Autonomous profiling float observations reveal the dynamics of deep biomass distributions in the denitrifying oxygen minimum zone of the Arabian Sea

Data from 13 autonomous profiling BGC-Argo floats, equipped with biogeochemical and bio-optical sensors deployed between 2011 and 2016, were used to explore the potential of bio-optical methods to map deep biomass distribution in the Arabian Sea oxygen minimum zone (OMZ). Dissolved oxygen sensors revealed concentrations below 5 μmol kg−1 for much of the depth range between 200 and 400 m and below 1 μmol kg−1 in the centre of the OMZ, which is well below climatological values. Optical particle backscatter sensors revealed intensities within the upper OMZ that were nearly as high as within the euphotic zone. The distribution of these particulate scatterers was confined to oxygen concentrations below 1 μmol kg−1, with vertical distributions that exhibited maxima just below the top of the OMZ and decreased downward with similar attenuation to that expected for sinking organic matter. These distributions were very similar to those of nitrate deficits (ΔNO3) estimated from a float with an ultra-violet nitrate sensor (R2 = 0.54, p < 0.01), suggesting backscatter may be a useful proxy to investigate the changing dynamics of denitrification. Weak correlations between the upper OMZ particulate backscatter signals and both backscatter and chlorophyll fluorescence in the overlying euphotic zone is consistent with previous suggestions that advection of organic matter from remote sources is likely to be an important control on denitrification in the Arabian Sea. These results offer a useful path for examining denitrification dynamics in OMZs, which can be augmented by additional sensors for pH to quantify carbon respiration and dissolved organic matter to examine its possible importance relative to particulate oxygen demand. They clearly demonstrate the powerful insights that can be gained from arrays of autonomous profiling floats equipped with biogeochemical sensors for understanding globally significant, large-scale biogeochemical patterns and processes.