Use of δ18O in the interpretation of hydrological dynamics in lakes

Stable isotopes offer an opportunity to investigate lake dynamics in the absence of whole catchment studies. This study considered multi-year δ18O values for six different lakes located in the Italian Alps. We studied δ18O values of two large (L. Caldonazzo and L. Levico) and four small lakes (L. Lavarone, L. Santo di Cembra, L. Serraia, and L. Tovel) characterised by different hydrological regimes (> or <1 year renewal time) and size (> or <50 ha). Differences in isotope composition along the water column, between years and layers were used to investigate the extend of lake water renewal, mixis, stratification, and groundwater infiltration. Lakes with short renewal time had significantly different spring overturn values, while lakes with longer renewal time had substantially constant values. The isotope content of the water column also clarified the extent of mixis for some lakes: Lavarone underwent complete mixing only in some years, while Tovel, previously considered meromictic, could no longer be considered as such. With the exception of L. Tovel, characterised by a peculiar hydrology, thermal stratification was reflected in yearly isotope patterns. Analysis of hypolimnetic isotope values indicated that in L. Lavarone and L. Serraia strong storm events altered summer hypolimnetic δ18O values suggesting groundwater infiltration. Furthermore, epilimnetic isotope enrichment was related to lake elevation by a linear relationship. The extraordinarily hot 2003 summer led to a higher evaporation with respect to other years considered. Multi-annual data on the isotopic values of rainfall for L. Tovel (1178 m a.s.l.) provided the necessary background for data interpretation since precipitation governs isotope inputs. Based on our data, we proposed a general scheme of the seasonal δ18O pattern in temperate lakes distinguishing between small and large lakes.