6. Conclusions
The time series data show that the studied estuarine systems exhibit a
clear seasonality within their water isotope values. This is comparable
to NE German lakes and rivers in both isotope amplitude and time
succession. The transect data further reveal complex isotope vs salinity
correlations: positive and significant correlations were observed in
March 2020 along all transects. In contrast, hyperbolic and partially
inverse correlations were found in the two sampled summers (June 2019
and July 2020).
We hypothesize that this is triggered by increased susceptibility of the
mostly shallow inland water to evaporative isotope enrichment in
summers, causing higher δ-values. Further, the discharge regime of
tributary rivers (lower δ-values) is an important factor. Additionally,
the influence of water intrusion from the Baltic Sea (higher δ-values)
can affect the outflow regions of the estuaries.
In summary, salinity is a fairly good predictor for water isotopes on
larger spatial scales in our study area. On regional scales, local
effects overprint the positive correlation between the two parameters
especially in summers, partly even leading to negative correlations.
This seasonality within the correlation of salinity vs water isotopes
needs to be considered when interpreting biogenic isotope data (of
plants or animals) because those might be similarly seasonally biased
(often towards the warm/growing season).