The air‐water CO₂ exchange of a coastal sea—A sensitivity study on factors that influence the absorption and outgassing of CO₂ in the Baltic Sea

Abstract

In this study, the BALTSEM model is used to estimate how air‐water CO2 fluxes in the Baltic Sea respond to parameterizations of organic alkalinity (Aorg), gas transfer, and phytoplankton growth, and further to changes in river loads. The forcing data include the most complete compilation of Baltic river loads for dissolved inorganic and organic carbon (DIC and DOC) and total alkalinity (TA). In addition, we apply the most recent estimates of internal TA generation in the system. Our results clearly demonstrate how air‐water CO2 fluxes of a coastal sea depend on river loads of carbon, TA, and nutrients as well the freshwater import itself. Long‐term changes in DIC loads are shown to be compensated by corresponding changes in air‐water CO2 exchange. By adding Aorg, a discrepancy in the carbonate system calculations was removed, and the simulated net CO2 absorption of the system decreased by 11%. A new parameterization for cyanobacteria growth significantly improved the seasonal development of pCO2 in the central Baltic Sea, although the net effect on CO2 fluxes was below 5%. By applying either a linear, quadratic, or cubic wind speed dependence for gas transfer, the long‐term net CO2 exchange was adjusted by less than 5%. There is no clear indication that any one of these parameterizations provides a more accurate estimate of CO2 fluxes than the other two. Our findings are applicable in other coastal areas that are heavily influenced by river loads of TA, DIC, and DOC.

The influence of short-term variability in surface water pCO₂ on modelled air–sea CO₂ exchange

“…However, short-term variability is not always incorporated in modelling studies. Some marine models ignore short-term variability in the atmospheric CO 2 concentration, when the air-sea CO 2 exchange is modelled (Omstedt et al 2009;Gypens et al, 2011;Kuznetsov and Neumann, 2013;Gustafsson et al, 2015;Valsala and Murtugudde, 2015). Similarly, when the air-sea CO 2 exchange is studied from an atmospheric perspective, i.e.…”

Key processes in the coupled carbon, nitrogen, and phosphorus cycling of the Baltic Sea

“…For that reason we have no estimates of basin-wide mean C concentrations based on observations. Simulated organic and inorganic carbon concentrations have however been validated in earlier publications (Gustafsson et al 2014a(Gustafsson et al , 2015, and average basin-wide simulated concentrations of TC, TOC, and DIC are presented in Table 3.…”

Structure and functioning of the acid–base system in the Baltic Sea

“…This A org contribution amounted from 1.5 to 3.5 % of the measured A T values. Even higher values of more than 50 µmol kg −1 were found by Hammer et al (2017) in the Baltic Sea surface mixed layer during the spring phytoplankton bloom. They reported also that A org decreases with depth and approaches 0 below the permanent halocline due to pH decrease.…”

“…According to the definition (Dickson, 1981), they should be considered strong acids. In experimental studies Hammer et al (2017) also found that humic and/or fulvic substances are more acidic than the bulk DOM naturally occurring in the Baltic Sea. This may also be a source of uncertainty in the A T measurements as the determination of the titration end-point requires the titration of the samples through this pH region.…”

“…This means that they are independent of pressure and temperature and follow the law of conservation of mass during mixing, which are prerequisites for variables transported in models. The first attempts to include an A org parameterization in biogeochemical models were made by Gustafsson et al (2015) and . However, the sensitivity of the models with regard to the inclusion/negligence of A org has not yet been reported.…”