Special issue  

The eastern tropical North Atlantic (ETNA) is characterized by a moderate O2 minimum zone (OMZ) with lowest O₂ concentrations just under 40μmol kg⁻¹ (Stramma et al., GRL, 2009). Current understanding is that the ETNA OMZ has been expanding over the past decades both in terms of vertical extent and intensity (Stramma et al., Science, 2008). The recent discovery of isolated low-oxygen (O2) eddy-associated water bodies, ranging from hypoxic (<20 μmol kg⁻¹) to even suboxic (<1 μmol kg⁻¹) conditions, in the generally well ventilated open ocean region near the Cape Verde Archipelago changed our understanding of oceanic processes in this area (Karstensen et al., BGD, 2014, doi:10.5194/bgd-11-17391-2014). In order to better understand the origin and development of these extreme dead-zone eddies as well as their biogeochemical and biological properties, a multi-facetted, multi-platform, interdisciplinary field study in the eastern tropical North Atlantic was designed to investigate biogeochemical and ecological processes in these eddies. This so-called "Dead Zone Eddy Hunt Study" was carried out most successfully in 2013/2014 and yielded a wide range of excellent results featuring unique and surprising findings. The special issue will cover the whole range of concerted hydrographic, biogeochemical, and biological observations carried out in the interdisciplinary study.