Magnitude of nitrate turbulent diffusion in contrasting marine environments

Beatriz Mouriño-Carballido,Emilio Marañón,Paloma Chouciño,Mariña Blazquez Maseda,Bieito Fernández Castro,Rocío Graña,Borja Aguiar-González,Marina Villamaña,José Luis Otero Ferrer,Víctor Moreira-Coello

doi:10.1038/s41598-021-97731-4

Abstract

Difficulties to quantify ocean turbulence have limited our knowledge about the magnitude and variability of nitrate turbulent diffusion, which constitutes one of the main processes responsible for the supply of nitrogen to phytoplankton inhabiting the euphotic zone. We use an extensive dataset of microturbulence observations collected in contrasting oceanic regions, to build a model for nitrate diffusion into the euphotic zone, and obtain the first global map for the distribution of this process. A model including two predictors (surface temperature and nitrate vertical gradient) explained 50% of the variance in the nitrate diffusive flux. This model was applied to climatological data to predict nitrate diffusion in oligotrophic mid and low latitude regions. Mean nitrate diffusion (~ 20 Tmol N y−1) was comparable to nitrate entrainment due to seasonal mixed-layer deepening between 40°N–40ºS, and to the sum of global estimates of nitrogen fixation, fluvial fluxes and atmospheric deposition. These results indicate that nitrate diffusion represents one of the major sources of new nitrogen into the surface ocean in these regions.

Highlights

Deposition and biological nitrogen fixation across a longitudinal section in the Mediterranean Sea, the contribution of nitrate diffusion to new production ranged from 0.1 to 25%19
We used a large dataset of microturbulence observations and a multivariable fractional polynomial method to investigate the relationship between nitrate turbulent diffusion and environmental variables that are routinely measured during oceanographic cruises
The PERFILC cruise sampled 27 stations in the Bay of Biscay in July 2008. hHydrographic and microstructure turbulence profiles collected during both cruises were averaged arithmetically to obtain mean profiles which were assigned to the mean geographical location of all the stations conducted during each cruise

Summary

Introduction

Deposition and biological nitrogen fixation across a longitudinal section in the Mediterranean Sea, the contribution of nitrate diffusion to new production ranged from 0.1 to 25%19. In the northeast subtropical Atlantic, Painter et al.[20] reported nitrate diffusive fluxes comparable to N 2 fixation rates. According to Caffin et al.21 N2 fixation was the major source of new nitrogen (> 90%), compared to nitrate diffusion and atmospheric deposition, in the western tropical South Pacific. We used a large dataset of microturbulence observations and a multivariable fractional polynomial method to investigate the relationship between nitrate turbulent diffusion and environmental variables that are routinely measured during oceanographic cruises. The observed relationship was used to build the first large-scale map of the distribution of the supply of nitrate into the euphotic zone through turbulent diffusion

Methods

Results

Conclusion