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

ABSTRACT

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 (N2) and argon (Ar) in air equilibrated with surface water, we were able to detect changes in the N2 concentration attributable to N2 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 N2/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 N2 deficits accumulated during periods of low wind and increasing surface water temperatures. A comparison of our results with the N2 deficits calculated from changes in the partial pressure of carbon dioxide in surface water indicated a similar trend. By demonstrating the ability of the N2/Ar approach to resolve N2 deficits in surface water caused by N2 fixation, our study contributes to assessments of the N2 fixation efficiency of cyanobacterial blooms.