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Viable diatom and dinoflagellate resting stages were recovered from sediments in Koljo$ Fjord on the west coast of Sweden. To determine the maximum survival time of buried resting stages, samples from sediment depths down to 50 cm were incubated at temperatures of 3, 10 and 18 mC. Sediment cores were dated by #"!Pb and the age of samples containing viable resting stages was determined using the constant rate of supply model. Dilution cultures of surface sediments allowed semiquantitative estimates of the potential seed bank. Dinoflagellate cysts from species such as Diplopsalis sp., Gymnodinium nolleri, Oblea rotunda and Protoceratium reticulatum were viable down to 15 cm depth, or 37 years old. Spores and resting cells of the diatoms Chaetoceros spp., Detonula confervacea and Skeletonema costatum were viable to over 40 cm depth, and may have been buried for many decades. The seed bank of living resting stages in surficial sediments was found to be rich (c. 57 000 diatom resting stages g −" wet weight and c. 200 dinoflagellate cysts g −" wet weight), and the percentage of viable resting stages was higher for spore-and cyst-forming species. The oxygen-deficient sediments in Koljo$ Fjord appear to be a natural conservator of cell viability, a condition not easily simulated in laboratory studies. These results are ecologically important since spores and cysts are a repository of genetic material able to repopulate waters if resuspended and exposed to suitable light, temperature and nutrients.

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Skeletonema marinoi Sarno et Zingone is a planktonic marine diatom with a widespread geographic distribution. Different populations of this species may show distinct genetic signatures. We have evaluated the utility of three common molecular methods for distinguishing clones of S. marinoi from different geographic regions. Clonal cultures were isolated from the Canadian west coast, south west Portugal, and the east and west coasts of Sweden. All strains originated from resting stages in sediment. More than 90% of the individually isolated chains grew to densities suitable for DNA extraction. Genetic signatures of clones from each sample location were assessed by sequencing variable domains (D1-D3) of the nuclear large subunit (LSU) rRNA gene and internal transcriber spacer (ITS) (ITS-1, 5.8S and ITS-2) regions, and also by analysis of randomly amplified polymorphic DNA patterns. Analysis of molecular variance showed that strains from the four geographic areas were significantly separated by all three methods but that differences among European samples were best resolved by ITS 2 sequences.

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 Detonula confervacea and Thalassiosira minima showed strong dependence on benthic propagules. Populations of Chaetoceros debilis and Thalassiosira nordenskioeldii were initiated by both benthic and planktonic cells, and benthic seeding was most successful when experiments were preceded by a planktonic bloom. Skeletonema costatum 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 Pseudo-nitzschia spp., were present only in planktontreated 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.