Water isotopes in estuarine lagoons at the German Baltic Sea coast

River estuaries are characterized by mixing processes between freshwater inflows and marine water masses, with lower/higher isotope values, respectively. Therefore, they often show linear correlation between salinity and water isotopes (δ¹⁸O and δ²H values). In this study, we evaluated spatial and seasonal isotope dynamics along three estuarine lagoon transects at the German Baltic Sea coast: a) the Schlei; b) the Darß-Zingst Bodden Chain; c) an eastern transect (Stettiner Haff - Peene Stream - Greifswalder Bodden - Rügener Boddens). The data show strong seasonality of isotope values even at locations located furthest from the river mouths. They further reveal a positive and linear salinity-isotope correlation in spring 2020, but hyperbolic and partially even reverse correlation in summers 2019 and 2020. We conclude that additional physical processes, such as evaporation from the shallow lagoons, overprint the two-phase mixing correlation during summers. Understanding of those water isotope dynamics is crucial in context of ecological studies, for example when interpreting oxygen and hydrogen isotope values in aquatic organisms that depend on ambient estuarine water.