Data presented here were collected between May 2015 to April 2017 within the BEFmate project (Biodiversity - Ecosystem Functioning across marine and terrestrial ecosystems; https://www.icbm.de/verbundprojekte/befmate/ ) of the Universities of Oldenburg and Göttingen and the Nationalparkverwaltung Niedersächsisches Wattenmeer. Experimental islands and saltmarsh control plots were created in the back barrier tidal flat and in the saltmarsh zone of Spiekeroog island. Data were measured with local installed conductivity logger within the experimental islands as well as in the saltmarsh control plots at one elevation level (pioneer zone). Logger were mounted within filter pipes to measure groundwater salinity. Conductivity logger used here are Hobo U24 Conductivity Logger U24-002-C (S/N 10570000, 10599255) (Onset Computer Corporation, Bourne, MA/USA). The Conductivity loggers were pre-calibrated by the manufacturer. Recorded data were internal logged till readout afield with the Hobo Underwater Shuttle (U-DTW-1) and following with the HOBOware Pro (V3.7.4) software. Processing was done using MATLAB (R2012b). Low tide data was removed. The position was derived from a portable DGPS-system. Date and Time is given in UTC.