5.1 Surface run-off and Baltic Sea dynamics drive estuarine salinity
The salinity in Baltic Sea estuarine lagoons is strongly influenced by the seasonal discharge patterns of the inflowing rivers, which is characterized by maximum runoff in late winter and spring (Cyberski, Wróblewski, & Stewart, 2000). During our sampling period, especially the major winter storms “Sabine” and “Victoria” in February 2020, lead to increased precipitation and consequently higher surface runoff of rivers and creeks (Figs. 3, 4 and 6).
The sampling spot A is located about 5 km west of the mouth of the Füsinger Au. This river delivers an annual freshwater input of about twice the total water volume of the entire Schlei (Gocke et al., 2003). Further, spot A it is located ca. 100 m eastwards from the inflow of the small creek Mühlenbach (Fig. 2a), and therefore susceptible to major freshwater discharge events. Likewise, the decreasing influence of freshwater inflow is clearly visible along the DZBC transect, with stronger impact on the seasonal salinity trends near the inflows of the river Recknitz (B and C) (Figs. 2a and 5a). Short-term salinity fluctuations in the DZBC 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).
In contrast, the areas close to the estuarine outflows towards the Baltic Sea are more susceptible to marine salinity dynamics. Those 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 into the Baltic Sea (V. Mohrholz, 2018) (Fig. 3), probably enforced by the same storms that contributed to enhanced precipitation amounts.
The outermost Rügener boddens (e.g. the WRBC including Vitter Bodden) are additionally characterised by complex dynamics within multiple influencing water-masses: water is partially intruding from the Baltic Sea, south and northeast of Hiddensee island (Bachor, 2005). Freshwater surface run off is derived from the NRBC (Helmut Hübel et al., 1998) (Fig. 6). The boddens are further susceptible to large-scale water exchange with more eastern parts of the bodden coast, via the Strela Sound (Birr, 1988; U. Schiewer & Gocke, 1996).
In both the Vitter Bodden and the easternmost parts of the DZBC, salt water intrusion events from the North Sea (Fig. 3) can explain the maximum salinities in February 2020 and 2021 (Figs 4 – 6). Strongly fluctuating values in February 2021 are probably influenced by the partial appearance and melting of an ice-cover across the boddens.