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.

Marine ammonia oxidizers that oxidize ammonium to nitrite are abundant in polar waters, especially during the winter in the deeper mixed-layer of West Antarctic Peninsula (WAP) waters. However, the activity and abundance of ammonia-oxidizers during the summer in surface coastal Antarctic waters remain unclear. In this study, the ammonia-oxidation rates, abundance and identity of ammonia-oxidizing bacteria (AOB) and archaea (AOA) were evaluated in the marine surface layer (to 30 m depth) in Chile Bay (Greenwich Island, WAP) over three consecutive late-summer periods (2017, 2018, and 2019). Ammonia-oxidation rates of 68.31 nmol N L−1 day−1 (2018) and 37.28 nmol N L−1 day−1 (2019) were detected from illuminated 2 m seawater incubations. However, high ammonia-oxidation rates between 267.75 and 109.38 nmol N L−1 day−1 were obtained under the dark condition at 30 m in 2018 and 2019, respectively. During the late-summer sampling periods both stratifying and mixing events occurring in the water column over short timescales (February–March). Metagenomic analysis of seven nitrogen cycle modules revealed the presence of ammonia-oxidizers, such as the Archaea Nitrosopumilus and the Bacteria Nitrosomonas and Nitrosospira, with AOA often being more abundant than AOB. However, quantification of specific amoA gene transcripts showed number of AOB being two orders of magnitude higher than AOA, with Nitrosomonas representing the most transcriptionally active AOB in the surface waters. Additionally, Candidatus Nitrosopelagicus and Nitrosopumilus, phylogenetically related to surface members of the NP-ε and NP-γ clades respectively, were the predominant AOA. Our findings expand the known distribution of ammonium-oxidizers to the marine surface layer, exposing their potential ecological role in supporting the marine Antarctic system during the productive summer periods.

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