Abstract
Abstract. The goal of the Arabian Sea section of the TARA oceans expedition was to study large particulate matter (LPM > 100 μm) distributions and possible impact of associated midwater biological processes on vertical carbon export through the oxygen minimum zone (OMZ) of this region. We propose that observed spatial patterns in LPM distribution resulted from the timing and location of surface phytoplankton bloom, lateral transport, microbial processes in the core of the OMZ, and enhanced biological processes mediated by bacteria and zooplankton at the lower oxycline. Indeed, satellite-derived net primary production maps showed that the northern stations of the transect were under the influence of a previous major bloom event while the most southern stations were in a more oligotrophic situation. Lagrangian simulations of particle transport showed that deep particles of the northern stations could originate from the surface bloom while the southern stations could be considered as driven by 1-D vertical processes. In the first 200 m of the OMZ core, minima in nitrate concentrations and the intermediate nepheloid layer (INL) coincided with high concentrations of 100 μm < LPM < 200 μm. These particles could correspond to colonies of bacteria or detritus produced by anaerobic microbial activity. However, the calculated carbon flux through this layer was not affected. Vertical profiles of carbon flux indicate low flux attenuation in the OMZ, with a Martin model b exponent value of 0.22. At three stations, the lower oxycline was associated to a deep nepheloid layer, an increase of calculated carbon flux and an increase in mesozooplankton abundance. Enhanced bacterial activity and zooplankton feeding in the deep OMZ is proposed as a mechanism for the observed deep particle aggregation. Estimated lower flux attenuation in the upper OMZ and re-aggregation at the lower oxycline suggest that OMZ may be regions of enhanced carbon flux to the deep sea relative to non OMZ regions.
Highlights
In the coming decades, the ocean’s biogeochemical cycles and ecosystems will become increasingly stressed by the extending spatial distribution of oxygen minimum zones (OMZ; Stramma et al, 2010)
We propose that observed spatial patterns in large particulate matter (LPM) distribution resulted from the timing and location of surface phytoplankton bloom, lateral transport, microbial processes in the core of the OMZ, and enhanced biological processes mediated by bacteria and zooplankton at the lower oxycline
Instead we suggest that this deep increase in suspended particulate matter (SPM) and LPM under the OMZ core results from local physical and biological processes
Summary
The ocean’s biogeochemical cycles and ecosystems will become increasingly stressed by the extending spatial distribution of oxygen minimum zones (OMZ; Stramma et al, 2010). One particular concern is the impact of low oxygen on the biological pump by affecting plankton communities and their ability to produce and transform the vertical flux of organic matter to the ocean’s interior. The most intense and the largest are in subsurface of the upwelling regions in the eastern Pacific and the Arabian Sea of the northern Indian oceans (Paulmier and Ruiz-Pino, 2008). In the semi-enclosed Arabian Sea, the development of oxygen-deficient conditions is attributed to excessive oxygen consumption triggered by the supply of organic matter from high surface productivity, combined with weak renewal of mesopelagic waters (Naqvi, 1987; Jayakumar et al, 2004; Anderson et al, 2007).
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