Oxygen utilization and downward carbon flux in an oxygen-depleted eddy in the eastern tropical North Atlantic

Björn Fiedler,Toste Tanhua,Johannes Karstensen,Hannes Wagner,Arne Körtzinger,Alexandra Loginova,Damian S Grundle,Rainer Kiko,Carolin R Löscher,Florian Schütte,Helena Hauss,Péricles Silva

doi:10.5194/bg-13-5633-2016

Abstract

Abstract. The occurrence of mesoscale eddies that develop suboxic environments at shallow depth (about 40–100 m) has recently been reported for the eastern tropical North Atlantic (ETNA). Their hydrographic structure suggests that the water mass inside the eddy is well isolated from ambient waters supporting the development of severe near-surface oxygen deficits. So far, hydrographic and biogeochemical characterization of these eddies was limited to a few autonomous surveys, with the use of moorings, underwater gliders and profiling floats. In this study we present results from the first dedicated biogeochemical survey of one of these eddies conducted in March 2014 near the Cape Verde Ocean Observatory (CVOO). During the survey the eddy core showed oxygen concentrations as low as 5 µmol kg−1 with a pH of around 7.6 at approximately 100 m depth. Correspondingly, the aragonite saturation level dropped to 1 at the same depth, thereby creating unfavorable conditions for calcifying organisms. To our knowledge, such enhanced acidity within near-surface waters has never been reported before for the open Atlantic Ocean. Vertical distributions of particulate organic matter and dissolved organic matter (POM and DOM), generally showed elevated concentrations in the surface mixed layer (0–70 m), with DOM also accumulating beneath the oxygen minimum. With the use of reference data from the upwelling region where these eddies are formed, the oxygen utilization rate was calculated by determining oxygen consumption through the remineralization of organic matter. Inside the core, we found these rates were almost 1 order of magnitude higher (apparent oxygen utilization rate (aOUR); 0.26 µmol kg−1 day−1) than typical values for the open North Atlantic. Computed downward fluxes for particulate organic carbon (POC), were around 0.19 to 0.23 g C m−2 day−1 at 100 m depth, clearly exceeding fluxes typical for an oligotrophic open-ocean setting. The observations support the view that the oxygen-depleted eddies can be viewed as isolated, westwards propagating upwelling systems of their own, thereby represent re-occurring alien biogeochemical environments in the ETNA.

Highlights

New technological advances in ocean observation platforms, such as profiling floats, gliders and in sensors have greatly facilitated our knowledge about physical, chemical and biological processes in the oceans, those occurring on small spatiotemporal scales (Johnson et al, 2009; Roemmich et al, 2009)
The temperature–salinity (TS) characteristics of the subsurface core of anticyclone mode-water eddies (ACMEs) in the open eastern tropical North Atlantic (ETNA) (Karstensen et al, 2015; Schütte et al, 2016b) were found to be nearly unchanged, compared to coastal regions. They resemble South Atlantic Central Water (SACW), the dominating upper layer water mass in the Mauritanian upwelling region, whereas the region around Cape Verde Ocean Observatory (CVOO) is dominated by high salinity North Atlantic Central Waters (NACW; Pastor et al, 2008)
As expected for a low-oxygen eddy, the TS characteristic in the 2014 eddy core for the two surveys matched very well with the characteristic found from the Mauritanian shelf reference stations (Fig. 2)

Summary

Introduction

New technological advances in ocean observation platforms, such as profiling floats, gliders and in sensors have greatly facilitated our knowledge about physical, chemical and biological processes in the oceans, those occurring on small spatiotemporal scales (Johnson et al, 2009; Roemmich et al, 2009). Satellite data and model studies show that eddies do play an important role in the offshore transport of organic matter and nutrients from the eastern boundary upwelling system (EBUS) into the open ocean. Considering their transport alone, eddies have been found to create a negative impact on productivity in the EBUS regions because of their net nutrient export (Gruber et al, 2011; Nagai et al, 2015; Rossi et al, 2009)

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Results