Nitrogen Fixation at the Mid-Atlantic Bight Shelfbreak and Transport of Newly Fixed Nitrogen to the Slope Sea

Authors

C. R. Selden, Old Dominion UniversityFollow
M. R. Mulholland, Old Dominion UniversityFollow
K. E. Crider, Old Dominion UniversityFollow
S. Clayton, Old Dominion University
A. Macías-Tapia, Old Dominion UniversityFollow
P. Bernhardt, Old Dominion UniversityFollow
D. J. McGillicuddy Jr., Woods Hole Oceanographic Institution
W. G. Zhang, Woods Hole Oceanographic Institution
P. D. Chappell, Old Dominion UniversityFollow

Document Type

Article

Publication Date

2024

DOI

10.1029/2023JC020651

Publication Title

Journal of Geophysical Research: Oceans

Volume

129

Issue

4

Pages

e2023JC020651

Abstract

Continental shelves contribute a large fraction of the ocean's new nitrogen (N) via N₂ fixation; yet, we know little about how physical processes at the ocean's margins shape diazotroph biogeography and activity. Here, we test the hypothesis that frontal mixing favors N₂ fixation at the Mid-Atlantic Bight shelfbreak. Using the ¹⁵N₂ bubble release method, we measured N₂ fixation rates on repeat cross-frontal transects in July 2019. N₂ fixation rates in shelf waters (median = 5.42 nmol N L⁻¹ d⁻¹) were higher than offshore (2.48 nmol N L⁻¹ d⁻¹) but did not significantly differ front frontal waters (8.42 nmol N L⁻¹ d⁻¹). However, specific N₂ uptake rates, indicative of the relative contribution of diazotroph-derived N to particulate N turnover, were significantly higher in frontal waters, suggesting that diazotroph-derived N is of greater importance in supporting productivity there. This study furthered captured an ephemeral shelf-water streamer, which resulted from the impingement of a warm core ring on the shelf. The streamer transported shelf-water diazotrophs (including UCYN-A and Richelia spp., as assessed by qPCR) offshore with sustained high N2 fixation rates. This feature injected >50 metric tons d⁻¹ of newly fixed N to the Slope Sea—a rate equivalent to ∼4% of the total N flux estimated for the entire Mid-Atlantic Bight. As intrusions of Gulf Stream meanders and eddies onto the shelf are increasing in frequency due to climate change, episodic lateral fluxes of new N into the Slope Sea may become increasingly important to regional budgets and ecosystem productivity.

Rights

© 2024. The Authors.

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.

Data Availability

CTD and sensor data are archived at the Biological and Chemical Oceanography Data Management Office (BCO-DMO) on the SPIROPA (‘Shelfbreak Productivity Interdisciplinary Research Operation at the Pioneer Array’) Project page: https://www.bco-dmo.org/project/748894. The MODIS Aqua sea surface temperature and chlorophyll a concentration data are publicly available courtesy of the NASA Goddard Space Flight Center: https://oceancolor.gsfc.nasa.gov/. All measurements required to calculate N₂ fixation rates, as well as calculated specific N₂ uptake and N₂ fixation rates, are available in the Supplemental Materials associated with this publication, and have been submitted for archival purposes to the BCO-DMO data repository.

Original Publication Citation

Selden, C. R., Mulholland, M. R., Crider, K. E., Clayton, S., Macías-Tapia, A., Bernhardt, P., McGillicuddy Jr, D. J., Zhang, W. G., & Chappell, P. D. (2024). Nitrogen fixation at the Mid-Atlantic Bight shelfbreak and transport of newly fixed nitrogen to the Slope Sea. Journal of Geophysical Research: Oceans, 129(4), 1-14, Article e2023JC020651. https://doi.org/10.1029/2023JC020651

Repository Citation

Selden, C. R.; Mulholland, M. R.; Crider, K. E.; Clayton, S.; Macías-Tapia, A.; Bernhardt, P.; McGillicuddy, D. J. Jr.; Zhang, W. G.; and Chappell, P. D., "Nitrogen Fixation at the Mid-Atlantic Bight Shelfbreak and Transport of Newly Fixed Nitrogen to the Slope Sea" (2024). OES Faculty Publications. 508.
https://digitalcommons.odu.edu/oeas_fac_pubs/508

ORCID

0000-0002-5721-3061 (Selden), 0000-0001-8819-189X (Mulholland), 0000-0001-7473-4873 (Clayton), 0000-0003-3848-7013 (Bernhardt), 0000-0001-5212-6228 (Chappell)