It's called nutrient pollution, which might seem to be something of an oxymoron. After all, nutrients such as nitrates and phosphates are essential to all living organisms and play a vital role in the functioning of life on Earth. And these are having devastating effects.
Algal blooms are natural phenomena, and an integral component of marine productivity.
Blooms of another toxic species, Gymnodinium breve, have been implicated in the deaths of humpback whales in 1987 and Florida manatees in 1982 and 1996. Some algal species bloom in such large quantities that they outstrip the ability of predators to feed on them. Huge numbers of dead algae then sink to the bottom, where they are decomposed by bacteria which consume oxygen in the water. Eventually, oxygen supplies in the water column and in sediments become depleted, choking resident marine life and causing mobile species to flee the area. In the Baltic Sea, the same phenomenon has led to the virtual extinction of bottom-dwelling animal life over an area of about 42,000 square miles.