What exactly causes red tides?

The answer

Red tides are caused by dense concentrations of certain dinoflagellate species, particularly Karenia brevis in the Gulf of Mexico, whose pigments can literally turn the water reddish-brown. These organisms produce brevetoxins that become aerosolized by wave action, causing respiratory irritation in humans along the shore. Fish kills during red tides can be massive, with millions of dead fish washing onto beaches. Despite the name, red tides are not related to actual tides and don't always appear red; they can be brown, green, or even invisible. They have been recorded throughout history, and some scholars suggest the biblical plague of blood-red water in the Nile may have been a red tide event.