Algal blooms are caused by a buildup of Nitrogen and Phosphorus in waterways, mostly from water runoff picking up nutrients while flowing over heavily fertilized land, like farms and golf courses. The nutrient increase causes algae to prosper. But with more algae, the water’s oxygen levels drop, and the algae release toxins that can kill off other aquatic life, like fish and plants. Toxins can also poison humans. Coming in contact with the water can cause nausea, vomiting and, in extreme cases, liver failure.