Excessive nutrient inputs into a water body induce biological, chemical and physical changes in aquatic plant and animal communities, often leading to oxygen depletion, especially in deeper waters. Greenish, murky water is typically the first sign of eutrophication as microscopic plants and algae grow profusely, consuming the surplus nutrients. But algae are short-lived, and when they die they sink to the bottom of the sea, where their decomposition uses up oxygen. The worst affected areas can become completely anoxic. In the absence of oxygen, decomposition can release toxic hydrogen sulphide, poisoning organisms and making the sea-bed lifeless.
