Coastal Eutrophication.
Nutrients and organic materials from plants, animals, and humans that enter coastal waters, either directly or indirectly, can stimulate a biological, chemical, and physical progression known as eutrophication. Coastal eutrophication is commonly observed in estuaries , bays, and marginal seas. In a broad sense, coastal eutrophication mirrors the eutrophication of lakes. For example, as increased nutrients stimulate algal and other plant growth, light transmission decreases. The eventual bacterial decay of algae and other plants lowers the dissolved oxygen level in the water. In extreme cases, all of the oxygen can be removed.
Human-accelerated eutrophication (known as cultural eutrophication) can be triggered by inputs of sewage, sludge, fertilizers, or other wastes containing nutrients such as nitrogen and phosphorus. As recently as the 1980s, for example, the New York Bight was essentially lifeless due to oxygen depletion, caused largely by decades of sewage and sludge disposal. As of 2002, Halifax Harbor was still receiving a daily influx of raw sewage, creating serious ecological and public health concerns.
Nutrient-enriched runoff from agricultural land in the midwestern United States is the primary cause of the well-known Gulf of Mexico "Dead Zone." Half of the U.S. farms are located in the Mississippi River Basin, whose entire drainage basin empties into the gulf. Much of the nitrogen reaching the gulf is from agricultural fertilizers, with lesser amounts from residential fertilizers and other sources. The water of the 20,000-kilometer (7,728-square-mile) Dead Zone, extending from the mouth of the Mississippi River Basin to beyond the Texas border, has so little oxygen that essentially no marine life exists.
If human-accelerated eutrophication is not reversed, the entire coastal ecosystem ultimately may be changed. Sensitive species may be replaced by more tolerant and resilient species, and biologically diverse communities may be replaced by less diverse ones. Further, nutrient enrichment and the associated eutrophication in coastal waters is implicated in some harmful algal blooms, in which certain species of algae produce biotoxins (natural poisons) that can be transferred through the food web, potentially harming higher-order consumers such as marine mammals and humans.
