Closing the knowledge gap surrounding nitrogen cycling in Minnesota lakes

Abstract

Nitrogen (N) is an element necessary for life, but in excess quantities in aquatic systems leads to hypoxia, loss of biodiversity and harmful algal blooms. Allochthonous inputs are typically the critical drivers of eutrophication, but it is also important to understand how those inputs interact with internal processes to characterize future N scenarios. Microbial processes such as N fixation and denitrification can add or remove N and understanding when and where these processes occur is essential to managing N in lakes. We demonstrated recently that lakes remove nitrogen through denitrification and loss of N2 (the final product of denitrification) to the atmosphere. Here, we aimed to examine how land use may impact this generalized pattern in summer by quantifying N2 losses in 17 lakes across a land use gradient. We found that lakes in agricultural regions had the highest levels of N2 saturation and all lakes showed the highest levels of supersaturation in June. Wet spring conditions and a late ice-off likely played an important role in the large nitrogen removal in June. We also found that all lakes were sources of N2 to the atmosphere throughout the entire growing season suggesting that N-deficiency could be prevalent even though N fixation is occurring. Peaks in N2 supersaturation occurred throughout the water column in areas with both low and high dissolved oxygen levels, the latter being somewhat paradoxical given that denitrification has traditionally been considered an anaerobic process. To further understand where and when these different processes are occurring, we quantified rates of N fixation and denitrification throughout the water column and sediments across 9 lakes in Minnesota and Wisconsin. We found that both processes were happening in the water column and sediments of lakes (N fixation rates: epilimnetic rate mean = 21.17 mmol N2-N/hr/m2, hypolimnetic rate mean = 8.85 mmol N2-N/hr/m2, sediment rate mean = 4.76 mmol N2-N/hr/m2; denitrification rates: epilimnetic rate mean=8.87 mmol N2O-N/hr/m2, hypolimnetic rate mean=144.62 mmol N2O-N/hr/m2, sediment rate mean=0.009 mmol N2O-N/hr/m2) with denitrification outpacing rates of N fixation by at least an order of magnitude- a statistic that supports our previous findings. Traditionally N fixation is thought to occur in the surface waters of lakes and performed primarily by photosynthetic cyanobacteria, and denitrification is thought be primarily in the sediments of lakes by facultative anaerobes. The results from this research highlight a complex story regarding the N cycle in freshwater systems where microbial processes occur in a variety of environments not limited by the presence of oxygen or the availability of light. Understanding where and when these microbial processes are occurring can lead to more accurate and effective management as we work to curb the effects of eutrophication.