Fig. 7: Interpolated δ18O values along the three sampled transects for March (left) and July (right) 2020. Baltic Sea excluded from interpolation. Panels a – c refer to subregions as indicated in Figure 1. Circles A-I: sampling points for time-series. Circles #1-68: shore samples. Arrows indicate exchange points with Baltic Sea water. Abbreviations as in Fig. 2.
Similar to salinity gradients (Fig. 2), the water isotope values in March 2020 increase from low to high at all three transects from river inlets to the outflows to the Baltic Sea (exemplarily plotted for δ18O values in Fig. 7). In July 2020, most regions show higher δ-values compared to Spring (Fig. 7; interpolated maps with isotope offsets in Supplementary Fig S1). Especially, the innermost waterbodies, such as the inner Schlei, the Saaler Bodden at DZBC, and the NRBC, exhibit relatively strong increase of isotope values.
Exceptions from these trends are areas under direct influence of Baltic Sea water, such as the outflow regions of the Schlei and DZBC, which show lower δ-values in July compared to March (Fig. 7). The latter can be explained by lower salinity and consequently lower isotope values of Baltic Sea water in summer, compared to the high values after the saltwater intrusion events in February and March 2020 (Fig. 3).
These dependencies lead to isotope gradients which are much less pronounced than in March. Specifically the DZBC shows almost homogenous isotope values along the whole bodden chain, with exception of the areas directly adjacent to river inflows (Fig. 7b). In the following, isotope-salinity correlations are closer examined.