Spatial Distribution of Dissolved Methane Over Extreme Oceanographic Gradients in the Subtropical Eastern South Pacific (17° to 37°S)

Methane (CH4) is one of the most powerful greenhouse gases with the capacity to influence the Earth's radiative budget as well as contribute to atmospheric chemistry. Natural oceanic production makes up to ∼4% of the overall global CH4 emissions, however, there is uncertainty around the accuracy of this value due to a lack of accurate measurements. Such is the case in the Subtropical Eastern South Pacific Ocean (SESP), a region with pronounced chlorophyll‐a and oxygen gradients, which in turn affect the microbial CH4 cycling. This study was conducted during spring‐summer (2014–2016) in the SESP. The region (∼17°–37°S/71°–110°W) is separated into (i) eutrophic, (ii) mesotrophic, and (iii) oligotrophic areas, according to oceanographic and biogeochemical criteria. The SESP presents high CH4 zonal variability with levels ranging from 0.63 to 33.4 nmol L−1, corresponding to 29% and 1,423% saturation, respectively. High CH4 concentrations (>1,000% saturation) are observed in the narrow eutrophic area subjected to coastal upwelling. These conditions clearly differ to those observed in the extended oligotrophic subtropical gyre (∼100% saturation). Furthermore, CH4 also tends to accumulate in the mesotrophic area (with upto 1,423% saturation), where oceanographic conditions as stratification, mesoscale eddies and island mass effect could trigger the presence of a microbial biomass that may be able to induce CH4 regeneration. The CH4 efflux is estimated to be between 0.13 and 19.1 µmol m−2 d−1 (mean ± SD = 4.72 ± 4.67) and the SESP has an emission rate of ∼87.9 Gg CH4 yr−1. Plain Language Summary Methane is a potent greenhouse gas that escapes from different natural and anthropogenic sources into the atmosphere and thus accelerates climate change. Atmospheric CH4 concentrations have risen 2.5 times since the beginning of the Industrial age. While much of this increase is attributed to human activities, natural sources can contribute between 35% and 50% of global CH4 emissions, Aquatic environments as estuarine, coastal, and open sea systems make up to ∼4% of the overall global CH4 emissions. Subtropical Eastern South Pacific Ocean (SESP), a region with different coastal (upwelling) and open sea (subtropical gyres) ecosystems and pronounced photosynthetic biomass and oxygen gradients. There, the majority of CH4 is created by microorganisms whose activities depend on both oxygen and organic matter variables. The SESP presents high CH4 spatial variability with levels