Evidence for net nitrogen gas production in Lake Erie surface waters
Two methods are commonly employed to quantify rates of N2-fixation in surface waters. Acetylene reduction assays (ARA) are an indirect measure of N2-fixation, relying on the conversion of acetylene to ethylene at a known mole ratio with respect to dissolved nitrogen gas. An alternative method, membrane-inlet-mass-spectrometry (MIMS), can directly quantify dissolved N2 gas concentrations in water.Here, surface sample grabs (0.5m depth) were collected at 6 sites in Sandusky Bay on 4 dates (N=24) and bioassays were performed using both techniques. Significant levels of N2-fixation were detected with ARA in 83% of samples, whereas net N2-fixation was not detected in any samples (0%) by analysis via MIMS. In contrasts, significant levels of N2-production were observed in 40% of the samples. N2-production is not possible to detect with ARA and would have been missed had MIMS not been employed. Notably, 36% of the samples yielded significant rates of N2-fixation via ARA and also net N2-production via MIMS. In order for analysis via MIMS to show significant increases in concentrations of dissolved N2, the rates of N2-production must exceed the rates of N2-fixation in those samples. It is well documented that members of the cyanobacterial order Nostocales produce specialized cells (heterocyst) that can fix nitrogen, converting inert gaseous N2 into ammonium ion (NH4+). Thus, detecting N2-fixation in Sandusky Bay is not atypical. However, N2-production is known to occur via two pathways within the microbial nitrogen cycle (i.e., anaerobic ammonium oxidation and denitrification). Currently, neither of these N2-production pathways are known to be associated with oxygenated surface waters, but rather anoxic lake sediments (denitrification) or groundwater (anaerobic ammonium oxidation). Additional research is needed to resolve the net N2-production observed by the methods employed in this study.
Boehler, Christopher T. and Bade, Darren L.(2019). Evidence for net nitrogen gas production in Lake Erie surface waters. Environmental Science & Design Research Initiative. Paper 5.