Effects of water mass mixing on diazotrophy at the New England shelfbreak front

Historically, it has been thought that dinitrogen (N₂) fixation occurs predominantly in the oligotrophic surface waters of the tropics and sub-tropics where the well-studied diazotroph Trichodesmium thrives. Recent work, however, has expanded the known range of N₂ fixation, and high rates have now been observed in several productive continental shelf/slope and upwelling environments, including along the Mid-Atlantic Bight. Diazotrophs living in these dynamic systems employ a diversity of physiological strategies to promote their growth, yet little is currently known about the environmental drivers of N₂ fixation in such places. Frontal mixing zones offer a means of studying the biogeography of and competitive interactions among distinct diazotrophic clades. Moreover, ocean fronts are thought to enhance biodiversity through water mass mixing, stimulate regional productivity via nutrient fertilization, and thereby promote organic matter export. We investigated diazotroph diversity and activity across the New England shelfbreak front in May and July, 2019, in concert with high-resolution hydrographic measurements. N₂ fixation rates were quantified using a variant of the ¹⁵N₂ tracer bioassay that accounts for the slow equilibration time of ¹⁵N₂. Shelf and slope water mass end-members and the mixing zone were repeatedly surveyed over the two 14-day cruises as the structure of the frontal zone shifted. This approach provided high temporal resolution, enabling examination of diazotroph community dynamics on the timescale of ocean mixing. This study elucidates the role of water mass mixing on N₂ fixation rates in a biogeochemically important region, and offers insight into the physical, chemical, and biological factors regulating the activity of distinct diazotroph communities.