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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

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The occurrence of the dinoflagellates belonging to the ciguatera-related genus Gambierdiscus is reported for the first time from the north-western Arabian Gulf and northern Red Sea in the Indian Ocean basin. Gambierdiscus yasumotoi was recorded from two sampling sites on the southern Kuwait coast, and co-occurrence of G. cf. belizeanus and G. yasumotoi was found also in the Gulf of Aqaba in Jordan. The morphology of cells is described on the basis of light microscopy and scanning electron microscopy examination.

Taxonomic data of free-living benthic fl agellates in Kuwait's intertidal sediments are summarized. A full list of the species composition is presented, including distribution on diff erent sediment types, species occurrence and light micrographs for each taxon identifi ed. A total of 67 fl agellate species were identifi ed, representing six classes. Most of them are reported from Kuwait for the fi rst time. Th e most abundant and diverse species were sand-dwelling dinofl agellates (43 taxa).

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