Control mechanisms on water isotopes and salinity
Water samples collected at the A. P. Møller Skolen (Kleine Breite,
Schlei; sampling point A), Biological Station Zingst (Zingster Stream,
E), and Hiddensee/Kloster (Vitter bodden; I) all exhibit clear seasonal
isotopic variability (Figs. 3-6). The seasonal isotope amplitude is in
range of ca. 10 – 5 ‰ (δ2H) and 2 – 2.5 ‰
(δ18O), reaching minimum/maximum values in Feb/Mar and
Sep/Oct, respectively. Thus we suggest, that time-series data even at
sampling points in the outermost boddens still mainly reflect a delayed
and attenuated precipitation signal, i.e. a seasonality pattern as
previously observed in German rivers (Reckerth et al., 2017) and
northern German lakes (B. Aichner et al., 2021) . Those trends in
δ2H and δ18O values are not fully
mirrored by seasonal salinity changes, especially in the inner Schlei
(A), the outer Zingster boddens (F – H), and the Vitter bodden (I).
This gives evidence, that other factors than mixing processes between
freshwater and marine water masses, exhibit additional control on local
water isotopes and salinity. For example, higher temperature and/or18O / 2H-enrichment due to
evaporation from lagoon surfaces are potential drivers behind higher
δ-values in summers. This is plausible, since potential evaporation has
been estimated to be in range of annual freshwater supply by
precipitation, at least in the DZBC (Chubarenko et al., 2005). In
winters, lower δ-values were observed in Feb/March 2021 compared to
Feb/March 2020. This could indeed be explained by temperature control,
i.e. the generally more cold conditions with partially ice-cover on all
sampling spots in winter / spring 2020/2021, compared to the relatively
mild winter/spring 2019/2020 (reflected in water temperatures; Figs 3 –
5).