A coastal N₂ fixation hotspot at the Cape Hatteras front: Elucidating spatial heterogeneity in diazotroph activity via supervised machine learning

Abstract

In the North Atlantic Ocean, dinitrogen (N 2 ) fixation on the western continental shelf represents a significant fraction of basin-wide nitrogen (N) inputs. However, the factors regulating coastal N 2 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 N 2 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 15 N 2 bubble release method and used to build empirical models of regional N 2 fixation via a random forest machine learning approach. N 2 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 N 2 fixation rates (> 100 nmol N L −1 d −1 ), which were likely driven by the haptophyte-symbiont UCYN-A. Above and below this hotspot, N 2 fixation rates were highest on the shelf due to the high particulate N concentrations there. Conversely, specific N 2 uptake rates, a biomassindependent metric for diazotroph activity, were enhanced in the oligotrophic slope waters. Broadly, these observations suggest that N 2 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.

UCYN-A/haptophyte symbioses dominate N2 fixation in the Southern California Current System

“…N 2 fixation by UCYN-A1 was more important in offshore waters, while UCYN-A2 contributed a higher share of the total N 2 fixation in nearshore waters, suggesting that even when these sublineages co-occur, their activity may be influenced by different environmental factors. More measurements in other coastal ecosystems are needed to better understand the variability and environmental controls on UCYN-A N 2 fixation, however, these findings lend support to speculation about its importance to N 2 fixation in other coastal systems [ 6 , 7 , 10 , 89 , 90 ]. In addition, although whole community N 2 fixation does not account for a large amount of new production in this system, this work establishes that N 2 fixation is a widespread process along the Baja California peninsula, during both upwelling and oceanic seasons and throughout the euphotic zone, with the highest volumetric rates in euphotic waters overlying the continental shelf.…”

Coastal upwelling enhances abundance of a symbiotic diazotroph (UCYN-A) and its haptophyte host in the Arctic Ocean

“…The gene concentration range for synthetic plasmid dilution curves ranged from 9.98 × 10 1 -10 8 gene copies L -1 for B. bigelowii 18S rRNA, and 3.56 × 10 1 -10 8 gene copies L -1 for UCYN-A1 and UCYN-A2. Effective limits of detection and quantification were calculated as described in Selden et al (2021) by assuming that the minimum detectable and quantifiable concentrations are 3 and 10 copies reaction -1 , respectively (Bustin et al, 2020). These limits varied due to slight differences in sample filtration volume, ranging from 170-220 gene copies L -1 and 575-735 gene copies L -1 , respectively.…”

Nitrogen fixation rates in the Guinea Dome and the Equatorial upwelling regions in the Atlantic Ocean

“…6) also shows higher rates at the southern end of the dome around 8°−9°N. Interestingly, it seems that the physicochemical processes at the boundaries of this atypical upwelling system enhance nitrogen fixation, likely supported by Trichodesmium, in a similar fashion to other boundary systems, e.g., the Cape Hatteras front (Mulholland et al 2019;Selden et al 2021).…”