Upper Midwest lakes are supersaturated with N₂

Little is known about the exchange of gaseous nitrogen (N₂) with the atmosphere in freshwater systems. Although the exchange of N₂, driven by excess or deficiencies relative to saturation values, has little relevance to the atmospheric N₂ pool due to its large size, it does play an important role in freshwater and marine nitrogen (N) cycling. N-fixation converts N₂ to ammonia, which can be used by microbes and phytoplankton, while denitrification/anammox effectively removes it by converting oxidized, inorganic N to N₂. We examined N₂ saturation to infer net biological nitrogen processes in 34 lakes across 5° latitude varying in trophic status, mixing regime, and bathymetry. Here, we report that nearly all lakes examined in the upper Midwest (USA) were supersaturated with N₂ (>85% of samples, n = 248), suggesting lakes are continuously releasing nitrogen to the atmosphere. The traditional paradigm is that freshwaters compensate for N-limitation through N-fixation, but these results indicate that lakes were constantly losing N to the atmosphere via denitrification and/or anammox, suggesting that terrestrial N inputs are needed to balance the internal N cycle.