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

Abstract

Biological nitrogen fixation is a key process balancing the loss of combined nitrogen in the marine nitrogen cycle. Its relevance in upwelling or high nutrient regions is still under debate, with the few available studies in these regions of the ocean reporting rates that vary widely from below detection limit to as high as 127nmol L−1d−1. In the eastern tropical Atlantic Ocean, two open ocean upwelling systems are active in boreal summer. One is the seasonal Equatorial upwelling, where the residual phosphorus associated with aging upwelled waters is suggested to enhance nitrogen fixation in this season. The other is the Guinea Dome, a thermal upwelling dome, where the information on the variability of nitrogen fixation is scarce and sparse. We conducted two surveys in the central Atlantic Ocean along 23ºW across the Guinea Dome and the Equator from ca. 16ºN to ca. 6ºS in September 2015 and August-September 2016 with high horizontal resolution (20 to 60 nmi between stations). The abundance of Trichodesmium colonies was characterized by an Underwater Vision Profiler 5 and the total biological nitrogen fixation in the euphotic layer was measured using the 15N2 technique. While the highest abundances of Trichodesmium colonies were found in the area of the Guinea Dome (9°−15°N) with a maximum of 3 colonies L−1 near the surface, colonies were almost absent in the Equatorial band between 2°N− 6°S. The highest nitrogen fixation rate was measured at the edges of the Guinea Dome in 2016 (ca. 31 nmol L−1 d−1), where a patchy distribution was unveiled by our high spatial resolution scheme, and diazotrophs thrive on a sufficient supply of both phosphorus and iron. In the Equatorial band, rates were considerably lower ranging from below detection limit to ca. 4 nmol L−1 d−1, with a clear difference in magnitude between 2015 (very low rates) and 2016 (average rates around 2 nmol L−1 d−1) probably due to a differential supply of phosphorus.