A MODEL OF THE IMPACT OF WINTER ICE COVER ON POLLUTANT CONCENTRATIONS AND FLUXES IN MOUNTAIN LAKES

摘要

High mountain lakes have been used in a number of recent studies concerned with measuring and modelling the dynamic response of remote ecosystems to environmental change (the AL:PE, MOLAR and EMERGE projects). One of the important features that distinguishes these lakes from lowland lakes at similar latitudes is fact that they may be isolated from the atmosphere for six months or more during the winter by a thick ice cover. Pollutants deposited during the winter are locked up in the ice and snow and released only during the spring thaw. These processes generate a very strong seasonal variation of pollutant concentrations in the water column, and fluxes through the water column to the sediment record. This article presents a mathematical model of these processes, and uses the model to assess data on fluxes and concentrations of fallout radionuclides in the water column of Redo Lake in the Spanish Pyrenees. Using fallout ~(210)Pb as a tracer, the model results show that concentrations in the water column immediately after ice melt are enhanced by a factor of up to 70%. Fluxes through the watercolumn show a similar pattern to the empirical results, though the theoretical values tend to be a little more conservative. By comparing fluxes through the water column with the measured atmospheric flux it is estimated that between 4-6% of annual fallout of ~(210)Pb onto the catchment of Redo is delivered to the lake, and that the mean ~(210)Pb residence time in the catchment is between 500-750 yr.