Total nitrogen consists of inorganic and organic forms. Inorganic forms include nitrate (NO₃^-), nitrite (NO₂^- ), unionized ammonia (NH₄), ionized ammonia (NH₃⁺), and nitrogen gas (N₂). Amino acids and proteins are naturally-occurring organic forms of nitrogen. All forms of nitrogen are harmless to aquatic organisms except un-ionized ammonia and nitrite, which can be toxic to fish. Nitrite is usually not a problem in waterbodies, however, because (if there is enough oxygen available in the water for it to be oxidized) nitrite will be readily converted to nitrate.

The Role of Nitrogen in Waterbodies
Like phosphorus, nitrogen is an essential nutrient for all plants, including aquatic plants and algae. In some cases, the inadequate supply of TN in waterbodies has been found to limit the growth of free-floating algae (i.e., phytoplankton). This is called nitrogen limitation, and occurs most commonly when the ratio of total nitrogen to total phosphorus is less than 10 (in other words, the TN concentration divided by the TP concentration is less than 10 or TN/TP < 10 ).

TN in water comes from both natural and man-made sources, including:

• air (some algae can "fix" nitrogen by pulling it out of the air in its gaseous form and converting it to a form they can use);
• stormwater run-off, including natural run-off from areas where there is no human impact (nitrogen is a naturally-occurring nutrient found in soils and organic matter);
• fertilizers;
• animal and human wastes (sewage, dairies, feedlots, etc.)

• Approximately 14% of these lakes (those with TN values less than 400 micrograms/L) would be classified as oligotrophic;
• About 25% of these lakes (those with TN values between 401 and 600 micrograms/L) would be classified as mesotrophic;
• 50% of these lakes (those with TN values between 601 and 1500 micrograms/L)would be classified as eutrophic; and
• nearly 11% of these lakes (those with TN values greater than 1500 micrograms/L)would be classified as hypereutrophic.

The location of a waterbody has a strong influence on its total nitrogen concentration. For example, lakes in New Hope Ridge/Greenhead Slope lake region in northwestern Florida (Washington, Bay, Calhoun, and Jackson counties) tend to have total nitrogen values below 220 micrograms/L, while lakes in the Lakeland/Bone Valley Upland lake region in central Florida (Polk and Hillsborough counties) tend to have values above 1700 micrograms/L.

Health Concerns
The concentration of total nitrogen in a waterbody is not a known direct threat to human health. It is the individual forms of nitrogen that contribute to the total nitrogen measurement and the use of the water that need to be considered. For example, nitrate in drinking water is a concern. Drinking water with nitrate concentrations above 45 mg/L has been implicated in causing blue-baby syndrome in infants. The maximum allowable level of nitrate, a component of the total nitrogen measurement, is 10 mg/L in drinking water. Concentrations of nitrate greater than 10 mg/L generally do not occur in waterbodies, because nitrate is readily taken up by plants and used as a nutrient.