Zingster Stream and boddens
In contrast to the Kleine Breite (Schlei), at Zingster Stream (sampling point E), the salinity values exhibited a more pronounced seasonal trend, with lower δ-values in winter/spring and higher values in summers (Fig. 4). This is a common pattern in Baltic Sea lagoons and related to the seasonal discharge pattern of inflowing rivers (Cyberski, Wróblewski, & Stewart, 2000) (Fig. 4). Short-term salinity fluctuations on the other hand can be explained by influence of different water masses from either the eastern or western side of the Zingster Stream, which is controlled by wind direction and water level differences (Schumann et al., 2006). The water isotope values mirror the seasonal salinity trend throughout the studied time period (24th March 2020 – 9th March 2021; Fig. 4), i.e. show lower values in winter/spring and higher in summers (Fig. 4). Comparable to the Schlei, the correlations between water isotopes and water / air temperature, as well as O2-saturation cannot rule out effects of these parameters onto δ-values. This is especially true for lower temperatures in the colder winter (2020/21) compared to the mild winter (2019/20), which could explain the lower δ-values in Jan-March 2021.
A similar seasonal trend in water isotopes as in Zingster Stream is discovered at all sampling locations at the DZBC, i.e. from the inflow of the Recknitz (sampling point B) to the outflow from the boddens towards the Baltic Sea (sampling point H) (Fig. 5). The decreasing influence of freshwater inflow along this transect is clearly visible at increasing salinities, despite the actual seasonal salinity trends are different in the outer boddens (F, G, H) compared to the innermost boddens (B, C) (Fig. 5a). Close to the outflow towards the Baltic Sea (point H), high salinities up to 14 psu were observed in February 2020 and March 2021. Those areas are more susceptible to salinity changes of the Baltic Sea, which in turn are influenced by salt water intrusion events from the North Sea, via the Little Belt, Great Belt and Øresund (Volker Mohrholz, 2018) (Fig. 1). Especially in February 2020 large amounts of salt water was driven by two major storms into the Baltic Sea (V. Mohrholz, 2018) (Fig. 3).