If you have ever been or lived in a coastal area  it is possible you have heard  of Red tides. Red tides are just a term for toxic algal blooms and its is slang fo harmful agal blooms. In certain conditions alge grows very quickly. But in some cases algal blooms are not toxic but only a few kinds the scienctific term for that species is Karenia brevis and it produces toxins. But not all Karenia brevis turns water red some turn yellow, orange, pink, or brown. That is why sciencetists do not like to use the term "Red tides". But some toxic algal blooms to not even make water a different color but, they are still harmful. When red tides happen they can kill millions of fish and other marine ecoysystems. Even people have been known to get sik by eating shellfish that comsume the alge. But, not all algal blooms are bad some actully benifit marine life by incresing food amount for marine animals.