Oceanographic Processes Favoring Deoxygenation Inside Patagonian Fjords

Abstract

Abstract. The dissolved oxygen (DO) levels of oceanic-coastal waters has decreased over the last decade owing to the increase in surface water temperature caused by climate change. In addition, biological and human activity in coastal zones, bays, and estuaries has contributed to the acceleration of current deoxygenation. The Patagonian fjord and channel system is one world region where low DO water (LDOW, 30 %–60 % oxygen saturation) and hypoxia conditions (< 30 % oxygen saturation, 2 ml L−1 or 89.2 µmol L−1) is observed. An in-situ data set of hydrographic and biogeochemical parameters (2017 stations), collected from sporadic oceanographic cruises between 1970 and 2021, was used to quantify the mechanism involved in the presence of LDOW and hypoxic conditions in northern Patagonian fjords. Results denoted two main areas with LDOW (e.g., Puyuhuapi Fjord-Jacaf channel, Comau Fjord, and the Reloncaví estuarine system) extending from 25–400 m depth. Simultaneously, hypoxia was recorded in the Puyuhuapi Fjord, Jacaf Channel, and Quitralco Fjord. Quitralco registered the lowest values of DO (9.36 µmol L−1 and 1.6 % oxygen saturation) of the entire Patagonian fjord system. Areas of LDOW and hypoxia coincided with the accumulation of inorganic nutrients. Water mass analysis confirmed the contribution of equatorial subsurface water in the advection of the LDOW to only the Puyuhuapi Fjord and Jacaf Channel. In addition, in Puyuhuapi Fjord, hypoxic conditions occurred when the community respiration rate (6.6 g C m−2d−1) exceeded the gross primary production estimate (1.9 g C m−2d−1) possibly due to the increased consumption of DO during the use of both autochthonous and allochthonous organic matter. Biogeochemical processes and circulation regimens also contribute to deoxygenation and will be part of the discussion of the present research.

Toxic Algal Bloom Recurrence in the Era of Global Change: Lessons from the Chilean Patagonian Fjords

“…In this sense, it is important to clarify that salmon was a non-indigenous species in Chile when the farms started in the area, and, therefore, the possible effects derived from salmon cultivation can be considered an anthropogenic effect, as it is eutrophication. Although some areas are at higher risk of eutrophication than others, e.g., due to high aquaculture loads [ 85 ], a recent study showed that the Patagonian fjords have not undergone significant changes in the nutrient concentrations of their water columns in the last 50 years [ 86 ], consistent with some features of system 2. Finally, it has been recently recognized that the seasonal upwelling processes on the adjacent oceanic shelf [ 87 , 88 ] give the fjord system conditions consistent with those of system 1.…”

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

“…World Ocean Atlas data are available at https://odv.awi.de/ and https://www.ncei.noaa.gov/products/world-ocean-atlas. In situ hydrographic profiles and time series collected along the Patagonian fjords are available at the Zenodo (Linford et al., 2023a; https://doi.org/10.5281/zenodo.7799354, and Linford et al., 2023b; https://doi.org/10.5281/zenodo.7799378).…”