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<p>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 N<sub>2</sub> 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 N<sub>2</sub> 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 N<sub>2</sub> saturation values, indicative of net denitrification. We observed that lakes in agricultural regions had the highest N<sub>2</sub> saturation and all lakes showed the highest levels of supersaturation in June when runoff was maximal. Although seasonal changes affected the degree of N<sub>2</sub> saturation, all lakes were sources of N<sub>2</sub> to the atmosphere throughout the sample period suggesting that denitrification was more impactful to internal processing than was N<sub>2</sub>-fixation. Peaks in N<sub>2</sub> supersaturation co-occurred with both low and high dissolved oxygen levels, the latter being somewhat paradoxical given that denitrification is an anaerobic process.</p>

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

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2025年9月2日

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  • Brianna M. Loeks,
  • James B. Cotner

摘要

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.