Nitrogen (N) is a chemical element occurring in several forms. Primary inorganic forms are ammonia and nitrate. Ammonia-N is the total of all N in the form of either dissolved gas (NH3) or ammonium ion (NH4+). Nitrate-N is nitrogen dissolved as nitrate ion (NO3-). A less abundant inorganic form is the nitrite ion (NO2-), which occurs in reducing environments (low DO). Elevated nitrate levels usually indicate bacterial nitrification, which is typical of sewage-contaminated waters. Total Kjeldahl-N (TKN) measures the total of all N in the form of either organic-N or ammonia-N. Organic-N includes particulate forms (such as cell matter from algae or bacteria, and sewage solids) and dissolved forms (such as proteins and peptides). Algae and other plants require N as a primary nutrient. Ammonia and nitrate N are the chief forms susceptible to algal and plant uptake, but certain dissolved organic forms can also be assimilated. Measurement of N provides insight into the total potential for algal and plant growth.