4.1 Time series
Water samples collected at the A. P. Møller Skolen (Kleine Breite, Schlei; sampling point A), Biological Station Zingst (Zingster Stream, E), and Hiddensee/Kloster (Vitter bodden; I) all exhibit clear seasonal isotopic trends (Figs. 3-6). The seasonal amplitude is in range of ca. 10 – 5 ‰ (δ2H) and 2 – 2.5 ‰ (δ18O), reaching minimum and maximum values in Feb/Mar and Sep/Oct, respectively.
In contrast to water isotope values, the salinities showed variable trends at the different sampling locations. At the Schlei (point A), values were relatively constant (ca. 6 – 7 psu) at most measuring days within the sampling period (26th March 2020 – 16th March 2021) (Fig. 3). Exceptions were two samples from 26th March 2020 and 19th Feb 2021, which showed decreased salinity values of 3.5 – 4.3 psu. Those are mirrored by lower δ-values in water samples collected at those two days.
In the DZBC (sampling points B to H), the salinity values exhibited pronounced seasonal trends throughout the studied time period (24th March 2020 – 9th March 2021) (Figs. 4 and 5). At the Zingster Stream (point E), they show lower values in winter/spring and higher values in summers, hence they resemble the seasonal water isotope trend (Fig. 4). This is also true for the sampling spots in the western part of the DZBC (B, C, D), i.e. close to the inflow of the river Recknitz. In the eastern part (F, G, H), this seasonality becomes less pronounced (Fig. 5). Close to the outflow towards the Baltic Sea (point H), peak salinities up to 14 psu were observed in February 2020 and March 2021.
At Vitter Bodden (Hiddensee/Kloster; sampling point I), the salinity measurements showed values around 9-10 psu, with a slight decreasing tendency throughout the period (18th March 2020 – 3rd March 2021) (Fig. 6). Maximum values up to 11.2 psu were observed in late winters 2020 and 2021. Hence, the seasonal trend in the Vitter Bodden resembles the conditions at the outflow of the DZBC (point H).