Program

The coastal zones of major oxygen minimum zones (OMZs) in the eastern tropical North Atlantic (ETNA) and South Pacific (ETSP) geographically overlap with upwelling systems. To assess the current state of OMZs and successfully predict their evolution in a changing climate, processes supplying and consuming oxygen need to be understood and quantified. Biological oxygen consumption corresponds mainly to respiration of dissolved (DOM) and particulate organic matter (POM). Within the frame of the German collaborative research project SFB 754, we conducted field studies in the ETNA and ETSP to investigate the episodic supply of labile DOM by submesoscale processes as well as by sinking of organic particles. To determine the abundance, export and chemical composition of organic matter, we used a variety of methods including glider and ship-based observations, deployments of surface tethered drifting sediment traps, as well as optical and chemical analyses. Our data suggest that subduction of submesoscale filaments contributes to ventilation of the upper OMZ, simultaneously supplying DOM to a zone of high microbial activity. In addition, diapycnal flux was identified as an important process supplying DOC on the order of 4±7 mol C m^-2 yr^-1 to the upper OMZ off Peru.  About 20% of diapycnal DOC flux was composed of semi-labile components, e.g. carbohydrates and amino acids.  Another important DOM supply mechanism was the sedimentary release of DOM, which we identified using glider based high resolution CDOM measurements. Subsequent upwelling of released DOC can contribute to high surface DOC concentration and may partly explain diapycnal supply to the upper OMZ. Whereas, rapid microbial consumption of DOM can help maintain the oxygen deficiency in the upper OMZ, degradation of organic particles sinking to the OMZ core is often thought to be reduced.  Our data from the ETNA suggest that attenuation of carbon export is stronger in areas with an OMZ compared to fully oxygenated waters, corroborating earlier observations. Our data also show that attenuation of carbon export fluxes depends on the component in which carbon is included. For instance, carbon included in transparent exopolymeric particles (TEP) appeared to degrade less during sinking than bulk POC. Predicting future expansion of OMZs and climate feedbacks will strongly depend on our ability to reveal the secrets of physical-bio-geo-chemical coupling in upwelling systems.