Nutrient Pollution - Eutrophication
Nitrates and phosphates are nutrients that plants need to grow. In small amounts they are beneficial to many ecosystems. In excessive amounts, however, nutrients cause a type of pollution called eutrophication. Eutrophication stimulates an explosive growth of algae (algal blooms) that depletes the water of oxygen when the the algae die and are eaten by bacteria. Estuarine waters may become hypoxic (oxygen poor) or anoxic (completely depleted of oxygen) from algal blooms. While hypoxia may cause animals in estuaries to become physically stressed, anoxic conditions can kill them.
Eutrophication may also trigger toxic algal blooms like red tides, brown tides, and the growth of Pfiesteria. Pfiesteria is a single-celled organism that can release very powerful toxins into the water, causing bleeding sores on fish, and even killing them. Although consuming fish affected by this toxin is not harmful to humans, exposure to waters where Pfisteria blooms occur can cause serious health problems (USEPA, 1998; Howarth et al., 2000).
Eutrophication is often devastating to animals and plants in estuaries as well as the economies of communities surrounding estuaries. Toxic algal blooms disrupt tourism due to foul odors and unsightly views, and poisoned fish and shellfish adversely affect recreational and commercial fisheries (Carpenter, 1998; Howarth et al., 2000).
Nutrient pollution is the single largest pollution problem affecting coastal waters of the United States (Howarth et al., 2000). Most excess nutrients come from discharges of sewage treatment plants and septic tanks, stormwater runoff from overfertilized lawns, golf courses and agricultural fields. Over 60 percent of the coastal rivers and bays in the United States are moderately to severely affected by nutrient pollution (Howarth et al., 2000).