A red tide, or Algae Bloom
Anthropogenically increased nitrogen and phosphorus input into water is a key cause of dead zones in our oceans, seas, and lakes. This is basically fertilizer ronoff from farms which eventually ends up concentrated in large bodies of water. The best native example being the deadzone which forms at the Mississippi delta at the Gulf of Mexico. This dead zone reaches out for miles and stretches down to Texas.
Eutrophication is a process whereby bodies of water receive an excess of nutrients that promote excessive plant growth. The presence of the nitrogen or phosphorus in the water promotes the growth of algae blumes, which end up dissolving the oxygen in the water, making the area uninhabitable for fish and other ocean life. Unlimately it becomes a dead zone. These dead zones fluctuate seasonally and are promoted by the spring planting seasons. Other factors that adjust the dead zones are flooding and hurricanes.
