Potential of Nitrogen/Argon Analysis in Surface Waters in the Examination of Areal Nitrogen Deficits Caused by Nitrogen Fixation

In marine systems, the loss of nitrogen caused by denitrification in oxygen deficient zones is balanced by nitrogen fixation mediated by cyanobacteria, which may form extensive blooms in surface waters. In this study, by determining the concentration ratio of nitrogen (N-2) and argon (Ar) in air equilibrated with surface water, we were able to detect changes in the N-2 concentration attributable to N-2 fixation. For this purpose, surface water was pumped continuously into a spray-type equilibrator while the air in the equilibrator's headspace was analyzed by mass spectrometry. After laboratory tests and model analysis to evaluate the sensitivity of our N-2/Ar approach, feasibility studies were conducted in the central Baltic Sea in the summer of 2015 during the development of a cyanobacterial bloom. Our results showed that N-2 deficits accumulated during periods of low wind and increasing surface water temperatures. A comparison of our results with the N-2 deficits calculated from changes in the partial pressure of carbon dioxide in surface water indicated a similar trend. By demonstrating the ability of the N-2/Ar approach to resolve N-2 deficits in surface water caused by N-2 fixation, our study contributes to assessments of the N-2 fixation efficiency of cyanobacterial blooms.