Temporal and lake-specific variations in oxygen and hydrogen stable isotopes in a boreal lake-chain during two hydrologically differing years

Hydrogen and oxygen stable isotope mass ratios (δ²H and δ¹⁸O) are conservative tracers that reflect hydrological conditions of watersheds and lakes, information that can be used for water budgets, such as estimating nutrient loading from the catchments and direct and indirect effects of climate change on waterbodies. We measured δ²H and δ¹⁸O values and estimated water mass balances over 2 hydrologically differing years in a chain of 10 boreal lakes. During a warm and dry summer, evaporative loss enriched the isotope values at lake surfaces. Thus, in lakes that were thermally stratified during summer, the isotope values differed distinctively between epilimnion and hypolimnion. During the following rainy summer, the within-lake variation of isotope values was much lower with no clear trend, presumably a result of a more unstable water column and faster water throughflow in the lake chain. After autumnal mixing the lake δ¹⁸O values were strongly correlated with lake area and proportion of inland waters of the catchment area. The interannual within-lake difference was then ∼1‰, indicating differences in evaporation. Isotope mass balance (IMB) calculations and those based on lake temperature and hydrological measurements in the nearby region yielded similar results for the rainy ice-free period. Presumably, prerequisites of steady-state conditions for IMB were better met in the rainy year than during the previous drier year. For some lakes the results suggested remarkable volumes of groundwater inflow, whose role in boreal lake water budgets deserves more attention in future studies.