Fjords and estuaries exchange large amounts of solutes, gases, and particulates between fluvial and marine systems. These exchanges and their relative distributions of compounds/particles are partially controlled by stratification and water circulation. The spatial and vertical distributions of N 2 O, an important greenhouse gas, along with other oceanographic variables, are analyzed from the Reloncaví estuary (RE) (~41°30′ S) to the gulf of Corcovado in the interior sea of Chiloé (43°45′ S) during the austral winter. Freshwater runoff into the estuary regulated salinity and stratification of the water column, clearly demarking the surface (<5 m depth) and subsurface layer (>5 m depth) and also separating estuarine and marine influenced areas. N 2 O levels varied between 8.3 and 21 nM (corresponding to 80 and 170 % saturation, respectively), being significantly lower (11.8 ± 1.70) at the surface than in subsurface waters in the Reloncaví estuary (14.5 ± 1.73). Low salinity and NO 3 − , NO 2 − , and PO 4 3− levels, as well as high Si(OH) 4 values were associated with low surface N 2 O levels. Remarkably, an accumulation of N 2 O was observed in the subsurface waters of the Reloncaví sound, associated with a relatively high consumption of O 2 . The sound is exposed to increasing anthropogenic impacts from aquaculture and urban discharge, occurring simultaneously with an internal recirculation, which leads to potential signals of early eutrophication. In contrast, within the interior sea of Chiloé (ISC), most of water column was quasi homohaline and occupied by modified subantarctic water (MSAAW), which was relatively rich in N 2 O (12.6 ± 2.36 nM) and NO 3 − (18.3 ± 1.63 μM). The relationship between salinity, nutrients, and N 2 O revealed that water from the open ocean, entering into ISC (the Gulf of Corcovado) through the Guafo mouth, was the main source of N 2 O (up to 21 nM), as it gradually mixed with estuarine water. In addition, significant relationships between N 2 O excess vs. AOU and N 2 O excess vs. NO 3 − suggest that part of N 2 O is also produced by nitrification. Our results show that the estuarine and marine waters can act as light source or sink of N 2 O to the atmosphere (air-sea N 2 O fluxes ranged from −1.57 to 5.75 μmol m −2 day −1 ), respectively; influxes seem to be associated to brackish water depleted in N 2 O that also caused a strong stratification, creating a barrier to gas exchange.

Within the earth's atmosphere, methane (CH 4 ) is one of the most important absorbers of infrared energy. It is recognized that coastal areas contribute higher amounts of CH 4 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 subsurface layers (>5 m depth), and also separates the estuarine area from the marine area. In the estuary, the highest CH 4 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 CH 4 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 CH 4 , and a positive correlation between Si(OH) 4 and CH 4 , suggesting that some of the CH 4 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 CH 4 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 CH 4 distribution (mean ± SD 9.84 ± 6.20 nM). In comparison with other estuaries, the Reloncaví fjord is a moderate source of CH 4 to the atmosphere, with effluxes ranging from 23.9 to 136 μmol m. This is almost double the levels obs e r v e d i n t h e I S C , w h i c h r a n g e s f r o m 2 2 . 2 t o 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.

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