Organic carbon budget for the eastern boundary of the North Atlantic subtropical gyre: major role of DOC in mesopelagic respiration

Yeray Santana-Falcón,María Dolores Pérez-Hernández,Evan Mason,Javier Arístegui,Xosé Antón Álvarez-Salgado,Alonso Hernández-Guerra

doi:10.1038/s41598-017-10974-y

Abstract

Transports of suspended particulate (POCsusp) and dissolved (DOC) organic carbon are inferred from a box-model covering the eastern boundary of the North Atlantic subtropical gyre. Corresponding net respiration rates (R) are obtained from a net organic carbon budget that is based on the transport estimates, and includes both vertical and lateral fluxes. The overall R in the mesopelagic layer (100–1500 m) is 1.6 ± 0.4 mmol C m−2 d−1. DOC accounts for up to 53% of R as a result of drawdown of organic carbon within Eastern North Atlantic Central Water (ENACW) that is entrained into sinking Mediterranean Overflow Water (MOW) that leads to formation of Mediterranean water (MW) at intermediate depths (~900 m). DOC represents 90% of the respired non-sinking organic carbon. When converted into oxygen units, the computed net respiration rate represents less than half the oxygen utilization rates (OUR) reported for the mesopelagic waters of the subtropical North Atlantic. Mesoscale processes in the area, not quantified with our approach, could account in part for the OUR differences observed between our carbon budget and other published studies from the North Atlantic, although seasonal or interannual variability could also be responsible for the difference in the estimates.

Highlights

Oceanic respiration occurs over the entire water column[1, 2], as opposed to photosynthesis, which is restricted to the epipelagic layer
In the present study, following the approach used by Alonso-González and coworkers in the southern Canary Current[20], we estimate the contribution of DOC and POCsusp fluxes to net respiration in the eastern boundary of the North Atlantic subtropical gyre (Fig. 1)
Eastern North Atlantic Central Water (ENACW) is entrained into the dense fast-flowing Mediterranean Overflow Water (MOW) that exits through the Strait of Gibraltar[27,28,29] and is subducted along the slope into the Gulf of Cádiz[30,31,32,33] to below 1000 m depth, where it spreads into the Atlantic as Mediterranean Water (MW)[27,28,29, 34,35,36,37]

Summary

Introduction

Oceanic respiration occurs over the entire water column[1, 2], as opposed to photosynthesis, which is restricted to the epipelagic layer (roughly above 150 m). It has been already shown that, using a box-model approach, suspended particulate organic carbon (POCsusp) could support up to 59% of the total mesopelagic respiration in the southwestern sector of the Canary Current[20], a region strongly affected by the coastal-ocean export of particulate material from the NW African coastal upwelling system[21, 22]. In a previous study[39], based on a rough estimation of organic carbon fluxes in a box region that included the Strait of Gibraltar, it has been suggested that DOC remineralization could explain about 90% of the inorganic carbon produced in the water column. We aim to contribute to the understanding of the organic carbon dynamics in this complex region, which is characterized by the confluence of water masses of different origin, water mass entrainment and subduction, and intermittent upwelling[24, 40,41,42,43]

Objectives

Methods

Results

Conclusion