Nitrates are a form of nitrogen found in water and are associated with nutrients. Nitrates generally come from excess fertilizer. In high concentrations nitrates can provoke an excessive growth of algae.
The long term concentration to assure the protection of the aquatic life in freshwater is 13 mg/L or 13 ppm. (3)
To convert NO3-N to NO3 in parts per million (ppm, or mg/L), multiply NO3-N by 4.42