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An investigation of vertical distribution, rhythms of vertical migration and division of epipelic free-living diatom algae was carried out at different sediment depths on intertidal sandflat in the Chernaya River Estuary (Kandalaksha Gulf, White Sea, Russia) during the summers of 1994-2001. The depth of penetration of viable microalgae cells in the sediment did not exceed 4.2 cm in the areas with clay sublayers, where zones with high negative Eh values were located closer to the surface. Diatom habitats in the areas with coarse sand sublayers extended to more than 8 cm depth. Approximately 40% of diatoms were present in the topmost 2 mm layer. The subsurface portion of diatom cells on average was concentrated within the layer with Eh values ranging from 0 to -80 mV. Eh values below -100 mV limited diatom penetration in deep anoxic sediment layers. The deep portion of diatom populations consisted basically of dividing cells. A cytological analysis of diatom cell distribution showed that the proportion of cells in the different phases of mitosis increased with increasing depth, and reached 80 to 90% at a depth of 2 to 3 cm. The regular presence of dividing diatom cells almost exclusively in aphotic anoxic sediments was noted first. The study of the dynamics of microphytobenthic vertical distribution revealed active diurnal vertical migration depending on temperature, light and tide regimes. The frequency of doublet cells (FDC) in epipelic diatom populations was in inverse proportion to cell concentrations in the photic zone. Diatom division was registered only during incubation in the porewater, which was rich in inorganic nutrients and dissolved organic matter. We suppose that a more stable habitat with higher concentrations of nutrients (especially nitrogen) in their more reduced forms (NH 4 + instead of NO 3 -) provides more favourable conditions for cell growth and division at the depth of several cm within the sediment compared, to the surface layer. Migration activity of diatom algae is an adaptation which permits cells to consume energy and to undergo fission under optimum conditions in spatially disconnected zones. KEY WORDS: Epipelic diatoms · Vertical distribution · Vertical migration · Cell cycle · Intertidal sandflatResale or republication not permitted without written consent of the publisher

investigations were carried out on an intertidal sandflat (Kandalaksha Gulf, the White Sea, Kuss~a) during the summers of 1988 to 1992. Analysis of samples collected from areas of different sizes furn~shed information on the distribution of microphytobenthic organisms (diatoms and dinoflagellates) over 3 spatial scales: microscale (10 to 1000 cm2), mesoscale (1000 cm2 to 100 m2) and macroscale (100 to 10000 m?). Analyses of the data disclosed the aggregated character of distribution on all spatial scales for the majority of dominating microalgal species and the existence of 2 orders of aggregation. The spatial structures formed by aggregates of the first and second orders are noticeably different from each other in the degree of aggregation (Cassie Index of 0.5 and > 2 on average, respectively), overlapping of species distributions (Pianka Index of 0.5 and 0.3 on average) and degree of similanty of species structure (Pianka Index of 0.9 and 0 5 on average). This indicates the existence of several structural associations on the studied intertidal sandflat which differed from each other in species composition and the character of spatial distribution of organisms. The studied community had a highly complicated spatial structure. The presence of several orders of aggregation probably results from several main biotic (interspecies interaction) and abiotic (granulometric composition of sediments and emersion period during low tide) factors. The degree of influence of these factors on the character of spatial d~stribution of microalgae is related to the selected spatial scale.

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