The effects of salinity and N : P on N‐rich toxins by both an N‐fixing and non‐N‐fixing cyanobacteria

Abstract Freshwater ecosystems are experiencing increased salinization. Adaptive management of harmful algal blooms (HABs) contributes to eutrophication/salinization interactions through the hydrologic transport of blooms to coastal environments. We examined how nutrients and salinity interact to affect growth, elemental composition, and cyanotoxin production/release in two common HAB genera. Microcystis aeruginosa (non‐nitrogen [N]‐fixer and microcystin‐LR producer [MC‐LR]) and Aphanizomenon flos‐aquae (N‐fixer and cylindrospermopsin producer [CYN]) were grown in N : phosphorus (N : P) 4 and 50 (by atom) for 21 and 33 d, respectively, then dosed with a salinity gradient (0–10.5 g L−1). Both total MC‐LR and CYN were correlated with particulate N. We found Microcystis MC‐LR production and release was affected by salinity only in the N : P 50 treatment. However, Aphanizomenon CYN production and release was affected by salinity regardless of N availability. Our results highlight how cyanotoxin production and release across the freshwater–marine continuum are controlled by ecophysiological differences between N‐acquisition traits