The annual and periodical recurrence of events and processes in marine plankton populations plays a central role in pelagic food webs. Phytoplankton in shallow coastal seas is subject to high seasonal variability which makes analyses of phenological changes and their classification as imprints of climatic changes difficult. The extent of natural variability relative to trend-like dynamics can only be determined if phytoplankton observations are both regular and long-term. We analyze phenological variability of diatom species monitored at Helgoland Roads during the period 1962-2013. We derive temporal indices based on the cumulative percentage of annual abundance to approximate the beginning and the end of individual annual growth periods and analyze their relationships to abiotic and biotic parameters. Timing of phytoplankton species is highly variable and only very few species exhibit significant trends in phenological characteristics over the entire period. Phytoplankton species can exhibit a wide range of environmental conditions associated with the beginning of growth. As a result, strong fluctuations in winter/spring temperatures do not automatically lead to changes in timing dates. Most species exhibit a broader timing window at the beginning of the growth period than at the end. Short phases of rapidly changing light conditions during spring and autumn seem to play an important role as a sort of ´climatic boundary` for phytoplankton seasonality. We will put the phenological response in context to long-term trends in abiotic determinants (e.g. temperature) observed at Helgoland Roads and discuss the potential of making assessments about consequences for the marine ecosystem.