Water Pollution Water pollution destroys important food sources and contaminates drinking water with chemicals that can cause immediate and long-term harm to human health. Water pollution also often severely damages aquatic ecosystems. Rivers, lakes, and oceans are used as open sewers for industrial and residential waste. Pesticides, herbicides, oil products, heavy metals (such as mercury, lead, and zinc), detergents, and industrial wastes can kill aquatic organisms outright or make the environment so inhospitable that species can no longer thrive. For instance, water pollution is a threat to 90% of the endangered fishes and freshwater mussels in the US. Unlike terrestrial dumps, whose effects are primarily local, toxic wastes in aquatic environments diffuse over a wide area. Many aquatic environments are naturally low in essential minerals, such as nitrates and phosphates, and aquatic species have adapted to the natural scarcity of minerals by developing the ability to process large volumes of water and to concentrate these minerals. When these species process polluted water, they concentrate toxic chemicals along with the essential minerals, which can eventually poison the plant or animal. Species that feed on these aquatic species then ingest these high concentrations of toxic chemicals. Essential minerals that are beneficial to plant and animal life can become harmful pollutants at higher levels. Human sewage, agricultural fertilizers, detergents, and industrial processes often release large amounts of nitrates and phosphates into aquatic systems, causing cultural eutrophication. For instance, humans release as much nitrate into the environment as do all natural processes, and this input is expected to increase in tandem with the increasing human population. Even small amounts of these nutrients can stimulate plant and animal growth, and high concentrations often result in thick blooms of algae at the surface of ponds, lakes, and coastal areas. These algal blooms may be so dense that they outcompete with other plankton species and shade out bottom- dwelling plant species. As the algal mat becomes thicker, its lower layers die and sink. Bacteria and fungi then decompose the dying algae, absorbing all the oxygen in the water. Without oxygen, much of the remaining animal life dies off, sometimes visibly in the form of
masses of dead fish floating on the water surface. The result is a greatly impoverished and simplified community consisting of only those species tolerant of polluted water and low oxygen levels. The spreading dead zone where the Mississippi River enters the Gulf of Mexico is an example of the direct consequences of water pollution. Such dead zones are increasing in number and size around the world as a result of human activities.