Biogeochemistry of Recently Discovered Oxygen-Depleted Mesoscale Eddies in the Open Eastern Tropical North Atlantic

Severely oxygen-depleted mesoscale features in the open eastern tropical North Atlantic, which are formed in the Mauritanian upwelling region, were discovered only recently. So far, few remote surveys conducted with autonomous platforms such as moorings, underwater gliders and profiling floats have provided a very first insight into these mesoscale eddies. Due to their hydrographic properties such water bodies are well isolated from ambient waters and therefore can develop severe near-surface oxygen deficits.

In this presentation we show results from the first-ever biogeochemical survey of one of these anticyclonic mode-water eddies conducted in spring 2014 at the Cape Verde Ocean Observatory (CVOO) off West Africa. Very low oxygen concentrations of 4.5 µmol kg^-1 associated with a CO₂ partial pressure of 1164 µatm were found close to the core of the eddy (at 100 m depth). Measurements for nitrate and phosphate also show exceptional high values. Findings point to rapid oxygen consumption through remineralization of organic matter along with depressed lateral mixing of this water body. Indeed, rates for oxygen utilization (OUR) were found to be enhanced when compared to known values in the Atlantic. A closer look into the carbonate system inside the eddy´s core revealed disadvantageous conditions for calcifying organisms with the pH dropping down to 7.6 and the Aragonite saturation level reaching 1 at the lower boundary of the euphotic zone.

Finally, strong indications for a shift in nitrogen cycling in the core of the eddy from nitrification towards denitrification were found based on gene abundance and N₂O-isotope analyses. To our knowledge such severe hypoxic and even suboxic near-surface conditions along with active denitrification have never been reported before in the open Atlantic Ocean.