Data presented here were collected between 2020-01 and 2023-06 at station BEFmate_I10low within the research unit DynaCom (Spatial community ecology in highly dynamic landscapes: From island biogeography to metaecosystems, https://uol.de/dynacom/ ) involving the Universities of Oldenburg, Göttingen, and Münster, the iDiv Leipzig and the Nationalpark Niedersächsisches Wattenmeer. Experimental islands and saltmarsh enclosed plots were established in the back-barrier tidal flat and in the saltmarsh zone of the island of Spiekeroog (Germany). Groundwater levels at different elevation zones were measured using pressure loggers deployed in dip wells within the experimental islands as well as in the saltmarsh enclosed plots. Measurements were obtained using Hobo U20L Water Level Loggers (Onset Computer Corporation, Bourne, MA/USA). All devices were pre-calibrated by the manufacturer. Logged data were retrieved in the field using a Hobo Underwater Shuttle (U-DTW-1) and were read out with the HOBOware Pro (V3.7.28) software, Subsequent data processing was done using MATLAB (R2024b). Atmospheric pressure correction for water-level calculations was applied using data from a nearby weather station. Post-processing and quality control included (a) the removal of data covering maintenance activities, (b) an outlier detection, and (c) visual checks. Outliers in water level and temperature time series were detected using a moving-median filter and a 3-sigma criterion, with additional cross-checking against a reference sensor. Identified outliers were removed, and height-corrected water level series were produced to ensure consistency across sensors and years.