Salinity-linked iron stress creates a finite risk window for cyanobacterial toxins in inland lakes

Cyanobacterial blooms are intensifying in shallow, nutrient-rich inland lakes, yet drivers of toxigenicity remain poorly predicted by nutrient-centric frameworks. We surveyed > 100 lakes across the Prairie Pothole Region spanning a broad salinity gradient (using specific conductivity as our salinity proxy, lakes ranged from 10² to > 10⁵ µS cm⁻¹). We measured macronutrients, metals, cyanobacterial biomass, siderophores, and cyanotoxins (ELISA screening with LC-MS/MS confirmation). Thirty-two eutrophic lakes were neither phosphorus- nor nitrogen-limited, indicating alternative constraints on biomass. Across these nutrient-replete lakes, modelled bioavailable free ferric iron (Fe'; expressed as pFe = -log₁₀[Fe³⁺]) exceeded a biological limitation threshold (pFe > 20), implicating iron limitation as a bottleneck to further biomass accumulation. Iron limitation intensified with specific conductivity, consistent with ionic-strength effects that reduce Fe' by shifting speciation and precipitation. Critically, iron-limited lakes showed elevated cyanotoxin potential, identifying a micronutrient-mediated paradox: cyanobacteria can be growth-limited yet increasingly toxigenic as specific conductivity rises. Toxin responses were nonlinear along the iron-stress axis, with a finite "risk window" at intermediate iron stress (pFe approximately 22-26) that is diagnosable from specific conductivity, pFe, and siderophores. These indicators provide a transferable, scalable screening framework to prioritize monitoring and mitigation in salinizing inland waters.