A Model of the Impact of Winter Ice Cover on Pollutant Concentrations and Fluxes in Mountain Lakes

High mountain lakes have been used in a number of recentstudies concerned with measuring and modelling the dynamicresponse of remote ecosystems to environmental change (theAL:PE, MOLAR and EMERGE projects). One of the importantfeatures that distinguishes these lakes from lowland lakes atsimilar latitudes is fact that they may be isolated from theatmosphere for six months or more during the winter by a thickice cover. Pollutants deposited during the winter are lockedup in the ice and snow and released only during the springthaw. These processes generate a very strong seasonalvariation of pollutant concentrations in the water column, andfluxes through the water column to the sediment record. Thisarticle presents a mathematical model of these processes, anduses the model to assess data on fluxes and concentrations offallout radionuclides in the water column of Redó Lake in theSpanish Pyrenees. Using fallout ²¹⁰Pb as a tracer, themodel results show that concentrations in the water columnimmediately after ice melt are enhanced by a factor of up to70%. Fluxes through the watercolumn show a similar patternto the empirical results, though the theoretical values tendto be a little more conservative. By comparing fluxes throughthe water column with the measured atmospheric flux it isestimated that between 4–6% of annual fallout of ²¹⁰Pbonto the catchment of Redó is delivered to the lake, and that the mean ²¹⁰Pb residence time in the catchment is between 500–750 yr.