Green waters, red alert: ecologically relevant eutrophic nitrate concentrations cause sublethal effects on crayfish...

Eutrophication, often caused by excess nitrogen, currently affects over 40% of water bodies globally. However, little is known about nitrate's sublethal toxicity on species considered hardy against this stressor, like crayfish. Pollutants can alter behaviour through long-term physiological changes or after short exposures by interfering with stimuli detection. To investigate whether nitrates affect crayfish behaviour, two experiments were conducted whereby crayfish were exposed to ecologically relevant moderate (50 mg NO₃^-/L) and high (150 mg NO₃^-/L) nitrate concentrations for 2 weeks or 30 min. They were then tested for their food-searching response to chemical food cues and their antipredator response to damage-released chemical alarm cues. Nitrates did not interfere with alarm cue responses but instead caused activity reductions in response to both alarm cues and water disturbances after high-level exposures for 2 weeks. Crayfish reduced food searching at both nitrate concentrations after two weeks while 30 min of nitrate exposure only caused effects at high concentrations. These results suggest that nitrates do not interfere with chemical cue detection, and likely elicit effects by decreasing oxygen availability for the musculoskeletal and sensory systems required for food-searching and antipredator behaviours. Furthermore, this is the first study demonstrating that crayfish can be affected by sublethal nitrate toxicity at ecologically relevant levels after exposures as short as 30 min, which could occur with point-source pollutant releases. Crayfish should not be overlooked when considering species vulnerable to eutrophication as it continues to spread, especially as sublethal toxicity can decrease fitness and result in indirect mortalities.