From a biogeochemical perspective, upwelling regions and the associated oxygen minimum zones are characterized by the complexity of their oceanographic conditions and the diversity of the biogeochemical transformations that take place along pronounced oxygen gradients. The biomass transportation and subsequent degradation in the sediment under anoxic conditions promotes the canonical microbial methanogenisis. In addition, aerobic methane (CH₄) production from bacterial cleavage of methylated compounds (like methylphosphonate, MPn) contributes to CH₄ supersaturation in the surface mixed layer. Recent studies have shown that coastal upwelling associated with oxygen minimum zones are considered as the largest pelagic CH₄ reservoirs in the ocean, making them marine hotspots for greenhouse gases. However, it is not yet clear whether climate change has the potential to cause an increase in greenhouse gas emissions from upwelling regions. In order to better predict the extent to which greenhouse gas sources and sinks in these areas will be affected by climate change, it is essential to (i) improve the quantification of the individual sources and sinks of relevant greenhouse gases and (ii) gain a mechanistic understanding of the underlying physical and microbial processes to allow better predictions of how a changing environment will affect the source strength of these gases. Based on our own studies in upwelling regions and other oxygen minimum zones, we hypothesize that (i) pelagic microbial CH₄ oxidation within the oxic/anoxic transition zone efficiently reduces CH₄ flux to the sea surface but is influenced by the intensity of vertical mixing between oxic surface water and CH₄-enriched deep water, and that (ii) degradation of MPn in surface waters contributes to CH₄ oversaturation and is controlled by the availability of inorganic phosphorus. To explore these questions, we plan to conduct a combination of oceanographic, biogeochemical and microbiological studies along a shelf transect off the coast of Concepción in cooperation with our Chilean partners from the University of Concepción, in order to define areas characterized by different mixing intensities within the oxic/anoxic transition zone. Parallel to high-resolution observations of turbulent mixing, we will assess microbial CH₄ oxidation, the abundance of methanotrophic bacteria, and their CH₄ oxidation activity. To verify the role of MPn degradation with respect to CH₄ production in surface waters, we will perform incubation experiments with surface water characterized by different nutrient availabilities.