Anomaly of total boron concentration in the brackish waters of the Baltic Sea and its consequence for the CO2 system calculations

Abstract

Abstract Borates are the third most important component of total alkalinity (A T ) in the oxic waters. Their concentrations are a function of the dissociation constant of boric acid and total boron (TB) concentration. The latter is approximated from salinity (S) as boron behave conservatively in the seawater. The linear dependencies between TB and S developed for the open ocean contain no intercept suggesting that river water contains no boron. Based on the historical data and our own measurements we identified a TB vs. S relationship specific for the Baltic Sea: TB [μmol kg −1 ] = 10.838 ∗ S + 13.821. In the series of the sensitivity tests we analysed what effect can have this anomaly on the determination of borate alkalinity (A B ) and on the calculations within the CO 2 system performed with A T as an input variable. Due to the high pK a for boric acid the influence of TB anomaly on A B exists only for pH > 8. The highest deviation in A B appears at low salinities. When salinity increases the effect becomes smaller and at salinities > 14, due to lower slope in TB vs. S dependency in the Baltic than in the open ocean, the effect on A B turns to negative and decreases further with the S increase. These uncertainties in A B influence calculations of pCO 2 (CO 2 partial pressure) and pH, when C T (total CO 2 concentration) and A T are used as input parameters (the combination used in biogeochemical models). For pCO 2 the discrepancies in calculations are not very much dependent on the A T . The highest are observed for low salinities and pH of 8.2–8.4, however they do not exceed 10 μatm. This relatively low influence of TB anomaly on pCO 2 calculations is a result of the high distance on the pH scale between high pCO 2 conditions (low pH) and the highest A B anomaly (high pH). In case of pH calculations the highest influence of TB anomaly is observed for the low A T and low S waters. For three different A T considered in our study the highest pH errors (up to 0.05 pH unit) were observed for A T = 500 μmol kg −1 , while the lowest (up to 0.01 pH unit) were observed for highly buffered waters (A T = 3000 μmol kg −1 ). Irrespective of the A T the highest errors were found for low C T simulating low pCO 2 (and thus high pH) conditions. This is due to the high pK a for boric acid that shifts the effects of the TB anomaly to high pH values. Although the observed discrepancies in pH and pCO 2 calculations due to TB anomaly manifest themselves only at the specific environmental conditions the use of experimentally obtained TB vs. S dependency will increase the accuracy of the CO 2 system calculations for the Baltic Sea and likely for other brackish systems.