Not all species of blue-green algae produce toxins; in fact, most do not. Unfortunately, it is impossible to distinguish harmful from harmless blooms based on appearance. Therefore, it’s safest to assume that all blooms have the potential to be poisonous. We do know the most hazardous toxins, microcystin and anatoxin, cause liver and nervous system injury. Other toxins may cause gastrointestinal damage or skin irritation. Dogs are susceptible to serious illness if they drink or swim in water containing these toxins.