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.氢与氧的稳定同位素质量比（δ²H和δ¹⁸O）是一类保守型示踪剂，可反映流域与湖泊的水文状况，相关信息可用于水收支研究，例如估算流域的营养盐负荷，以及气候变化对水体的直接与间接影响。本研究针对10个串联分布的北方湖泊，在两个水文特征迥异的年度内测定了其δ²H与δ¹⁸O值，并估算了水体质量平衡。在暖干夏季，湖面蒸发损耗会使水体的同位素值富集；因此，在夏季出现热分层的湖泊中，湖上层与湖下层的同位素值存在显著差异。在随后的多雨夏季，湖泊内部的同位素值变异程度大幅降低，且无明显变化趋势，这大概率是因为湖泊群的水体水柱更不稳定，且水体流经速率更快。在秋季全层对流混合之后，湖泊的δ¹⁸O值与湖泊面积及流域内内陆水域占比呈显著相关。两年间湖泊内部的同位素差值约为1‰，这反映了蒸发作用的差异。同位素质量平衡（IMB）计算结果，以及基于邻近区域湖泊温度与水文观测的计算结果，在多雨无冰期得到了相近的结论。相较此前的干旱年份，多雨年份更易满足同位素质量平衡稳态条件的各项前提要求。部分湖泊的计算结果显示其地下水补给量可观，地下水在北方湖泊水收支中的作用值得未来研究进一步关注。