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The EPA water quality criteria state that phosphates should not exceed .05 mg/l if streams discharge into lakes or reservoirs, .025 mg/l within a lake or reservoir, and .1 mg/l in streams or flowing waters not discharging into lakes or reservoirs to control algal growth (USEPA, 1986). Surface waters that are maintained at .01 to .03 mg/l of total phosphorus tend to remain uncontaminated by algal blooms.

It is primarily caused by the leaching of phosphate or nitrate containing fertilisers into lakes or rivers. Some algae and blue-green bacteria thrive on the excess ions and a population explosion known as algal bloom occurs. This growth is unsustainable, however. Perhaps because another nutrient becomes limiting, massive death among the bacteria happens, and they float to the surface.

Oxygen is required by all respiring animals in the water and it is replenished by photosynthesis of green plants. The oxygen level is already low because of the population explosion and further oxygen is taken up by microorganisms which feed off the dead algae. The layer of dead algae also forms a surface layer through which little light can pass, meaning the rate of photosynthesis decreases and the oxygen level becomes depleted. Due to this, fish suffocate.

Freshwater algal blooms are the result of an excess of nutrients, particularly phosphorus.[1] The excess of nutrients may originate from fertilizers that are applied to land for agricultural or recreational purposes, these nutrients can then enter watersheds through water runoff.

When phosphates are introduced into water systems, higher concentrations cause increased growth of algae and plants. Algae tend to grow very quickly under high nutrient availability, but each alga is short-lived, and the result is a high concentration of dead organic matter which starts to decay. The decay process consumes dissolved oxygen in the water, resulting in hypoxic conditions. Without sufficient dissolved oxygen in the water, animals and plants may die off in large numbers.