The Skagerrak - Kattegatt - Baltic Proper - Bothnian Sea - Bay of Bothnia system provides a strong chemical gradient from seawater to organic-rich brackish waters. Modelling the chemistry of this system provides a significant challenge in that no model that covers this salinity range, and includes dissolved organic matter, is available off the shelf. The dissolved organic matter is important in that recent measurements have shown that it can contribute several per cent of the alkalinity (buffer capacity) of the water (Kulinski et al., 2014): it is not, however, included in current models for acid-base chemistry. Inclusion of natural organic matter in these models is therefore essential to understanding the effects on acidification in the Baltic Sea. The dissolved organic matter in natural waters is known to comprise a complex mixture of different molecular structures. In fresh waters, where the concentrations of dissolved organic matter are relatively high, this material has been extracted and concentrated. While little progress has been made in identifying the components making up this complex mixture, laboratory studies have characterised the acid-base chemistry, resulting in generic models that can be used to assess the buffer capacity. These models have been applied in fresh water environments, where the very low dissolved salt concentrations simplify the treatment of the inorganic chemistry. Seawater, in contrast, contains several dissolved salts (not only sodium chloride) at relatively high concentrations, requiring a much more complex chemical model than fresh water. We have now developed in prototype form a generic fresh water model for dissolved organic matter with a state of the art seawater model. This provides for the first time a state of the art chemical model covering the full range of conditions from Baltic Sea to seawater and including natural organic matter. This model is now being used to examine the organic buffer capacity that modifies the response Baltic waters to SOX and NOX inputs.
K. Kulinski, B. Schneider, K. Hammer, U. Machulik and D. Schulz-Bull (2014). The influence of dissolved organic matter on the acid–base system of the Baltic Sea. Journal of Marine Systems, in press, doi: http://dx.doi.org/10.1016/j.jmarsys.2014.01.011