Nitrogen - Understanding Lake Data

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Nitrogen is second only to phosphorus as an important nutrient for plant and algae growth. A lake's nitrogen sources vary widely. Nitrogen compounds often exceed 0.5 mg/l in rainfall, so that precipitation may be the main nitrogen source for seepage and some drainage lakes.

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In most cases, however, the amount of nitrogen in lake water corresponds to local land use. Nitrogen may come from fertilizer and animal wastes on agricultural lands, human waste from sewage treatment plants or septic systems, and lawn fertilizers used on lakeshore property. Nitrogen may enter a lake from surface runoff or groundwater sources.

Nitrogen exists in lakes in several forms. Analysis usually includes nitrate (NO3-) plus nitrite (NO2-), anunonium (NH4+), and organic plus ammonium (Kjeldahl nitrogen). Total nitrogen is calculated by adding nitrate and nitrite to Kjeldahl nitrogen. Organic nitrogen is often referred to as biomass nitrogen.

Nitrogen does not occur naturally in soil minerals, but is a major component of all organic (plant and animal) matter. Decomposing organic matter releases ammonia, which is converted to nitrate if oxygen is present. This conversion occurs more rapidly at higher water temperatures. All inorganic forms of nitrogen (NO3-, NO2- and NH4+) can be used by aquatic plants and algae. If these inorganic forms of nitrogen exceed 0.3 mg/l (as N) in spring, there is sufficient nitrogen to support summer algae blooms.

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.

FIGURE 6. Sources and cycling of nitrogen in a lake

Sources of cycling of nitrogen in lake - Sketch of a cloud, rain, hills, stream, and pond cross section showing groundwater table with nitrogen cycling in lake and its sediments

In about 10% of Wisconsin's lakes, nitrogen (rather than phosphorus) limits algae growth. This occurs when the ratio of total nitrogen to total phosphorus is less than 10:1. Values between 10:1 and 15:1 are considered transitional, while lakes with values greater than 15:1 are considered phosphorus limited-algae growth is controlled by the amount of phosphorus.

Low nitrogen levels do not guarantee limited algae growth in the same way low phosphorus levels do. Nuisance blue-green algae blooms are often associated with lakes that have low nitrogen to phosphorus (N:P) ratios. These algae use atmospheric nitrogen gas (N2) dissolved in lake waters as a nitrogen source; other more desirable types of algae and plants depend on the inorganic nitrate and ammonium forms of nitrogen.

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Larger plants also need nitrogen and may depend on spring runoff for septic systems to recharge the sediments with nitrogen. Growth of Eurasian milfoil has been correlated with such fertilization of the sediment.

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For more information on this topic, contact:

James Vennie
Watershed Management
(608) 266-2212


Lakes Partnership | Watershed Management | Fish Wisconsin | Fisheries Management

Last Revised: Thursday January 25 2007