A Coastal N₂ Fixation Hotspot at the Cape Hatteras Front: Elucidating Spatial Heterogeneity in Diazotroph Activity Via Supervised Machine Learning

Authors

Corday R. Selden, Old Dominion UniversityFollow
P. Dreux Chappell, Old Dominion UniversityFollow
Sophie Clayton, Old Dominion UniversityFollow
Alfonso Macías-Tapia, Old Dominion UniversityFollow
Peter W. Bernhardt, Old Dominion UniversityFollow
Margaret R. Mulholland, Old Dominion UniversityFollow

Document Type

Article

Publication Date

2021

DOI

10.1002/lno.11727

Publication Title

Limnology & Oceanography

Volume

66

Issue

5

Pages

1832-1849

Abstract

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 N₂ fixation 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 N₂ fixation 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 the ¹⁵N₂ bubble release method and used to build empirical models of regional N₂ fixation via a random forest machine learning approach. N₂ fixation 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 N₂ fixation rates (> 100 nmol N L⁻¹ d⁻¹), which were likely driven by the haptophyte‐symbiont UCYN‐A. Above and below this hotspot, N₂ fixation rates were highest on the shelf due to the high particulate N concentrations there. Conversely, specific N₂ uptake rates, a biomass‐independent metric for diazotroph activity, were enhanced in the oligotrophic slope waters. Broadly, these observations suggest that N₂ fixation 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.

Rights

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Original Publication Citation

Selden, C. R., Chappell, P. D., Clayton, S., Macías‐Tapia, A., Bernhardt, P. W., & Mulholland, M. R. (2021). A coastal N₂ fixation hotspot at the Cape Hatteras front: Elucidating spatial heterogeneity in diazotroph activity via supervised machine learning. Limnology & Oceanography, 66(5), 1832-1849. https://doi.org/10.1002/lno.11727

Repository Citation

Selden, Corday R.; Chappell, P. Dreux; Clayton, Sophie; Macías-Tapia, Alfonso; Bernhardt, Peter W.; and Mulholland, Margaret R., "A Coastal N₂ Fixation Hotspot at the Cape Hatteras Front: Elucidating Spatial Heterogeneity in Diazotroph Activity Via Supervised Machine Learning" (2021). OES Faculty Publications. 403.
https://digitalcommons.odu.edu/oeas_fac_pubs/403

ORCID

0000-0002-5721-3061 (Selden), 0000-0001-5212-6228 (Chappell), 0000-0001-7473-4873 (Clayton)