Algal blooms in lakes and rivers may be 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[?].
Red tides are an example of naturally occuring marine algal blooms. These are caused by various dinoflagellates, some of which produce large quantities of neurotoxins. These result in the deaths of large numbers of fish, and in filter feeding organisms such as mussels and oysters becoming poisonous through bioaccumulation of the toxin. Black water was another marine bloom, of unknown cause.