The huge increase in nitrogen loading of the environment has had a number of consequences. There has been a large rise in the nitrogen levels of drinking water supplies, resulting mainly from agricultural run-off and wastewater

Although an increased rate of fertilizer nitrogen application has been advocated to meet the growing demand for food, when they are not applied judiciously there are problems of environmental pollution. This is because plants are capable of taking up only a relatively small portion of the applied nitrogen, a substantial amount being lost through various chemical and biological processes

Between 30% and 60% of the nitrogen deposited on land is thought to be absorbed by the ecosystem. The degree to which a watershed can retain nitrogen is a function of the soil characteristics, topography, underlying geology, the amount and type of surface vegetation, and the degree of impervious cover (Paerl, 1993). Inevitably, a significant amount of deposited nitrogen will be transported during a precipitation event, via overland or subsurface flow, into a freshwater system. Usually freshwater systems are phosphorus- limited and will not use the excess nitrogen. Thus, most of the nitrogen will be delivered to estuarine systems

A 1999 assessment performed by the Gulf of Mexico Hypoxia Working Group under the auspices of the US National Science and Technology Council's Committee on Environment and Natural Resources, has confirmed that the hypoxia condition in the Gulf of Mexico is caused primarily by excess nutrients delivered to those waters from the Mississippi-Atchafalaya River Basin. Dramatic increases in fertilizer nitrogen input into the basin have occurred since the 1950s, and since 1980 about 1.6 million metric tons of nitrogen have been deposited in the Gulf each year. In particular, nitrates have greatly increased as river basins drain agricultural land in Minnesota, Iowa, Illinois, Indiana, and Ohio

In some major rivers of the northeastern United States, for example, nitrate concentrations have risen up to tenfold since the beginning of the century, necessitating costly purification systems to protect human health (Carpenter and others 1998). Globally, human activities have increased the amount of riverine transport of dissolved inorganic nitrogen by a factor of 2-4 (Seitzinger and Kroeze 1998)

There is compelling evidence that nutrient enrichment is at least partly to blame for damage to estuaries and coastal seas, and some of the decline in coastal fisheries production. In brackish water, nitrogen is usually the limiting nutrient for algal activity and plant growth. River discharges laden with nitrogen-rich sewage and fertilizer run-off therefore tend to stimulate algal blooms, which can lead to oxygen starvation in coastal waters at lower depths. This has caused significant fish kills in the Baltic Sea, Black Sea and Chesapeake Bay (Vitousek and others 1997). Biodiversity can also be reduced as 'nuisance' algae come to dominate marine ecosystems. The world's oceans are being plagued by a rising incidence of algal blooms - known as brown or red tides