Nutrient pollution, which is predominantly composed of nitrates and phosphates, comes from agricultural, industrial, and sewage pollution. The source of nutrient pollution varies among nations. In general, industrialized countries are plagued with agricultural runoff, whereas developing countries suffer from nutrient runoff due to industry. As most sewage treatment systems do not treat for nitrates and phosphates, most nations experience nutrient runoff from sewage.
In western France and other European nations, the critical load of nutrient nitrogen has been exceeded by over 1200 equivalents per hectare and year (Pollution- Biodiversity Information System for Europe, n.d.). Duke University biochemist William Schlesinger estimated that 60 percent of all fixed nitrogen deposited on land can be attributed to human actions (Moffett, 1998). These numbers show that humans have a clear impact on the amount of nutrients introduced into the environment, which has a variety of consequences. In the United States, 10 percent of assessed freshwater ecosystem miles had nutrient enrichment problems. In general, nutrient pollution accounts for about 30 percent of water quality problems in the United States (Zheng, Lei et al., n.d.).
Nitrogen overloading in the soil can remove cations from the soil, making the soil less nutritious for plants. This has been known to cause "forest dieback" in Europe, where nutrient overloading in the soil is prevalent (Willison et al., 1990). In addition to this, nutrient overloading in the soil can cause acidification which then results in aluminum dissolution. This inhibits a plant's ability to absorb cations such as calcium. The most devastating effect nutrient runoff has on biodiveristy is the eutrophication of marine and other aquatic ecosystems. Nutrient pollution accumulates in terrestrial soil. But, as one study by Science explained, much land has already reached its threshold ability to absorb and break down nutrients. This means that nutrients such as nitrates and phosphates are easily flushed into waterways (Moffett, 1998), causing algae blooms. Algae blooms consume dissolved oxygen that other organisms depend on, thereby creating "dead zones" where little or no animal life exists.
Dead zones can occur in lakes as streams bring in nutrients, as well as in coastal areas. The number of eutrophic lakes is on the rise as nutrient runoff increases. In China, for instance, one study estimated that the percentage of eutrophic lakes increased from 5.0 percent to 55.01 percent between 1978 and 1987 (Jin, 2005) . Marine dead zones can be found off of most continents and are spreading over large areas of the sea floor (Creeping Dead Zones NASA). One of the most well-known of these marine dead zones is in the Gulf of Mexico. This hypoxic zone, encompassing 7728 square miles, contains less than two milligrams of oxygen per liter of water, a value too low to support animal life at the bottom of the ocean floor (Carlisle, 2000). This zone forms each spring and lasts until September (Carlisle, 2000), preventing life from establishing itself on the ocean floor in that area. Scientists have identified 415 other marine dead zones worldwide, with another 233 areas of concern (Dead Zone, n.d.). In the diagram below from the World Resources Institute (Lash. 2007), one can see that dead zones are distributed all around the globe, including in biodiversity hotspots such as the coast of Thailand.
