We combine the temperature and phytoplankton data from one of the longest aquatic data sets in history, the Helgoland Roads (North Sea, 54°11.3'N, 7°54.0'E) timeseries to evaluate the effects of climate change on the base of marine food webs. The data shows that, despite an obvious warming of 1.1°C since 1962, the mean diatom day of the algal spring bloom is delayed and shifted to the end of the first quarter of the year. This is apparently related to a warming of the autumn (October-December) temperatures. It is the first indication of a warming related shift in phytoplankton succession, the consequences of which would range from lifecycle/food resource mismatches to regime shifts in the North Sea system. © Springer-Verlag and AWI 2004.