The impact of variable DOC concentrations on acidification assessments

Dissolved organic carbon (DOC) affects the acidity in soil and lake waters and buffers both against acidification and against recovery from acidification. Current acidification assessments, based on the widely used MAGIC (Model of Acidification of Groundwater In Catchments) model, often assume constant DOC concentrations through time, but observations suggest this assumption may not be warranted. Here we used MAGIC to investigate the effect of four different scenarios of variable DOC concentrations on acidification assessments of 75 Swedish lakes during peak acidification (1980), in the recent past (2012) and the near future (2030). In all scenarios, DOC concentrations varied to the same extent in soil and lake waters, so the effects of DOC on soil cation exchange and water acid–base chemistry were accounted for. To capture the possible span of the variable DOC effects, modelled variations covered a large range of concentration levels suggested by earlier studies. A higher proportion of the modelled lakes in the scenarios with variable DOC concentrations were assessed as acidified at the time of peak acidification compared to in the scenario with constant DOC concentration. In 2012 and 2030, the proportion of acidified lakes was in the same range regardless of historical and future DOC scenarios. This means that for management purposes, the MAGIC model with constant DOC concentration assessed the extent of acidification for the recent past and near future without bias.