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In inorganic chemistry, a nitrate is a salt of nitric acid with an ion composed of one nitrogen and three oxygen atoms. In organic chemistry the esters of nitric acid and various alcohols are called nitrates.
Nitrates should not be confused with nitrites (NO<sub>2</sub><sup>-</sup>) the salts of nitrous acid. Organic compounds containing the nitro functional group (which has the same formula and structure as the nitrate ion save that one of the O<sup>−</sup> atoms is replaced by the R group) are known as nitro compounds.
In freshwater or estuarine systems close to land, nitrate can reach high levels that can potentially cause the death of fish. While nitrate is much less toxic than ammonia or nitrite, levels over 30 ppm of nitrate can inhibit growth, impair the immune system and cause stress in some aquatic species. In most cases of excess nitrate concentrations, the principle pathway of entering aquatic systems is through surface runoff from agricultural or landscaped areas which have received excess nitrate fertilizer. These levels of nitrate can also lead to algae blooms, and when nutrients become limiting (such as potassium, phosphate or nitrate) then eutrophication can occur. As well as leading to water anoxia, these blooms may cause other changes to ecosystem function, favouring some groups of organisms over others. Consequently, as nitrates form a component of total dissolved solids, they are widely used as an indicator of water quality.
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