Nitrogen fixation (NFix) is an important, yet understudied, microbial process in aquatic ecosystems, especially in the Laurentian Great Lakes (LGL). To date, a dearth of nitrogen fixation rate measurements exists in the LGL, are from temporally isolated studies, and were collected primarily from near-shore and surface water environments. Evidence of nitrogen accumulation across the Laurentian Great Lakes suggest that we do not have a firm grasp on nitrogen cycling in large lakes. Thus, we sought to quantify the spatial variability of NFix in the LGL. We found lakes are significantly different in NFix rates from one another and that rates are depth dependent. Overall mean surface NFix rates of Lakes Superior, Michigan, Huron, Erie and Ontario were 0.024, 0.020, 0.069, 0.145, and 0.078 (nmol N2/L/hr), respectively. Likewise, we found the Western, Central and Eastern basins of Lake Erie are significantly different in NFix rates (0.1540, 0.1032, 0.0738 nmol N2/L/hr). However, we found no significant difference in NFix rates between near and offshore sites in Lake Erie, which may have been biased due to a cyanobacterial bloom containing a nitrogen-fixing Dolichospermum sp. Linear regression models indicate NFix is generally positively correlated with chlorophyll-a concentration and negatively correlated with oxidized nitrogen species concentrations. However, Lakes Erie and Huron exhibited a positive linear relationship with oxidized nitrogen, suggesting that NFix may persist to meet cellular and community nitrogen demands. Together, our data highlight NFix is important despite the presence of abundant nitrogen in all LGL.
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We would like to thank the Captains and crews of the R/V Lake Guardian and R/V Blue Heron for all their hard work in helping us gather our samples. Likewise, we would like to thank the Environmental Protection Agency (EPA) Great lakes National Program Office's (GLNPO) Cooperative Science Monitoring Initiative (CSMI) for granting us ship time to collect our samples. The authors have no conflicts of interests. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
© 2021 The Author(s)
- Laurentian Great Lakes
- Nitrogen fixation