| 01 Jun 2026
Vertical Fluxes of Dissolved Oxygen Driven by Turbulence and Diapycnal Mixing in Patagonian Fjords
Abstract. A global inventory of oceanic dissolved oxygen (DO) indicates that only 0.6 % of the ocean's oxygen originates from the atmosphere. This makes the ocean highly sensitive to both natural and anthropogenic disturbances that can disrupt the physical and biogeochemical processes governing oceanic DO dynamics. The impact of ocean deoxygenation has accelerated globally, driven by warming and diminishing deep-water ventilation. Therefore, the primary objective of this work is to quantify, for the first time, the contribution of the dissipation of turbulent kinetic energy and the diapycnal eddy diffusivity (diapycnal mixing) to water ventilation, as evidenced by the occurrence of upward oxygen transport from deep to subsurface layers denoted as positive dissolved oxygen fluxes in the northern Patagonian fjords. A vertical microstructure profiler was used to measure, with high vertical resolution, the dissipation rate of turbulent kinetic energy and the oxygen characteristics of water at approximately 160 stations in the northern Patagonian fjords during seasonal campaigns in 2023. The results showed a range of dissipation between 10-9 and 10-4 W kg-1, and diapycnal mixing values ranging from 10-5 to 10-2 m2 s-1. The highest values of both variables were recorded in the Chiloé Inner Sea, where previously reported intense mixing has been attributed to tidal energy. Regarding oxygen flux records, larger positive fluxes were reported in the surface layer (10-5 and 10-3 µmol L-1s-1). Still, significant events of positive fluxes were recorded subsurface and in the deep layers of the Puyuhuapi Fjord due to intense diapycnal mixing forced by the advection of dense ocean waters. Moreover, intense turbulence contributes to positive oxygen fluxes over more of the water column than just the surface layer, especially in the Chiloé Inner Sea, driven by the interaction between the semidiurnal tides with the complex topography of the region. Moreover, double-diffusive convection appeared to be another mechanism favoring deep-water ventilation. Our results highlight the importance of incorporating turbulence measurements into fjord studies to understand sensitivity to significant oxygen variability.
Dear authors,
The article describes large dataset of extensive VMP profiles for almost a year in the Patagonian fjords, accompanied by a numerical model. It focuses on the pathways of oxygen including seasonal hypoxia and ventilation, and aims to identify its drivers.
The general topic of oxygen conditions and dynamics in the Patagonian fjords worked by you is indeed interesting and not well documented in literature. Taking into account turbulence measurements to assess oxygen pathways as well as modeling data to search for physical drivers of said oxygen pathways seems like a valid approach.
Unfortunately, there are multiple major and minor concerns regarding the scientific quality of the MS:
Eq. 2 and 3 are neither well introduced nor referenced correctly. Lefor et al. (2012) is not listed in the bibliography and I could not find a proper reference during an online search. Valle-Levinson (2010) is also not listed in the bibliography. This sloppiness goes through the whole MS (mixing of lower and upper case k and K, different Ks in text and equation (e.g. lines 145, 252), velocities and velocity fluctuations mixed up) and makes the reading exhausting.
From my point of view a major inconsistency is the interpretation of the time tendency of oxygen (∂O/∂t) as an upward or downward transport based on its sign: Line 153 "When ∂O/∂t > 0, oxygen is being transported upward.". This might be the case, but must not be and there is no reason to assume that. Since the whole MS is based on these assumptions the MS cannot be suggested for publication at this stage.
Some minor details are listed below:
Line 18: “A global inventory of oceanic dissolved oxygen (DO) indicates that only 0.6% of the ocean’s oxygen originates from the atmosphere.” Is there a source for this statement?
Line 138: Where does equation 1 come from?
Line 146 - 147: "The diapycnal eddy diffusivity was referred to along the manuscript as diapycnal mixing.": This makes no sense, what is the purpose to rename the well established term "eddy diffusivity" with a term that is as also well defined but used here completely different?
Line 148: K_{rho} was not introduced or defined before and does not show up in any equations in the MS. Why name it at all, if it is directly replaced by K_{shear}?
Line 194-195: Where does equation 6 and the factor 0.25 come from?
Line 239: The large sigma symbol for the turbulent mixing is likely an error, or not defined/repeated in the MS
Line 290-291: Is it enough to analyse surface currents to characterize the general circulation in the study site?
Despite all criticism, due to the beautiful dataset and the relevance of the topic I encourage the authors to resubmit after careful reanalysis.