Dissolved Methane Distribution in the Reloncaví Fjord and Adjacent Marine System During Austral Winter (41°-43° S)

Within the earth's atmosphere, methane (CH₄) is one of the most important absorbers of infrared energy. It is recognized that coastal areas contribute higher amounts of CH₄ emission; however, there is a lack of accurate estimates for these areas. This is particularly evident within the extensive northern fjord region of Chilean Patagonia, which has one of the highest freshwater runoffs in the world. Oceanographic and biogeochemical variables were analyzed between the Reloncaví fjord (41° S) and the Interior Sea of Chiloé (ISC) (43° S), during the 2013 austral winter. Freshwater runoff into the fjord influences salinity distribution, which clearly delimits the surface (5 m depth), and also separates the estuarine area from the marine area. In the estuary, the highest CH₄ levels are generally observed in the cold and brackish nutrient-depleted surface waters (N- and P-depleted), ranging from 16.97 to 151.4 nM (mean ± SD 52.20 ± 46.49), equivalent to 640-4537% saturation except for the case of Si(OH)₄. Conversely, subsurface waters have lower CH₄ levels, fluctuating from 14.3 to 29.6 nM (mean ± SD 22.75 ± 4.36 nM) or 552-1087% saturation. A significant negative correlation was observed between salinity and CH₄, and a positive correlation between Si(OH)₄ and CH₄, suggesting that some of the CH₄ in estuarine water is due to continental runoff. Furthermore, the accumulation of seston and/or plankton at the pycnocline may potentially generate the accumulation of CH₄ via microbial processes, as observed in estuarine waters. By contrast, the marine area (the ISC), which is predominantly made up of modified subantarctic water, has a relatively homogenous CH₄ distribution (mean ± SD 9.84 ± 6.20 nM). In comparison with other estuaries, the Reloncaví fjord is a moderate source of CH₄ to the atmosphere, with effluxes ranging from 23.9 to 136 μmol m⁻² day⁻¹. This is almost double the levels observed in the ISC, which ranges from 22.2 to 46.6 μmol m⁻² day⁻¹. Considering that Chilean Patagonia has numerous other fjord systems that are geomorphologically alike, and in some cases have much greater freshwater discharge, this study highlights their potential to be a significant natural source of this greenhouse gas.