5.2 Control mechanisms on water isotopes
All isotope time-series exhibit strong seasonality of δ-values. This
suggests, that even the sampling points most distant from the freshwater
inflows 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. We hypothesize, that 18O /2H-enrichment due to evaporation from surfaces of the
shallow lagoons is a potential driver 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 and estuarine lagoons on the Polish Baltic Sea coast (Chubarenko et
al., 2005).
Further, the correlations between water isotopes and water / air
temperature, as well as O2-saturation cannot rule out
effects of these parameters onto δ-values. 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 colder
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, 4, 6).
In summer, the maximum δ-values are reached in September and early
October (Fig. 3, 4, 6), i.e. 1-2 months delayed to maximum air and water
temperatures. On the other hand, this timing is synchronous to maximum
O2-depletion (i.e. minimum
O2-concentration) at the sampling spot (Figs 3 and 4).
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 cannot be fully excluded, but do
not explain synchronous trends in δ2H values.
Near the estuarine outflows, the direct influence of Baltic Sea water
onto the δ-values becomes apparent. Following the salt water intrusion
events in February/March 2020, marine water with higher salinity and
consequently higher δ-values shape the outflow environments of the
estuaries. In contrast, lower salinities in July also lead to decreased
δ-values in those realms (Fig. 7).