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• δ2H and δ18O of 135 lakes along transect from North Atlantic Ocean to Bothnian Bay. • All lakes are sensitive to distillation processes during moisture transport. • Inflow isotopic composition and evaporation are equally important for inland lakes. • Interannual variability likely reflects seasonal atmospheric circulation changes. • Variable sensitivity to inflow and evaporation needs consideration in proxy studies. High-latitude lakes are sensitive to climate change and store information about large-scale circulation changes and catchment-integrated processes. Lakes are mainly recharged by meteoric water, meaning that some lake sediment proxies may indirectly archive the stable isotopic composition of hydrogen (δ2H) and oxygen (δ18O) of past precipitation. Yet, despite similar precipitation input, lakes within a region may exhibit a wide range of isotopic values due to the varying influence of inflow seasonality and evaporation. Moreover, the relative sensitivity of each lake to these controls may vary through time, something that is difficult to account for. Here, we evaluate the impact of variable inflow δ2H and evaporation on the lake water isotopic composition across northern Fennoscandia (Norway, Finland, and Sweden). We measured lake water δ2H and δ18O of 135 lakes spanning from the north Norwegian coast along a 460 km transect to the Bothnian Bay, sampled from 2018 to 2020. Our data show that both coastal and inland lakes are sensitive to distillation during moisture transport, and that lakes farther from the Atlantic Ocean are additionally impacted by evaporation. We estimated the isotopic composition of lake water inflow values for evapo-concentrated transect lakes (δ2H I) using a Bayesian method. Resampled transect lakes had more depleted δ2H I in 2020 than in 2019, indicating either that precipitation was 2H-depleted or that more winter precipitation contributed inflow to the lakes in 2020 compared to in 2019. We suggest that the more 2H-depleted values in 2020 were a response to a snow-rich winter, associated with extremely positive Arctic Oscillation (AO+) conditions and increased moisture supply from the North Atlantic. We find evidence that lake water isotopic variability in this region reflects a combination of seasonal precipitation changes associated with atmospheric circulation changes, and catchment-integrated evaporation. Careful consideration of the variable sensitivity to these processes is essential when making inferences about past climate based on lake water isotope proxies. [ABSTRACT FROM AUTHOR]