Nitrogen Fixation Provides a Significant Source of Biologically Available Nitrogen in the Great Lakes

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Nitrogen Fixation Provides a Significant Source of Biologically Available Nitrogen in the Great Lakes

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Nitrogen fixation (NFix) is an important yet understudied microbial process in aquatic ecosystems and especially in the Laurentian Great Lakes (LGL). Nitrogen fixation (NFix) or diazotrophy is a lynchpin in the nitrogen cycle that converts dinitrogen (N2) gas to ammonia (NH3) using the nitrogenase protein complex (Nif). Nitrogen fixation re-mobilizes atmospheric nitrogen and N lost to denitrification processes. Early work suggested the contribution of NFix in the LGL is minimal to the nitrogen budget. However, recent work has shown during bloom events, NFix can help alleviate nitrogen limitations. Evidence of nitrogen accumulation in the LGL, like in Superior, suggest that we do not have a firm grasp on the nitrogen cycle in large lakes. Furthermore, for most of the Great Lakes, there is a dearth of nitrogen fixation rates from near-shore and surface water environments. Thus, we sought to revisit NFix in the LGL and comprehensively sample from near and offshore stations and with depth to understand the spatial variability of NFix. In our quest to survey the Great Lakes, we carried out a series of preliminary studies to determine our methodology and understanding of nitrogen fixation is aqueous samples. Preliminary studies exploring seasonality and size fractionation showed detectable nitrogen fixation rates in Lake Superior, counter to previous expectations as a result of readily available dissolved inorganic nitrogen. Likewise, preliminary studies on nitrogen speciation and concentrations were found to inhibit nitrogen fixation rates in pure culture experiments. Inorganic nitrogen exemplified a significant threshold where nitrogen fixation rates decreased. In our Great Lakes survey, we found that each lake is significantly different in NFix rates from one another and that rates are depth dependent. Linear regression models show that chlorophyll-a and oxidized nitrogen species are correlated, directly and inversely, with NFix but with the caveat that lakes like Erie and Huron do not fully adhere to the model. Together this suggest that traditional controls on diazotrophy may not always adhere and that NFix is a significant source of N.


University of Minnesota M.S. thesis. August 2020. Major: Water Resources Science. Advisor: Cody Sheik. 1 computer file (PDF); iv, 47 pages.

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