Total nitrogen is calculated by adding nitrate and nitrite to Kjeldahl nitrogen. Organic nitrogen is often referred to as biomass nitrogen. Decomposing organic matter releases ammonia, which is converted to nitrate if oxygen is present. This conversion occurs more rapidly at higher water temperatures. Figure 6 shows the various ways that nitrogen enters and cycles within a lake.
Sediments clearly cause nitrogen to undergo a number of changes.
Nitrogen recycled back into overlying water at spring and fall turnover will often increase ammonia levels in samples taken during turnover. Nitrogen can be lost from the lake to the atmosphere by denitrification as shown in the figure. This only occurs if oxygen is depleted, allowing nitrate to be converted back to nitrogen gas. Low nitrogen levels do not guarantee limited algae growth in the same way low phosphorus levels do. Page - 11 Larger plants also need nitrogen and may depend on spring runoff for septic systems to recharge the sediments with nitrogen.