Argo Float Reveals Biogeochemical Characteristics Along the Freshwater Gradient Off Western Patagonia

Abstract

The coastal region off Chilean Patagonia has been poorly studied due to the lack of available observations. Here we analyzed, by the very first time, biogeochemical (BGC) data to elucidate the role that biological and physical processes play on nitrate, oxygen, pH and hydrographic variables, along a salinity gradient off central Patagonia. Argo float profiles covering the upper ocean from December 2015 to July 2019 reveal that offshore waters are characterized by low temperatures and high salinities related to high oxygen and medium-high values of pH and nitrate. As the Argo float drifted onshore, freshwater influences the upper 50–100 m with low salinity and high temperature. Waters under the influence of the continental runoff were characterized by medium-to-high oxygen and pH levels, and the lowest nitrate concentrations. Interestingly, oxygen-deficient waters located beneath the freshwater-modified layer showed the lowest pH and highest nitrate. A comprehensive analysis of the temporal and vertical variability of the oxygen:nitrate ratio, in conjunction with biological-related and physical parameters, indicates that the BGC variability seems to be the result of a synergistic interaction between physical and biological processes, where the stratification sets up the environment and promotes the biological response that, in turn, is auto-regulated by modifying the chemical composition in the freshwater-influenced zone. The arrival of future floats with additional sensors (Chlorophyll/Fluorescence, Photosynthetically Active Radiation, Backscatter, etc.) will add new BGC properties that improve our understanding of the coastal marine response to the increasing freshwater input off western Patagonia in the context of climate change.

Recent Deoxygenation of Patagonian Fjord Subsurface Waters Connected to the Peru–Chile Undercurrent and Equatorial Subsurface Water Variability

“…The association between DO changes and ESSW occurs at both intraseasonal and seasonal timescales, as evidenced by the out-of phase relationship between seasonal DO and PCUC magnitude: minimum DO values during summer-fall coincide with the maximum PCUC through GM (Figures 10 and 12). However, average DO records registered in the open waters of the adjacent Pacific Ocean showed values around the hypoxic limit (Galán et al, 2021;Silva et al, 2009), as was observed in the deep waters of GM (Figures 3 and 4) and CTD-DO measurements (Figure S1 in Supporting Information S1).…”

“…Moreover, an essential variability to oceanographic processes in this region comes from circulation and surface wind regimes (Aguirre et al, 2019;Pérez-Santos et al, 2019;Strub et al, 2019). The South Pacific current (∼40°S) forced by the westerly wind belt (e.g., Garreaud, 2007) participated in the origin of the Humboldt and Cape Horn current systems, aiding the distribution of the water mass properties (Galán et al, 2021;Schneider & Bravo, 2006;Schneider et al, 2005;Strub et al, 2019). Recently, Pujol et al (2022) reported an increase in the frequency and day-duration of marine heatwave events along with central and south Chile due to the advection of warmer water from the extratropical region and by changes in the atmospheric-ocean conditions.…”

“…Moreover, an essential variability to oceanographic processes in this region comes from circulation and surface wind regimes (Aguirre et al., 2019; Pérez‐Santos et al., 2019; Strub et al., 2019). The South Pacific current (∼40°S) forced by the westerly wind belt (e.g., Garreaud, 2007) participated in the origin of the Humboldt and Cape Horn current systems, aiding the distribution of the water mass properties (Galán et al., 2021; Schneider & Bravo, 2006; Schneider et al., 2005; Strub et al., 2019). Recently, Pujol et al.…”

Satellite-Derived Variability of Sea Surface Salinity and Geostrophic Currents off Western Patagonia

“…Two cores of minimum salinities continue in summer (from January to March) and are centered around 43-48°S and 49-52°S (Figure 4a). The presence of the northern core may be attributed to the contribution of freshwater from the Gulf of Penas (e.g., [22]), which accounts for the freshwater discharges from the Pascua and Baker rivers (e.g., [42,43]). This input of freshwater is highly significant in the region as it alters the physical and biogeochemical characteristics of the upper ocean [9,22].…”

“…An extensive low-salinity tongue extending from approximately 40ºS to 56ºS, with the freshwater band extending up to 5º offshore, has principally been documented [8,21]. An extended analysis showed the contrasting seasonal variability and vertical structure of the freshwater wedge along the meridional gradient off western Patagonia [9], whereas the dominant biogeochemical conditions were described along the freshwater gradient off central Patagonia by Galán et al [22]. To the best of our knowledge, this is the first study analyzing the spatio-temporal variability of the low-salinity waters off western Patagonia based on the improved OISSS product, whereas the transport of the Cape Horn Current has been recently inspected based on sea level variability [14].…”