Zooplankton distribution in a hypoxic eddy in the Eastern Tropical North Atlantic - an “open ocean dead zone'?

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The recent discovery of mesoscale eddies in the Eastern Tropical North Atlantic (ETNA) that harbor an intense oxygen minimum zone (OMZ) just below the mixed layer has led us to conduct an interdisciplinary eddy hunt. In spring 2014, an anticyclonic mode water eddy passing north of Cape Verde was tracked using satellite data and gliders, followed by ship-based sampling. The eddy was characterized by increased nitrate and Chl-a, along with a 1.5 to 2-fold increase in total area-integrated zooplankton abundance. O2 concentrations were as low as 4.5 μmol kg-1 (85 to 120 m depth). In this depth range, a marked reduction in target strength (shipboard ADCP, 75 kHz) at nighttime was evident. Acoustic scatterers were avoiding this zone and were compressed at the surface. However, vertically stratified multinet hauls and Underwater Vision Profiler (UVP5) image data revealed that this depth range was not completely void of metazoan life. Many of the smaller and/or less mobile organisms targeted by the multinet were able to tolerate conditions in the shallow OMZ. In particular, Oncaea spp., ostracods, eucalanoid copepods and siphonophores seemed to favor this zone, while e.g. euphausiids appeared to avoid it.