Monitoring total nitrogen in lakes

Nitrogen is an essential nutrient for the growth of plants and algae. Whilst nitrogen is present naturally in lakes, concentrations may be elevated due to diffuse discharges from agricultural and horticultural land (e.g., due to fertiliser use) and wastewater discharges. Elevated nitrogen concentrations can cause eutrophication (excess nutrients), a process that can lead to excessive growth of macrophytes, or algae and cyanobacteria blooms, which can impact the ecological, recreational and cultural values of lakes.

Total nitrogen (TN) is the sum of all forms of nitrogen in water: nitrate nitrogen (NO3 –N), nitrite nitrogen (NO2 –N), ammoniacal nitrogen (NH3–N and NH4 –N) and organic nitrogen (nitrogen in amino acids and proteins, which may be present in dissolved or particulate form). Nitrogen gas dissolved in water is not included as part of the TN.

Nitrogen enters lakes from inflows (rivers, streams and groundwater) and atmospheric deposition. It can also come from within the lake, from decaying plant debris and from the bottom sediments (for example, due to wave or wind action causing resuspension of particulate organic nitrogen). Total nitrogen concentrations can vary within a lake, particularly when there is seasonal stratification in deep lakes, leading to marked differences in concentrations and forms of nitrogen between the warm surface waters and cooler bottom waters.