The strong and dynamic horizontal chemical gradients in the southeastern Pacific (SEP), from ultra-oligotrophic waters to coastal upwelling, exhibit highly variable combinations of low O2 and pH/high pCO2 and include strong vertical changes in 02 concentrations, from saturation levels at the surface to undetectable levels in the nucleus of minimum oxygen zone (MOZ). For fieldwork, our Institute will conduct observations and experiments during expeditions. In situ estimations will be made of physiological/metabolic rates and communities, gene expression and particle flows (in particular associated with the carbon cycle, which in minimum oxygen zones is relatively unknown). As well, culture-independent genomic analysis of representative species will allow for putting to the test the associations between genomic and physiological variations. The composition and functioning of the communities will be analyzed and compared to the genomic population among the specific representatives of functional plankton groups.