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 along all three transects, i.e.
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 March (Fig. 7; interpolated maps with
isotope offsets between the two seasons in Supplementary Fig S1).
Especially, the innermost waterbodies, such as Kleine and Große Breite
(Schlei), the Saaler Bodden (DZBC), and the NRBC, exhibit relatively
strong increases of isotope values (Fig. 7, S1). 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).
These dependencies lead to isotope gradients which are much less
pronounced in July 2020, compared to March. Specifically, the DZBC shows
almost homogenous isotope values along the whole bodden chain in July
2020, with exception of the areas directly adjacent to river inflows
(Fig. 7b). In the following, isotope-salinity correlations are closer
examined.