The SYLT ROADS Long Term Ecological Research (LTER) pelagic time series covers physical and hydrochemical parameters at five neighboring stations in the Rømø Bight, Wadden Sea, North Sea. Since the time series beginning in 1973 sea surface temperature (SST), salinity, ammonium, nitrite, nitrate and soluble reactive phosphorus (SRP) were measured on a biweekly basis. Other parameters were introduced later. Since the start of continuous sampling in 1984 the sea surface temperature in the Bight has risen by 1.16 oC, with the highest increases during the fall months while the pH decreased by 0.26 units. Summer and fall salinities are generally significantly elevated compared to spring and winter conditions. Dissolved nutrients (ammonium, nitrite, nitrate and SRP) showed periods of elevated eutrophication (1973 – 1998) and de-eutrophication since 1999 while silicate displayed a different development showing higher levels of Si in the winter data since 1999. Interestingly phytoplankton parameters did not mirror these large nutrient changes, as a seasonal comparison of the two eutrophication periods showed no significant differences with regard to chlorophyll. This phenomenon might be triggered by an important change in nutrient limitation during the time series: Until 1988 the phytoplankton was probably primarily limited by silicate, and since 1999 SRP limitation got increasingly important.