potential saxitoxin-producing cyanobacteria in the water column (years 2018-2022) and deployed nutrient diffusing substrata (NDS) to determine the impact of nutrients, depth, and season on potential-STX producing benthic cyanobacteria (years 2018 & 2019). The water column datasets showed that "hotspots" of sxtA lasted only a few weeks. sxtA gene copies per mL did not correlate with Dolichospermum or Aphanizomenon biovolume, which have been associated with sxtA elsewhere. In the NDS, saxitoxin (ng/cm²) and cyanobacteria chlorophyll were inversely correlated with the highest saxitoxin in September and at the deeper depth, whereas cyanobacteria chlorophyll was highest during June and at the shallower depth. This research suggests continued monitoring is needed to determine drivers of saxitoxin in the western basin, and we recommend that future Lake Erie cyanobacteria research should not solely focus on microcystins and planktonic blooms.