Fig. 9: Correlations between a) δ18O values and salinity. Dashed grey trendline for March 20. Dashed black circle indicate Rügener Bodden and Baltic Sea samples; b) δ18O values and latitude or longitude. Dashed grey/black trendlines for March/July 20. Grey arrows indicate direction of isotopic change from March to July; c) Δ δ18O (March - July) and latitude or longitude; d) Δ δ18O and Δ Sal (March - July) for the three sampled transects. DZBC: Darß-Zingst Bodden Chain. East Transect comprises Stettiner Haff to GW Bodden and Rügener Boddens.
When analysing the three transects individually and in more detail, they all show a significant correlation between isotope values and salinity in March 2020 (Fig. 9a). Especially in the Schlei, an almost linear correlation between δ-values and salinity is observable. Outliers with significant higher δ-values are samples taken from adjacent lakes and noors, which not or only weakly connected to the Schlei (Bültsee #14, Schnaaper See #21, Ornumer Noor #17, and Windebyer Noor #19) (Fig. 7a). Further, a sample from the close-by Baltic Sea location Eckernförde (#21) might be influenced by local mixing process different than in the Schlei system and its adjacent Baltic Sea outflow (e.g. #27, #29). In the DZBC, the March 2020 samples can be assigned to two clusters: 1) low saline samples taken from rivers or close to their outflows (#30/#31, #35/#36) and from Saaler Bodden (#32). 2) samples with salinity >6 psu from all other locations (Fig. 9a and 7b). In the Eastern Transect, again a linear salinity - isotope correlation is observed from the Stettiner Haff to Rügener Boddens, with one outlier (#51) derived from the inner NRBC (Fig. 9a and 7c).
In contrast, in the Schlei and DCBZ, the two sampled summers (June 2019 and July 2020), reveal constant isotope values or even a reverse correlation from salinities 4-6 psu and higher on (Fig. 9a). In the eastern transect, however, a positive correlation along the Stettiner Haff-Peenestrom-Greifswalder Bodden transect is observable, while samples from the WRBC, NRBC and adjacent Baltic Sea samples show mixed values clustering around -4.9 to -6.2 ‰ δ18O and salinity values 8.1 to 9.6 psu (dashed black circles in Fig. 9a and b).
These patterns come out more clearly when correlating δ-values to latitude / longitude, following their geographical orientations along the transects (Fig. 9b), especially when considering the March-July offset of isotope values (Δδ) (Fig. 9c). This approach eliminates the influence of larger salinity ranges in March compared to July, which is strongest in the Schlei, and makes isotope values from specific spots better comparable.
As noted above, most samples show isotope enrichment in the warmer season (i.e. negative Δδ March-July values) (Fig. 7). Exceptions are a number of samples from the Baltic Sea, whose isotope signatures are rather driven by mixing processes than by seasonal trends, or samples under direct influence of Baltic Sea water (e.g. close to the outflows of the Schlei and DZBC). In general, samples tendencially show stronger isotope enrichment further inlands. Thus, they show increasing Δδ (March-July) values from the rivers inflow towards the Baltic Sea (Fig 9b, c; Fig. 7; interpolated salinity and Δδ maps in Supplementary Figure S1). Obviously, the shallow lagoons inlands (such as Kleine Breite at the Schlei, and Saaler and Bodstedter Boddens at DZBC) are more susceptible to undergo stronger isotopic enrichment in summers, than locations more adjacent to the Baltic Sea. This is most pronounced in the Schlei, here the Δδ values are balanced (i.e. similar isotope values in March and July) at ca. 75% of the distance between the inner Schlei and the outflow towards the Baltic Sea (Fig. 9c). Beyond this place (which lies around the narrow passage aside the town Arnis, #24/#25), Δδ values become even positive due to lower δ-values in July compared to March. (Fig. 7a and Supplementary Figure S1a).
The major inference from these data set is, that sampling points that undergo larger salinity changes between the maximum and minimum freshwater inflows in spring and summers (i.e. most negative ΔSal Mar-Jul) are also more susceptible to larger isotopic enrichment in the summers (more negative Δδ values Mar-Jul). This systematic is most pronounced in the Schlei, which is characterized by the most linear salinity gradient among the studied transects (Fig. 9d). A similar trend is visible in the DZBC, but here it is weakened due to outliers as mainly derived from the Barther Bodden and Grabow samples. In the Eastern transect, no clear systematics between ΔSal and Δδ could be observed. While almost all samples show isotopic enrichment in summers (negative Δδ), the stronger influence of mixing processes of multiple water sources (i.e. from rivers, inner Rügen boddens, Baltic Sea water intrusion from both the east and the north-west) can explain the heterogenous isotope values in this study area. Here, increasing sample density along sub-transects could potentially facilitate interpretation of isotope data and the control mechanisms behind.