Dominance of the invasive diatom Mediopyxis helysia (Kühn et al., 2006) irrespective of prevailing Si:N ratios

Background/Question/Methods

Invasions of non-indigenous phytoplankton species are increasing in the North Sea, potentially affecting community structure of autotrophs and other trophic levels.

The invasive diatom Mediopyxis helysia was first observed in spring 2009 in the backbarrier tidal flats of Spiekeroog. Close-meshed sampling in the observational study revealed spring blooms mainly being composed of M. helysia as well as a pronounced dominance throughout all seasons of three consecutive years. M. helysia´s rapid establishment in the German Bight led to the question, which advantageous traits this species has and by which environmental conditions its expansion is facilitated.

Therefore, we conducted experiments with artificial communities composed of equal biovolumes of 11 species, cultured under six different Si:N ratios in the presence and absence of M. helysia for one month. We hypothesized that M. helysia reaches dominance under experimental conditions as well and thus impacts the performance of other algae groups.

Results/Conclusions

During the observational study, dominance of M. helysia increased during three consecutive spring blooms (2009-2011), while total biovolume, species richness and evenness declined.

At the end of the experiment, total biovolume was highest at high Si | high N. The phytoplankton community was dominated by the chlorophyte Dunaliella salina, whereas presence of M. helysia enhanced total biovolume, contributing up to 23%. M. helysia was neither significantly influenced by Si nor N and thus produced comparable biovolumes in all treatments. By contrast, summed biovolumes of all other diatoms increased significantly with increasing Si:N ratios and equaled M. helysia’s biovolume at the end of the experiment. Hence, M. helysia was the most successful diatom and led to changes in the phytoplankton community composition as it tended to diminish growth of chlorophytes, confirming our hypothesis.

Taken results from both studies together, a successful establishment of M. helysia in the North Sea could have far-reaching consequences for the whole ecosystem. Besides alternations in species richness and evenness in the phytoplankton community, it may exert cascading effects up the food chain, since it potentially cannot be ingested by all zooplankton species due to its large size.