The SYLT ROADS LTER pelagic time series covers physical and hydrochemical parameters in the Sylt-RØmØ Bight, Wadden Sea, North Sea. Since its inception in 1973 Sea Surface Temperature (SST), Salinity, Ammonia, Nitrite, Nitrate, Soluble Reactive Phosphorus (SRP) and Silicate were measured on a biweekly basis. The other parameters were introduced later (pH - since 1979, dissolved organic nitrogen (DON) - since 1996, dissolved organic phosphorus (DOP) - since 2001, chlorophyll a - since 1979, suspended particulate matter (SPM) - since 1975) and in case of dissolved oxygen were discontinued (1979-1983). During the years 1977, 1978 and 1983 no sampling took place. 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 0.248 units. Summer and fall salinities are generally significantly elevated compared to spring and winter conditions. Dissolved nutrients as ammonia, nitrite, nitrate and reactive phosphorus (SRP) showed periods of elevated eutrophication (1973 - 1998) while silicate displayed a different development with significantly elevated levels of the winter data since 1999. Interestingly phytoplankton sum parameters did not mirror these huge nutrient changes, as a seasonal comparison of the two eutrophication conditions showed no significant differences with regard to chlorophyll, which is ultimately triggered by an important change in nutrient limitation during the time series: Till 1988 the phytoplankton was primarily limited by silicate, while since 1999 SRP limitation became increasingly important.