Modelling the Baltic Sea acid–base (pH) balanceRising atmospheric carbon dioxide levels due to human activity have been shown to reduce the ocean pH by 0.1 units and are believed likely to reduce it even more in the future – according to some recent estimates, by up to 0.4 pH units during the coming 100 years (IPCC 2007). There are also suggestions that acid precipitation may increase the acidification of coastal seas, which then may lead to more severe conditions in these areas (Doney et al. 2007) in the absence of other processes damping coastal acidification.
The acid–base (pH) balance is somewhat more complicated than the heat or water balances due to the buffer effect. The balance is controlled by the amount of protons added to (donors) or subtracted from (acceptors) the balance. The protons, however, also interact with the buffer system, which can be understood as the total alkalinity (AT). The total alkalinity is a major component of the ocean carbon system, and can be defined as “the excess of proton acceptors over proton donors” (Dickson 1981). Typical proton donors are carbon acid, sulphuric acid, and nitric acid while a typical proton acceptor is limestone. This is illustrated in Fig. 1, which indicates that if we add, for example, limestone to the marine system, we increase the amount of basic material while also strengthening the buffer system, resulting in only a minor increase in pH. On the other hand, if we add carbon acid, the pH does not change as long as the buffer system is strong. The problem, however, is that proton donors also influence and weaken the buffer system, which may lead to a drastic pH reduction if a critical total alkalinity level is reached. This has been observed in lakes due to airborne sulphur acid, though lakes often have much weaker buffer capabilities than do marine systems.
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Figure 1. The acid–base (pH) balance depicted as a balance between the concentration of proton donors and proton acceptors, damped by the buffer system.
Biogeochemical processes have been built into our Baltic Sea numerical model (Omstedt et al. 2008), which has been extensively explored and validated. The model is an advanced process-oriented coupled ocean-basin model, allowing the effective modelling of fully coupled physical–biogeochemical processes.
The carbon chemistry dynamics are depicted in Fig. 2. The total alkalinity is modelled at the correct level, but displays less variability than do the observations. The observed and modelled pH values are in close agreement with each other: In the studied period, the mean pH is 8.2 and the seasonal variations are within ±0.3.
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Figure 2. Observed (circles) and calculated (black lines) surface layer properties (top: total alkalinity; bottom: pH-value) of the central Baltic Sea (from Omstedt et al. 2008).
The present model system captures major physical–chemical and biological response patterns, evaluated based on observations from the central Baltic Sea, with interesting implications for the coupling between climate change, eutrophication, and the acid–base (pH) balance. In our work, however, we have not yet considered acid precipitation, changes in land use, or future possible projections, nor have we considered the carbon chemistry of anoxic waters. These matters will be addressed in BALTEX II and the upcoming BONUS Baltic-C research, which will also examine air–sea exchange processes as well as river and ocean inputs of organic and inorganic carbon, total alkalinity, and nutrients.
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