Upper Midwest lakes are supersaturated with N₂.

Area of Science:

• Environmental Science
• Limnology
• Biogeochemistry

Background:

• Gaseous nitrogen (N₂) exchange in freshwater systems is poorly understood.
• N₂ cycling is vital for aquatic ecosystems, involving nitrogen fixation and denitrification/anammox.
• Atmospheric N₂ exchange influences nutrient availability for aquatic life.

Purpose of the Study:

• To investigate N₂ saturation levels in lakes to infer net biological nitrogen processes.
• To understand the role of N₂ supersaturation in the nitrogen balance of freshwater systems.
• To evaluate the prevailing paradigm of nitrogen fixation compensating for N-limitation in lakes.

Main Methods:

• Analysis of N₂ saturation in 34 lakes across a 5° latitude range.
• Inclusion of lakes with diverse trophic statuses, mixing regimes, and bathymetry.
• Statistical analysis of N₂ supersaturation across a large sample size (n=248).

Main Results:

• Nearly all surveyed lakes in the upper Midwest (USA) exhibited N₂ supersaturation (>85% of samples).
• This supersaturation indicates a net release of nitrogen from lakes to the atmosphere.
• Observed N₂ release suggests denitrification and/or anammox processes dominate over nitrogen fixation.

Conclusions:

• Freshwater lakes are significant sources of atmospheric nitrogen, contrary to some traditional views.
• The continuous loss of nitrogen implies that internal processes are not sufficient to balance the nitrogen cycle.
• Terrestrial nitrogen inputs are likely essential for maintaining nitrogen balance in these lake ecosystems.