nutrient levels are important in controlling the abundance of many species of algae. the relative abundance of nitrogen and phosphorus can in effect determine which species of algae come to dominate. algae are a very important source of food for aquatic life, but at the same time, if they become over-abundant, they can cause declines in fish when they decay. similar over-abundance of algae in coastal environments such as the gulf of mexico produces, upon decay, a hypoxic region of water known as a dead zone.