Grids of 17 to 50 stations in the German Bight were sampled 18 times within the framework of the multidisciplinary programmes ZISCH and PRISMA in winter and spring of 1988/89 and from April 1991-April 1992. The frequent abundance of Coscinodiscus wailesii Gran & Angst, a recently established large diatom, was noteworthy, as it dominated the phytoplankton biomass over long periods (e.g. 12/88-3/89 and 8/91-11/91). The bulk of the phytoplankton carbon during these periods (up to 90%) could be attributed to this species. Blooms of Coscinodiscus wailesii producing up to 1400 μg carbon 1-1, were recorded in early spring of 1989 and autumn of 1991. The potential consequences for the whole ecosystem refer to the huge size of this organism, which may cause reduced exploitation of its primary production by native consumers. Furthermore, sedimentation and remineralisation processes may be affected. For C. wailesii, a doubling of biomass in 70 h could be estimated on the basis of data from four successive surveys in 1991. Results from laboratory cultures under comparable conditions confirm this rate. In the survey area, no increase in biomass was recorded for the related Coscinodiscus granii Gough, although there was sufficient silicate supply for growth. Field data showed 8-10 times lower copper and 10-20 times lower cadmium and zinc accumulation in C. wailesii, compared to concentrations found in native phytoplankton species. Subsequent laboratory tests suggested that one reason for the remarkable success of C. wailesii, mainly in inshore regions, may be derived from its tolerance of higher doses of heavy metals due to very low sorption. © 1995 Biologische Anstalt Helgiland.