Nitrates are a form of nitrogen found in aquatic environments.  Forms of nitrogen in water include:

            Nitrate:  NO3
            Nitrite:  NO2
            Ammonia:  NH3

Nitrate is necessary to aquatic plant life, but can result in excessive plant growth, especially algae, if available in disproportionate amounts.  Excess nitrates can lead to algae blooms, which increase turbidity and water temperature.  When the algae dies, it is consumed by bacteria that significantly decrease the amount of oxygen in water.  All of these affects can make habitat uninhabitable for aquatic life.  Sources of nitrates include sewage treatment plants, septic systems, fertilizers, industrial discharges, and animal manure.