There are two distinct frontal systems in the German Bight: a tidal mixing front north of the East Frisian Islands and a river plume front deriving mainly from the Elbe river in the East. Phytoplankton species composition revealed significant differences between these two types of fronts. The distribution of chlorophyll a and phytoplankton carbon showed maxima at both fronts in spring; however, the standing crop at the tidal mixing front was dominated to more than 90% by autotrophic nanoflagellates with low photosynthethic rate (0.72 mg C mg Chl a^-1 l^-1), whereas neritic diatoms with relatively high photosynthetic activity (4.1 mg C mg Chl a^-1 l^-1) formed more than 90% of the peak at the river plume front. It is supposed that the diatom peak at the Elbe river plume front was generated and maintained there by increased water column stability in conjunction with an improved nutrient supply, whereas the nanoflagellate maximum at the tidal mixing front is traced back to convergent accumulation. In summer, during periods of calm weather, even small frontal gradients may lead to high biomass accumulations of particularly the dinoflagellates Ceratium fusus and Noctiluca scintillans. Both species are typical for the summer plankton in this area. The Ceratium fusus biomass reached values of more than 1 mg C 1^-1 in the surface waters adjacent to the front, whereas the Nocriluca concentration amounted to as much as 4 mg C l^-1. The former showed an unusually high percentage of aberrant forms in the frontal peak, which may be a result of chemical interference.