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

Abstract

AbstractContinental 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.