Each year we get the Spawning in Spring and each year people cross themselves and mutter about an invasion from Mars.
This phenomenen can either be a desove or Marea Roja; The former is the product of millions of fish releasing their eggs into the water to be fertilised, which is inocuous. The Latter is really algal blooms; i.e., large concentrations of aquatic microorganisms, such as protozoans and unicellular algae.
The biggest difference is that fish spawning might be unpleasant (unless you’re a fish egg) whilst the algal blooms can be toxic, which is why they shut down the mussel farms when it appears.
The production of natural toxins such as brevetoxins and ichthyotoxins are harmful to marine life. As far as humans go, it is only a problem if you consume contaminated shellfish, the chances of which are slight as all shellfish is tested for toxins.
The end result is that it’s best not to swim in it otherwise you’ll end up looking like a baddie out of Stingray… just kidding!
(News: Herradura, Costa Tropical, Granada, Andalucia)