Dominance of the non-indigenous diatom Mediopyxis helysia in Wadden Sea phytoplankton can be linked to broad tolerance to different Si and N supplies

•We address consequences of the invasion of the diatom M. helysia into the Wadden Sea.•Performance of M. helysia at different Si and N supply was examined experimentally.•M. helysia reached dominance in the natural and artificial community.•Its presence changed the community composition in all chosen Si and N concentrations.

The non-indigenous diatom Mediopyxis helysia was first observed in spring 2009 in the backbarrier tidal flats of Spiekeroog, North Sea (53°45′01.00″N and 007°40′16.30″) and has dominated spring blooms throughout the years 2009–2011. It also prevailed during the remainder of the season and – compared to 2009 – the total biovolume during spring blooms decreased by 36% and phytoplankton species richness as well as evenness declined concomitantly. M. helysia's rapid establishment led to the questions: (1) Which advantageous traits does this species possess? (2) Which environmental conditions facilitate its expansion? Therefore, we here amend our field observations with an experiment, in which artificial communities were cultured under six different combinations of Si and N concentrations in presence and absence of M. helysia for one month. The relative biomasses of M. helysia with regard to different nutrient concentrations in an artificial community were examined with exclusion of grazing. At the end of the experiment, the phytoplankton community was dominated by the only lab culture, the chlorophyte Dunaliella salina, but M. helysia was the most successful diatom. Its presence changed the community composition irrespective of the chosen Si and N concentrations. M. helysia significantly diminished chlorophytes and dinoflagellate percentages, whereas diatom growth was supported and total biovolume increased in the experiment. Moreover, M. helysia enhanced the evenness in the artificial community. The broad persistence of M. helysia under different resource conditions may have contributed to the successful establishment of M. helysia in the North Sea and its dominance throughout the year.