Between Practicality and Precision: Evaluating Equilibration Methods for CO₂ in Eutrophic Tropical Waters

Carbon dioxide (CO₂) is one of the major greenhouse gases, whose increasing atmospheric concentration intensifies global warming. In tropical coastal waters under intense anthropogenic pressure, nutrient enrichment, high temperatures, and year-round solar radiation intensify the metabolic processes of photosynthesis and respiration, which consume and produce CO₂, respectively, promoting marked temporal variability in its partial pressure (pCO₂). Accurate determinations are essential to reduce uncertainties in estimates of CO₂ exchange at the air–water interface in these environments. This study evaluated the performance of manual equilibration compared with continuous equilibration for determining pCO₂ in eutrophic surface waters of Guanabara Bay (RJ). Sampling was carried out simultaneously using both methods during seasonal campaigns and diel cycles between June 2018 and January 2020, with discrete sampling every two hours. In manual equilibration, 1,000 mL bottles were filled with water without introducing bubbles, and a 60 mL N₂ headspace was introduced; after five minutes of agitation to achieve equilibrium between the liquid and gas phases, the gas was extracted and CO₂ was quantified by gas chromatography. Continuous equilibration, adopted as the reference method, was performed in a closed circuit using a “marble” -type equilibrator coupled to an infrared CO₂ analyzer, with data acquisition at intervals of approximately one minute. To ensure temporal correspondence between the methods, each manual determination was compared with the mean of the continuous values recorded during the 15 minutes before and after sampling. Biogeochemical conditions varied widely, with pH ranging from 7.1 to 9.1. The results showed that pCO₂ ranged from 63 to 1,630 µatm using continuous equilibration and from 320 to 9,668 µatm using manual equilibration. Manual equilibration yielded a greater range and more extreme values, and the two methods showed no significant correlation in the overall set of observations (Spearman correlation, n = 124; p > 0.05). The difference between the methods showed a positive median of 623 µatm (155.6%) and an interquartile range from 281 to 2,910 µatm (44% to 692%), indicating substantial dispersion among individual determinations. Potential sources of uncertainty include sample handling, gas exchange, and ongoing metabolic processes after sampling. Thus, although it offers operational advantages in the field, manual equilibration did not adequately represent the magnitude of pCO₂ recorded by the reference method under the conditions studied. Continuous equilibration therefore proved more appropriate for determining pCO₂ in eutrophic tropical coastal waters, combining direct measurements, high temporal resolution, and reduced susceptibility to interference associated with sample handling.