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.