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The occurrence of mesoscale eddies that develop suboxic environments atshallow depth (about 40–100 m) has recently been reported for the eastern tropical North Atlantic (ETNA). Their hydrographic structure suggests thatthe water mass inside the eddy is well isolated from ambient waterssupporting the development of severe near-surface oxygen deficits. So far,hydrographic and biogeochemical characterization of these eddies was limitedto a few autonomous surveys, with the use of moorings, underwater gliders andprofiling floats. In this study we present results from the first dedicatedbiogeochemical survey of one of these eddies conducted in March 2014 near theCape Verde Ocean Observatory (CVOO). During the survey the eddy core showedoxygen concentrations as low as 5 µmol kg−1 with a pH ofaround 7.6 at approximately 100 m depth. Correspondingly, the aragonitesaturation level dropped to 1 at the same depth, thereby creating unfavorableconditions for calcifying organisms. To our knowledge, such enhanced aciditywithin near-surface waters has never been reported before for the openAtlantic Ocean. Vertical distributions of particulate organic matter and dissolved organic matter (POM and DOM), generally showed elevated concentrations in the surfacemixed layer (0–70 m), with DOM also accumulating beneath the oxygenminimum. With the use of reference data from the upwelling region where theseeddies are formed, the oxygen utilization rate was calculated by determiningoxygen consumption through the remineralization of organic matter. Inside thecore, 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 viewedas isolated, westwards propagating upwelling systems of their own, therebyrepresent re-occurring alien biogeochemical environments in the ETNA

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