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.