Entrainment, annual circulation and groundwater inflow in a chain of lakes as inferred by stable 18O isotopic signatures in the water column

The stable oxygen isotopic signature δ18O of water has been proved to be a useful marker in hydrological lake studies. However, it is applied only sporadically to determine the extent of mixis and stratification, which is vital for all hydrological and limnological studies. We present a study of the changes in δ18O, temperature and Schmidt stability in the water column of three adjacent lakes of very different mixis types (polymictic to meromictic) over almost one year (May 2008 to April 2009). The response of δ18O to important hydrologic processes (entrainment, stratification, circulation, groundwater inflow) and weather influences (cooling period, ice cover and melt water inflow) is discussed. The lakes are part of the Osterseen chain of lakes south of Munich, Germany. Although hydrologically connected, these lakes show various mixis types (polymictic to meromictic) due to large differences in size, groundwater inflow and water renewal time. Polymixis and the strong subsurface inflow of groundwater in Lake Waschsee (25.6 × 103 m3) were indicated by the same trends in the δ18O signature throughout all water layers and by the mean overall signature (-9.94‰) being very close to δ18O of local groundwater (-10.01‰ ± 0.06). δ18O signatures of the larger dimictic Lake Fohnsee (2298.3 × 103 m3) revealed a highly significant trend towards lower values of δ18O in its hypolimnion, indicating inflow of groundwater. A cooling period during the summer stratification characterised by high wind speeds resulted in a considerable drop of lake surface temperatures and Schmidt stability (up to 25%) in lakes Fohnsee and Eishaussee and was followed by a deepening of the mixed upper water layer and entrainment of hypolimnetic water layers. This was clearly shown by a signal change in deeper water layers formerly constant in δ18O. The permanent meromixis present in Lake Eishaussee (297.0 × 103 m3) could also be confirmed by isotopic signatures, as bottom water layers remained significantly isolated in δ18O from the remaining water column over the whole study period. We summarize that the oxygen isotopic signature of water is an easy to interpret, excellent indicator of important hydrologic processes in a lake and can readily be integrated into routine sampling. The present findings will further contribute to the analysis of hydrological data as well as to the interpretation of paleoclimatic reconstructions using proxies of lake water δ18O.