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).