5.2 Control mechanisms on water isotopes
All isotope time-series exhibit strong seasonality of δ-values. This suggests, that even the sampling points most distant from the freshwater inflows reflect a delayed and attenuated precipitation signal, i.e. a seasonality pattern as previously observed in German rivers (Reckerth et al., 2017) and northern German lakes (B. Aichner et al., 2021). Those trends in δ2H and δ18O values are not fully mirrored by seasonal salinity changes, especially in the inner Schlei (A), the outer Zingster boddens (F – H), and the Vitter bodden (I).
This gives evidence, that other factors than mixing processes between freshwater and marine water masses exhibit additional control on local water isotopes. We hypothesize, that 18O /2H-enrichment due to evaporation from surfaces of the shallow lagoons is a potential driver behind higher δ-values in summers. This is plausible, since potential evaporation has been estimated to be in range of annual freshwater supply by precipitation, at least in the DZBC and estuarine lagoons on the Polish Baltic Sea coast (Chubarenko et al., 2005).
Further, the correlations between water isotopes and water / air temperature, as well as O2-saturation cannot rule out effects of these parameters onto δ-values. In winters, lower δ-values were observed in Feb/March 2021 compared to Feb/March 2020. This could indeed be explained by temperature control, i.e. the generally colder conditions with partially ice-cover on all sampling spots in winter / spring 2020/2021, compared to the relatively mild winter/spring 2019/2020 (reflected in water temperatures; Figs 3, 4, 6).
In summer, the maximum δ-values are reached in September and early October (Fig. 3, 4, 6), i.e. 1-2 months delayed to maximum air and water temperatures. On the other hand, this timing is synchronous to maximum O2-depletion (i.e. minimum O2-concentration) at the sampling spot (Figs 3 and 4). The mechanistic relationship between phytoplankton blooms, local oxygen depletion and oversaturation, carbonate precipitation, and pH values, and their effects on oxygen isotopes in lacustrine waters have long been debated (e.g. (Dietzel, Tang, Leis, & Köhler, 2009; Li et al., 2020; Quay et al., 1995)). Likewise, an effect of this parameters on the measured δ18O values cannot be fully excluded, but do not explain synchronous trends in δ2H values.
Near the estuarine outflows, the direct influence of Baltic Sea water onto the δ-values becomes apparent. Following the salt water intrusion events in February/March 2020, marine water with higher salinity and consequently higher δ-values shape the outflow environments of the estuaries. In contrast, lower salinities in July also lead to decreased δ-values in those realms (Fig. 7).