Monitoring nitrogen in rivers

Nitrogen in water is an essential nutrient for the growth of plants and algae. Whilst nitrogen is naturally present at low levels in rivers, excessive nitrogen can result in eutrophication of water bodies. Eutrophication (excess nutrients) can lead to extensive growth of aquatic plants and algae, which can impact the ecological health, recreational, cultural and food-gathering values of a river.

In a river, nitrogen can be present in various forms (chemical species). Total nitrogen (TN) is the sum of all forms of nitrogen: nitrate nitrogen (NO3 –N), nitrite nitrogen (NO2 –N), ammoniacal nitrogen (NH4 –N) and organic nitrogen (nitrogen in amino acids and proteins). Nitrate nitrogen is a stable form of nitrogen found in freshwater ecosystems; it is highly soluble and can be readily used by vascular plants and algae for growth. However, at high concentrations, nitrate can be toxic to aquatic life.

The major sources of nitrate in freshwater systems include diffuse discharge from agricultural areas and wastewater discharges (from municipal sewerage treatment plants, septic tanks and industry). Atmospheric deposition and decaying plant debris are also sources of nitrate in freshwater bodies. Ammoniacal nitrogen is the concentration of nitrogen present as either ammonia (NH3) or ammonium (NH4 + ) and, at high concentrations, is toxic to aquatic life. Elevated ammoniacal nitrogen concentrations in rivers and streams are often due to point source discharges, such as raw or poorly treated sewage or dairy shed effluent. The toxicity of ammoniacal nitrogen increases as water pH and temperature increase.