Schlei (Kleine Breite, Schleswig)
At the inner Schlei, water isotopes show a seasonal trend (higher/lower values in summer/winter) while in contrast salinities were relatively constant (ca. 6 – 7 psu) at most measuring days within the sampling period (26th March 2020 – 16thMarch 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.
During, both February/March 2020 and February 2021, major storms lead to increased precipitation and consequently higher surface runoff of rivers and creeks discharging into the Schlei (Figs. 2a and 3). The sampling spot A is located about 5 km west of the mouth of the river Füsinger Au, the annual freshwater input of which is about twice the total water volume of the entire Schlei (Gocke et al., 2003). Further it is located ca. 100 m eastwards the inflow of the small creek Mühlenbach, and therefore susceptible to major discharge events, delivering freshwater with lower δ-values.
The maximum δ-values are reached in September and early October (Fig. 3), i.e. 1-2 months delayed to maximum air and water temperatures. On the other hand it is synchronous to maximum O2-depletion (i.e. minimum O2-concentration) at the sampling spot. The mechanistic relationship between phytoplankton blooms, local oxygen depletion and oversaturation, carbonate precipitation, and pH values, and their effects on oxygen isotopes in lacustrine waters have long been debated (e.g. (Dietzel, Tang, Leis, & Köhler, 2009; Li et al., 2020; Quay et al., 1995)). Likewise, an effect of this parameters on the measured δ18O values at Schlei cannot be fully excluded, but do not explain synchronous trends in δ2H values.