681 / 2024-09-18 22:54:19
Inferring depth-integrated nitrogen fixation rates from surface measurements in global ocean
Marine nitrogen fixation,Depth-integrated,Global ocean
Session 3 - The nitrogen cycle towards a sustainable ocean: from microbes to global biogeochemistry
Abstract Accepted
Weili Chen / Xiamen University
Ya-Wei Luo / Xiamen University
Biological nitrogen (N2) fixation is a key process, contributing nearly one half of the bioavailable N input to the ocean. Limited spatiotemporal coverage of depth-integrated nitrogen fixation rates potentially induces uncertainty in estimations of N input. Previous studies have found depth-integrated N2 fixation rates had significant relationships with surface measurements. However, the relationships were limited in specific regions. We proposed a globally applicable relationship between depth-integrated and surface measurements based on 635 profiles, (depth-integrated NFR) = 71 × (surface NFR)0.83 (R2 = 0.88), and provided operational guidelines, making in-situ work for integral values convenient. And we categorized the profiles into three cases (well-mixed, monotonous decrease and subsurface maximum) and computed a mean vertical profile for each after quality control. Most profiles predicted well except those where N2 fixation rates increased with depth strongly and iron may play a role. Furthermore, we offered adjustment coefficients for ocean basins. The empirical formula appeared to overestimate some NFR observations sampled at depths shallower than euphotic zone depth (1% of surface light). It may suggest that we need to sample at least at euphotic zone depth to avoid neglecting N2 fixation rates in deep layer. Based on this relationship, we used boosted random forest to estimate global oceanic N2 fixation rates (64 - 127 Tg N yr -1; ranges based on one geometric standard error). Recently, a method for high frequency underway N2 fixation measurements has been used in field. The relationship might be potential for underway measurements and we expect this will improve our understanding of biogeography of marine N2 fixation rates.