Nitrate (NO3) and nitrite (NO2) are two forms of nitrogen. Nitrate is easily dissolved in water and more common in streams. Nitrite, another form of nitrogen, is rare because it's quickly changed by bacteria into nitrate or returned back to the atmosphere as nitrogen gas. In lakes, most nitrate/nitrogen is in NO3 form. It is measured in milligrams per liter (mg/L).

Nitrogen is an essential plant nutrient, but excess nitrogen can cause water quality problems. Too much nitrogen and phosphorus in surface waters causes nutrient enrichment, increasing aquatic plant growth and changing the types of plants and animals that live in a stream. This process, called eutrophication also can affect other water quality parameters such as temperature and dissolved oxygen. For humans a pollution concern is a high nitrate concentration in drinking water, which can result in methemoglobinemia (blue baby syndrome). Elevated levels of nitrates/nitrogen are often caused by over application of fertilizers that leach into waterbodies.

Source: MPCA