Internal processing and N₂ balance in Upper Midwest lakes across a land use gradient

Excess nitrogen (N) in freshwater systems is harmful and can lead to eutrophication, loss of biodiversity and toxic cyanobacterial blooms. External loading of N is an important driver of eutrophication, however, internal processing can either exacerbate or relieve excess N through N₂ fixation or denitrification, respectively. Here, we aimed to determine how variation in N loading and hydrologic setting affect internal N processing in lakes in summer by quantifying N₂ saturation in 17 lakes across a land use gradient in Minnesota and Iowa. We hypothesized that lakes with the highest N loading rates would have the highest N₂ saturation values, indicative of net denitrification. We observed that lakes in agricultural regions had the highest N₂ saturation and all lakes showed the highest levels of supersaturation in June when runoff was maximal. Although seasonal changes affected the degree of N₂ saturation, all lakes were sources of N₂ to the atmosphere throughout the sample period suggesting that denitrification was more impactful to internal processing than was N₂-fixation. Peaks in N₂ supersaturation co-occurred with both low and high dissolved oxygen levels, the latter being somewhat paradoxical given that denitrification is an anaerobic process.