The Helgoland Roads long-term datasets, with their daily sampling frequency, provide some of the most detailed data on phytoplankton composition and bacterial dynamics as well as the physico-chemical environment in the North Sea. Such data are invaluable for a wide range of topics from foodweb interactions to the effects of climate change on ecosystem dynamics and even ecosystem services. The Helgoland Roads data have shown an increase of 1.7 degrees in sea surface temperature as well as increases in salinity since 1962. These changes have also been reflected in changes in phytoplankton composition, the timing of the spring bloom and species seasonality. Examples are the extended occurrence of the diatom Guinardia delicatula and recent high biomass blooms of Mediopyxis helysia. The diatom bloom in general has been shown to be delayed after warm autumns preceeding it and even changes in niche breadth could be shown for individual species. In addition, a regime shift could also be shown for the period around 1988. While often related to salinity and temperatures, the causes of such changes are rarely simple. Here we will give further examples of changes in phytoplankton dynamics and describe the complex analyses used to illucidate ‘cause and effect’.