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.