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).