The Sylt Roads pelagic time series covers physical and hydrochemical parameters in the Sylt–Rømø bight, Wadden Sea, North Sea. Sea surface temperature (SST), salinity, pH, ammonium, nitrite, nitrate, dissolved (in)organic nitrogen (DIN, DON), silicate, soluble reactive phosphorus (SRP), dissolved organic phosphorus (DOP), chlorophyll a and suspended matter concentrations (SPM) are measured twice a week. These parameters are used to identify long-term changes in the Northern Wadden Sea.