Abstract
Abstract. Four carbonate system variables were measured in surface waters during a cruise aimed at investigating ocean acidification impacts traversing northwestern European shelf seas in the summer of 2011. High-resolution surface water data were collected for partial pressure of carbon dioxide (pCO2; using two independent instruments) and pH using the total pH scale (pHT), in addition to discrete measurements of total alkalinity and dissolved inorganic carbon. We thus overdetermined the carbonate system (four measured variables, two degrees of freedom), which allowed us to evaluate the level of agreement between the variables on a cruise whose main aim was not intercomparison, and thus where conditions were more representative of normal working conditions. Calculations of carbonate system variables from other measurements generally compared well with direct observations of the same variables (Pearson's correlation coefficient always greater than or equal to 0.94; mean residuals were similar to the respective accuracies of the measurements). We therefore conclude that four of the independent data sets of carbonate chemistry variables were of high quality. A diurnal cycle with a maximum amplitude of 41 μatm was observed in the difference between the pCO2 values obtained by the two independent analytical pCO2 systems, and this was partly attributed to irregular seawater flows to the equilibrator and partly to biological activity inside the seawater supply and one of the equilibrators. We discuss how these issues can be addressed to improve carbonate chemistry data quality on future research cruises.
Highlights
Accurate determination of the inorganic carbon system is a key requirement for ocean acidification studies, as it forms the basis for assessments of biological and biogeochemical responses to changes in ocean carbonate chemistry as a result of rising atmospheric CO2 concentrations
The 95 % confidence interval of the slope does include the value 1 and the 95 % confidence interval of the intercept does include the value 0. The comparison between both sets of pCO2 data revealed a mean residual of −2 μatm
The analysis indicated that the correlation of pCO2 with Teq and Photosynthetically active irradiance (PAR) was statistically significant (p < 0.0001; F = 587.6) and that the two parameters together explained 38 % of the pCO2 variance
Summary
Accurate determination of the inorganic carbon system is a key requirement for ocean acidification studies, as it forms the basis for assessments of biological and biogeochemical responses to changes in ocean carbonate chemistry as a result of rising atmospheric CO2 concentrations. It is essential for the determination of the air–sea fluxes of CO2, calculation of carbon budgets and estimation of anthropogenic CO2 concentrations in different water masses. Körtzinger et al (1996) carried out what may have been the first intercomparison study in coastal waters between two designed underway pCO2 systems They found a remarkable agreement between the two simultaneously
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