Biogeochemical shelf sea modelling has a long tradition. Most models include early diagenesis sediment modules for remineralization of organic matter. The model presented here also simulates alkalinity, which is exported into the pelagic system. There the produced alkalinity joins in the carbonate system and is able to buffer invading atmospheric CO₂. The input of nitrate via rivers stimulates alkalinity generation within the sediment, which in turn reduces the acidification of coastal areas.