Nitrogen availability limits marine productivity across large ocean regions. Diazotrophs can supply new nitrogen to the marine environment via nitrogen (N2) fixation, relieving nitrogen limitation. The distributions of diazotrophs and N2 fixation have been hypothesized to be generally controlled by temperature, phosphorus, and iron availability in the global ocean. However, even in the North Atlantic where most research on diazotrophs and N2 fixation has taken place, environmental controls remain contentious. Here we measure diazotroph composition, abundance, and activity at high resolution using newly developed underway sampling and sensing techniques. We capture a diazotrophic community shift from Trichodesmium to UCYN-A between the oligotrophic, warm (25–29 °C) Sargasso Sea and relatively nutrient-enriched, cold (13–24 °C) subpolar and eastern American coastal waters. Meanwhile, N2 fixation rates measured in this study are among the highest ever recorded globally and show significant increase with phosphorus availability across the transition from the Gulf Stream into subpolar and coastal waters despite colder temperatures and higher nitrate concentrations. Transcriptional patterns in both Trichodesmium and UCYN-A indicate phosphorus stress in the subtropical gyre. Over this iron-replete transect spanning the western North Atlantic, our results suggest that temperature is the major factor controlling the diazotrophic community structure while phosphorous drives N2 fixation rates. Overall, the occurrence of record-high UCYN-A abundance and peak N2 fixation rates in the cold coastal region where nitrate concentrations are highest (~200 nM) challenges current paradigms on what drives the distribution of diazotrophs and N2 fixation.
In the North Atlantic Ocean, dinitrogen (N2) fixation on the western continental shelf represents a significant fraction of basin‐wide nitrogen (N) inputs. However, the factors regulating coastal N2fixation remain poorly understood, in part due to sharp physico‐chemical gradients and dynamic water mass interactions that are difficult to constrain via traditional oceanographic approaches. This study sought to characterize the spatial heterogeneity of N2fixation on the western North Atlantic shelf, at the confluence of Mid‐ and South Atlantic Bight shelf waters and the Gulf Stream, in August 2016. Rates were quantified using the15N2bubble release method and used to build empirical models of regional N2fixation via a random forest machine learning approach. N2fixation rates were then predicted from high‐resolution CTD and satellite data to infer the variability of its depth and surface distributions, respectively. Our findings suggest that the frontal mixing zone created conditions conducive to exceptionally high N2fixation rates (> 100 nmol N L−1d−1), which were likely driven by the haptophyte‐symbiont UCYN‐A. Above and below this hotspot, N2fixation rates were highest on the shelf due to the high particulate N concentrations there. Conversely, specific N2uptake rates, a biomass‐independent metric for diazotroph activity, were enhanced in the oligotrophic slope waters. Broadly, these observations suggest that N2fixation is favored offshore but occurs continuously across the shelf. Nevertheless, our model results indicate that there is a niche for diazotrophs along the coastline as phytoplankton populations begin to decline, likely due to exhaustion of coastal nutrients.
more » « less- NSF-PAR ID:
- 10452113
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Limnology and Oceanography
- Volume:
- 66
- Issue:
- 5
- ISSN:
- 0024-3590
- Page Range / eLocation ID:
- p. 1832-1849
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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