Quantitative importance of alkalinity flux from the sediments of acid lakes

Abstract

The recognition of lake acidification as an important environmental problem has recently stimulated the development of predictive models^1–4 linking strong acid loading to lake pH or alkalinity. Among these models, the mass balance approach² has been used to predict lake alkalinity from measured stream and precipitation chemistry and from watershed hydrology. This approach requires that all important positive or negative alkalinity fluxes within the system be considered. The diffusive loss of H⁺ to the sediments and loss of HCO₃⁻ from the sediments constitute potentially important sources of alkalinity in acid lakes. However, the relative importance of these fluxes has generally been overlooked and the presently available information appears to be limited to a few studies on the generation of alkalinity in anoxic hypolimnia^3,4. The flux of alkalinity across the sediment–water interface is estimated here for one lake subject to relatively high acid deposition. For this lake, the estimated annual diffusive loss of alkalinity from the sediment to the overlying water was found to be as large as the combined measured acidity input from the watershed and from direct precipitation to the lake, thus showing the significant role of sediment–water interactions in the alkalinity balance of some acid lakes.