The Helgoland Roads phytoplankton long-term data set is one of the longest and the most detailed data sets in Europe, having provided continuous work-daily observations of phytoplankton abundance since 1962. These high frequency counts have undergone and are continuously subject to a high level of quality control. The Helgoland data set thus is useful in the evaluation of new records in phyto- and zooplankton. Here, we report the first appearance of the relatively recently described diatom Mediopyxis helysia in the Helgoland Roads counts. This species was first detected in Helgoland samples in March 2009. Importantly, it has rapidly become a prominent member of the Helgoland phytoplankton community. While in 2009 it only produced a moderate spring peak of 4,000 cells l-1, it was one of the dominant diatoms in the 2010 spring bloom with Mediopyxis reaching cell densities above 300,000 cells l-1 and total chlorophyll concentrations exceeding 40 μg l-1. In 2010, this species was repeatedly present throughout the year. There was no clear relationship between temperature and cell abundance with all Mediopyxis cells occurring at temperatures between 3 and 12°C. However, the extensive peak in 2010 was associated with a sudden drop in salinity, which could indicate that this bloom might have been the result of inflow of low salinity water into the area. This was supported by a laboratory growth experiment in which a clonal culture of M. helysia grew fastest at a salinity of 27 and slowest at a salinity of 31.5. Further long-term observations will be required to establish whether this species will become a regular feature at Helgoland and how this might affect the local food web. © 2011 Springer-Verlag and AWI.