Preliminary studies on dinoflagellates epiphytic on macrophytes in a bight of Peter the Great Bay, Sea of Japan revealed the presence of two species of the genus Ostreopsis, O. cf. ovata and O. cf. siamensis. Morphometric features, such as the length of the pore plate, and thecal ornamentations were used for species identification. The two species differed from each other in cell size and the length of the pore plate. Cells of O. cf. ovata were smaller in length and width, with a dorsoventral diameter of 36-60 mm and a width of 24-45 mm, in comparison with the cells of O. cf. siamensis, which were 63-78 mm in dorsoventral diameter and 36-54 mm in width. Cells of O. cf. siamensis are slightly flattened in comparison with O. cf. ovata; the dorsoventral to anteroposterior diameter ratio varied from 1.9 to 2.7 for O. cf. ovata and from 1.8 to 3.5 for O. cf. siamensis. The length of the pore plate in O. cf. ovata (6.3-8.3 mm) is shorter than that in O. cf. siamensis (11-13 mm), while the diameter of the thecal pores in O. cf. ovata and O. cf. siamensis overlapped markedly (0.12-0.25 mm and 0.16-0.20 mm, respectively). The two species were presentfrom August to October at water temperatures of 7-258C on almost all macrophytes collected. The maximum abundance of Ostreopsis spp. reached 10,970 cells g -1 dry weight in late September at a water temperature of 98C. This is the first occurrence of the genus Ostreopsis in Russian waters, which are characterized by cold temperatures during the winter season.

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The morphology of Chaetoceros salsugineus Takano is described using both light and electron microscopy. This species possesses typical characteristics of the subgenus Hyalochaete. Our investigations show that intercalary valves having processes are actually terminal valves, in the area of contact of which the colony disintegrates into separate short chains. Chaetoceros salsugineus occurred annually in the summer period and was abundant during the summer bloom of phytoplankton. Maximum densities (up to 12 millions cells L" 1 ) were recorded near the water surface after heavy rains when stratification of the water column occurs. Some taxonomic and autoecological questions are discussed.