We analysed coccolithophorid and calcareous dinoflagellate assemblages from an 18-month (June 1990-August 1991) sediment trap record in the Bay of Biscay. With three trap deployments, the sampling resolution ranged from five to eight days. Characterisation of the assemblage dynamics is based on the use of statistical tools such as principal component analysis (PCA). The assemblages record seasonal and short-term events, implying that despite the dominance of lateral transport from the shelf, the traps faithfully record ecological dynamics. Summer species are Braarudosphaera bigelowii, Calcidiscus leptoporus (small), Coccolithus pelagicus, Emiliania huxleyi (closed), small Gephyrocapsa, Helicosphaera carteri, Pontosphaera japonica, Syracosphaera gr. molischii, Thoracosphaera heimii, Umbellosphaera tenuis. Autumn assemblages are characterised by the high frequency of C. leptoporus (large), Syracosphaera pulchra, Florisphaera profunda and E. huxleyi (open). Pontosphaera discopora and G. muellerae are most abundant during the winter whereas Umbilicosphaera sibogae peaks during spring. No other species shows a clear seasonal abundance pattern. The changes in assemblage composition are correlatable with changes in environmental parameters, such as wind, wave and light. Environmental dynamics, linked to seasonal succession, trigger a shift in relative abundance of morphotypes of E. huxleyi and C. leptoporus. The relations between species in the coccolithophorid community are characterised by a strong species dominance: (1) E. huxleyi dominated the assemblages (54-94%) and (2) the community followed a geometric distribution (in ranked dominance of species). The dominance increases during higher phytoplankton production as indicated by higher fluxes of diatoms, silicoflagellates and coccospheres of E. huxleyi. (C) 2001 Elsevier Science B.V. All rights reserved.