Originally Posted by Buc210
Biologists do say that Red Tide is a naturally occurring Algae but human activities (fertilizers, sewage, etc) exacerbate it.
Correct. They are a naturally occurring phenomenon. For example (there are many possible scenarios), when there is a period of no wind
and high insolation after a previous period of intense rainfall. The rain carries high concentration of nutrients by runoff into the shallow coastal areas. Dead calm produces vertical stratification in the water column. Nutrients concentrate in the upper layers. Red tides are most often produced by dynoflagelates. These are a group of unicellular algae that have one or more flagellae (like a sperm cell). This means they can swim quite effectively. They concentrate in the upper layers of the water column, making use of available nutrients and sunlight to reproduce, and out-competing most other phytoplankton species.
Problem with this, is that they deplete the dissolved oxygen in the water, causing mass mortality of fish and crustaceans. Some species also produce toxins. The discomfort from being by the sea during a red tide (pic of masked guy posted posted by DeepFrz), is not caused by toxins, but by -most likely- ammonia. Ammonia is a byproduct of the bacterial metabolism of nitrous compounds (all those fish and crabs, and much more decomposing on the bottom). Low concentration of ammonia on sea spray are still very irritating. Dynoflagellate toxins are too low in concentration in the water, even if you are swimming in it or swallow a bit of it. What is not recommended, is to eat shellfish (oysters, mussels, prawns) during or immediately after a red tide. Most shellfish are filter feeders. They accumulate the toxins, without harm to themselves, in their tissues, at concentrations that land us in the hospital.
But the phenomenon in natural, albeit exacerbated by organic pollution. If it is not caused by dynoflagellates but by phytoplankton green algae, it's called simply and algal bloom. It happens in all oceans above the 30th parallel in spring and early summer.