Eutrophication

Eutrophication is the process by which nutrients build up in a body of water and spur excessive growth of algae and other organisms, growth that can in turn deplete the water's dissolved oxygen. It occurs naturally over long periods as sediment and nutrients from dissolved phosphate minerals and decomposing plant matter accumulate, but cultural eutrophication caused by human activity, chiefly agricultural runoff carrying fertilizer and animal waste, sewage and industrial discharge, and atmospheric deposition from burning fuel, speeds the process far beyond its natural pace; phosphorus is usually the limiting nutrient in fresh water, while nitrogen plays that role at sea. The resulting algal blooms block sunlight from reaching organisms living on the bottom, and as the algae respire at night and decompose, they consume dissolved oxygen, a chain of effects that can kill fish, invertebrates, and other bottom-dwelling life, create anaerobic dead zones empty of most life that needs oxygen, and open the water to invasive species able to exploit the sudden abundance of nutrients.