Photo credit: FL Dept. EPA
Blue-green algae is actually not an algae at all. Algae is harmless and part of the food chain, but what is called blue-green algae is a microscopic organism called cyanobacteria. These produce toxins that are indeed harmful to people, especially children and pets, since they are more likely to ingest some water. High levels can last for months, and the toxins can remain in the water up to three weeks after a bloom is no longer visible. These can produce skin rashes, nausea, runny eyes and nose, difficulty breathing, muscle weakness, vomiting, and diarrhea. More severe reactions may include dizziness, fever, headaches, and liver and nervous system damage. If someone comes in contact with the toxins, they should shower and wash their clothes as soon as possible. If camping along such bodies of water, use municipal water supplies, as even boiling will not destroy the toxins.
There are two things that cause cyanobacteria blooms: high levels of phosphorous and nitrogen coupled with warm water. Since identifying and controlling nutrient run-off from surrounding lands is difficult, the first impact on toxin levels will usually occur with falling temperatures and substantial precipitation.