Algal blooms occur in many water bodies where there is too much "food"
Harmful Algal Blooms
USGS scientists identified potentially harmful levels of cyanobacteria in lakes they sampled in Illinois during the drought of 2012. Toxins produced by the cyanobacteria were measured at harmful concentrations in four lakes. Cyanobacteria, also called blue-green algae, are capable of generating toxins (cyanotoxins), such as microcystin, that are potent enough to poison animals and humans.
Problems with excess levels of nitrogen in the environment
Excess nitrogen can harm water bodies
Excess nitrogen can cause overstimulation of growth of aquatic plants and algae. Excessive growth of these organisms, in turn, can clog water intakes, use up dissolved oxygen as they decompose, and block light to deeper waters. Lake and reservoir eutrophication can occur, which produces unsightly scums of algae on the water surface, can occasionally result in fish kills, and can even "kill" a lake by depriving it of oxygen. The respiration efficiency of fish and aquatic invertebrates can occur, leading to a decrease in animal and plant diversity, and affects our use of the water for fishing, swimming, and boating.