Algal bloom

Algal blooms are sudden spurts of algae growth caused by greatly increased amounts of phosphorus or nitrogen entering an aquatic ecosystem from either sewage systems or agricultural fertilizers.

Excessive growth of the algae causes destruction of many of the higher links of the local food web. Algae that die and sink to the bottom at the end of the growing season stimulate massive growth of decomposers and bacteria the following year. This results in the depletion of oxygen in the deeper water layers (see anoxic).

These conditions may result in fish kills and replacement with less valuable species who may be more tolerant of increased phosphorus and lower oxygen levels. Deoxygenation also may cause chemical changes in the mud on the bottom, producing increased quantities of chemicals and toxic gases. All these changes further accelerate the eutrophication (aging) of the aquatic ecosystem.

The so-called Red tides found in the Gulf of Mexico are a marine example of algal blooms: these are massive growths of a marine alga, Karenia brevis. This particular organism produces a potent neurotoxin, and the red tide results in the deaths of large numbers of fish. Filter feeding organisms such as mussels and oysters can become poisonous through bioaccumulation of the toxin during a red tide.