Nitrogen fixation at the Mid‐Atlantic Bight shelfbreak and transport of newly fixed nitrogen to the Slope Sea

Selden, C. R.; Mulholland, M. R.; Crider, K. E.; Clayton, S.; Macías‐Tapia, A.; Bernhardt, P.; McGillicuddy, 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). 10.1029/2023JC020651

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Abstract/Summary

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.

Item Type:	Publication - Article
Digital Object Identifier (DOI):	10.1029/2023JC020651
ISSN:	2169-9275
Additional Keywords:	N2 fixation, diazotroph, front, Gulf Stream, Mid-Atlantic Bight, warm-core ring, streamer
Date made live:	25 Apr 2024 14:50 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/537341

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