<jats:p>Although phytoplankton blooms are major events in aquatic systems, the importance of benthic resting stages in seeding planktonic blooms is still unclear. Using microcosms, we tested the influence of benthic versus planktonic inocula on the development and taxonomic composition of diatom communities in a temperate fjord. Experiments in early spring 2002, fall 2002, and late spring 2003 showed that the type and quantity of inoculum influenced bloom development and composition. Species composition was vastly different when seeded by cells from the benthos. Species such as <jats:italic>Detonula confervacea</jats:italic> and <jats:italic>Thalassiosira minima</jats:italic> showed strong dependence on benthic propagules. Populations of <jats:italic>Chaetoceros debilis</jats:italic> and <jats:italic>Thalassiosira nordenskioeldii</jats:italic> were initiated by both benthic and planktonic cells, and benthic seeding was most successful when experiments were preceded by a planktonic bloom. <jats:italic>Skeletonema costatum</jats:italic> was abundant in all treatments but showed variations in size, depending on the type of inoculum. Species that do not have a resting stage, such as <jats:italic>Pseudo‐nitzschia</jats:italic> spp., were present only in plankton treated microcosms. Seasonal factors were especially important in determining the successful growth of newly seeded populations. Our results indicate that benthic resting stages provide an important source for some species. Because the introduction of benthic resting stages to surface waters can greatly influence species composition of the plankton, it is important that studies of plankton blooms include life‐history stages from both the sediments and the water column.</jats:p>