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

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 N-15(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 biomass-independent 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.

Automated summary

This study analyzed the spatial heterogeneity of N-2 fixation on the western North Atlantic shelf and found exceptionally high rates in the frontal mixing zone driven by haptophyte-symbiont UCYN-A. N-2 fixation is favored offshore but occurs continuously across the shelf, indicating a niche for diazotrophs along the coastline as phytoplankton populations decline.