Salinity-driven shifts in cyanobacterial toxin concentrations and profiles in prairie lakes

While harmful cyanobacterial blooms are typically linked to nutrient enrichment, the influence of salinity on cyanobacterial toxin occurrence, concentration, and composition across inland waters remains poorly understood. We surveyed >100 Canadian prairie lakes spanning freshwater to hypersaline conditions and quantified cyanobacterial biomass (phycocyanin), community structure (FlowCAM imaging), and congener-resolved toxin profiles (LC-MS/MS). Toxin concentrations varied nonlinearly across the specific conductivity (SpC) gradient, with a first toxin maximum near 1000 µS cm⁻¹ where microcystin concentrations approached ∼400 µg L⁻¹, and a second maximum near 10,000 µS cm⁻¹ where total toxin concentrations approached ∼2000 µg L⁻¹. The lower-SpC maximum coincided with shifts in cyanobacterial community structure and increasing relative abundance of Microcystis near the freshwater-subsaline transition, whereas the upper maximum occurred within the hyposaline range where halotolerant Nodularia emerged and toxin composition shifted from predominantly microcystin profiles toward mixed microcystin + nodularin assemblages. In contrast, cyanobacterial biomass and toxin concentrations declined sharply at higher SpC, and toxins were not detected in mesosaline or hypersaline lakes (> 30,000 µS cm⁻¹). Although nutrients and dissolved organic carbon increased strongly with SpC, toxin concentrations did not track these variables monotonically, indicating that ecological reorganization across the SpC gradient was not captured by biomass or nutrients alone. Collectively, the results suggest that SpC acts as an ecological filter that reorganizes cyanobacterial community structure and toxin composition across inland waters and support the incorporation of a broader toxin characterization, including nodularin, into monitoring and forecasting frameworks for salinity-affected lakes under intensifying hydroclimatic variability.