Originally posted by Meadowlark:
In salt water, there is a dreaded "red tide" of algae bloom that sometimes appears without warning to kill every fish in its path. It kills supposedly by DO crash. I have never believed that.
Your instincts are good. According to a National Geographic (IIRC!)program I saw, red tide is very toxic, and kills by poisoning. It's so toxic that humans can be sickened by eating fish and shellfish caught within the blooms. Really bad stuff! It's not impossible that red tide has a fresh water analog(s) that hasn't yet been observed. Toxins can be subtle, and hard to find if researchers don't know exactly what they are looking for.
