Upper Midwest lakes are supersaturated with N.

Little is known about the exchange of gaseous nitrogen (N) with the atmosphere in freshwater systems. Although the exchange of N, driven by excess or deficiencies relative to saturation values, has little relevance to the atmospheric N pool due to its large size, it does play an important role in freshwater and marine nitrogen (N) cycling. N-fixation converts N to ammonia, which can be used by microbes and phytoplankton, while denitrification/anammox effectively removes it by converting oxidized, inorganic N to N We examined N 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 N (>85% of samples, = 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.