An algal bloom is a sudden increase in the population of algae in a freshwater or marine habitat. The bloom generally looks like a scum on the surface of the water and may colour it blue-green, red, or yellow, depending upon the species of algae involved. An algal bloom is caused by enrichment in the nutrient content of the water known as eutrophication. High levels of nitrogen and phosphorus in atrophic waters encourage the growth of algae.
Algal blooms have various impacts on freshwater and marine ecosystems. First, they make the water cloudy and reduce the depth to which sunlight can penetrate the surface layers. Their sheer number may irritate and block the gills of fish.
Algal blooms and eutrophication tend to increase the population of oxygen-consuming bacteria called decomposers that feed on dead algae. The presence of such organisms increases the biochemical oxygen demand (BOD) of the water, a measure of the amount of dissolved oxygen they consume. The higher the BOD, the less dissolved oxygen is available for other organisms, so it is a measure of pollution.