Simultaneous CO2 and O2 Supersaturation in Waters of Southern Patagonia? The Importance of Evaluating Overall Carbonate System Parameters Uncertainty and External Consistency. A Comment to Vargas et al., 2018

Vargas and collaborators (Vargas et al., 2018), therein V2018, reported measurements of pH, total alkalinity (A(T)), dissolved inorganic carbon (C-T), dissolved oxygen (DO) and other hydrographic variables along a portion of the Strait of Magellan and associated fjords during the spring of 2010. Using the measured values of A(T) and pH to calculate the partial pressure of CO2 (pCO(2)), they showed that DO supersaturated waters in the upper 50 m depth were often associated with pCO(2) higher than atmospheric CO2 levels. Here we show that this anomalous pattern arises because V2018's carbonate system parameters were apparently incorrectly calculated, resulting in positively biased pCO(2) values. A(T) measurements in this layer were often higher than the marine end-member, which could indicate an additional source of bias. Biased carbonate system parameters used by V2018 imply a sub-estimation of the capacity of CO2 uptake, with important consequences for local capacity to modulate the potential deleterious effects of freshening and ocean acidification on fjord biota. The critical analysis of the global sources of uncertainty in carbonate-system parameters, and evaluation of internal and external consistency (i.e., with complementary hydrographic information or with previous reports), are necessary to detect bias, and generate data with sufficient quality to study the effect of ocean acidification on marine ecosystems (GOA-ON, 2019).

Automated summary

The study revealed errors in the calculation of carbonate system parameters by Vargas et al. during measurements in the Strait of Magellan and associated fjords, leading to overestimated pCO(2) values. These errors may have significant implications for the local modulation of potential deleterious effects of freshening and ocean acidification on fjord biota.