5. Conclusions
The time series data show that the studied estuarine systems exhibit a
clear seasonality within their water isotope values, which is comparable
NE German lakes rivers in both isotope amplitude and time succession.
The transect data further reveal complex isotope vs salinity
correlations: widely positive linear correlations were observed in March
2020 along all transects. In contrast, hyperbolic and partially inverse
correlations were found in the two samples 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) and influence of water intrusion from
the Baltic Sea (higher δ-values) obviously plays a role.
In summary, salinity is a fairly good predictor for water isotopes on
larger spatial scales. On regional scales, local effects overprint the
positive correlation between the two parameters especially in summers,
partly even leading to negative correlations. This seasonality of
salinity vs isotope correlation 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).