Does salt affect nitrate toxicity?
Yes, there may be some evidence for this. Westin (1974) has shown that nitrate is more toxic in brackish water than in freshwater. New research on nitrate toxicity in freshwater and marine conditions has been published in Chemosphere. This suggests that the main toxic effect of nitrate is due to its ability to convert oxygen-carrying pigments to forms that are incapable of carrying oxygen. It says that nitrate is technically less toxic in saltier water.
