Microalgal assemblages were collected from drifting sea-ice and sea-surface waters in the coastal waters off Mombetsu, Hokkaido, in the Okhotsk Sea during the winter of 2011 and 2012, and the biomass and species composition of the assemblages were examined. Chlorophyll a concentration and cell density in the colored parts of the drifting sea-ice varied in the range of 24.1–104.0 mg m^-3 and 5.10–15.38×10⁶ cells L^-1, respectively. Based on cell density, the dominant taxa were the pennate diatoms Achnanthes taeniata and Fragilariopsis spp., the centric diatoms Thalassiosira spp. and Chaetoceros socialis. Pennate diatoms commonly dominated in the microalgal assemblages from sea-ice samples. On the other hand, Thalassiosira spp. was the most dominant taxon in all sea-ice samples based on carbon biomass. From the results of the present and previous studies, it is suggested that Thalassiosira spp. play an important role in the ice-edge and spring phytoplankton blooms.

Seasonal occurrence of pelagic chaetognaths and some characteristics of the population structure of the dominant species, Zonosagitta nagae (Alvariño, 1967) were described based on zooplankton samples collected by vertical hauls (0-ca. 600 m depth) of a Norpac net (mesh opening: 0.33 mm) at the entrance of Tokyo Bay from November 1992 to January 1995. Twenty-four species belonging to 13 genera of Chaetognatha were identified, of which 15 species belonging to 9 genera were epipelagic and 7 species belonging to 5 genera were mesopelagic, and only one was bathypelagic. The most dominant species (Z. nagae) in the epipelagic layer appeared mainly from March to August with a peak in June, and its relative contribution to total chaetognath abundance was 51％ on average. The second-most dominant species, Mesosagitta minima, occurred mainly in summer and autumn. Two species were equally next-most dominant, Flaccisagitta enflata, which was mainly found from September to November, and Serratosagitta pacifica, which occurred abundantly from November to January. It is a common characteristic with regards to their distribution that the above four epipelagic species found in Tokyo Bay are also predominant in both Suruga and Sagami Bays. Furthermore, it is noteworthy that Aidanosagita crassa, which may have been transported out from the inner part of Tokyo Bay, mainly occurred in summer. In the mesopelagic layer, Eukrohnia kitoui Kuroda, 1981 was dominant for seven months of the year. It is supposed that the mass occurrence of this species may be due to the submarine topography at the survey point. The ovaries of Z. nagae started to appear at about 6 mm body length, began to mature from around 15 mm BL, and seemed to spawn after reaching 16 mm BL from March until about October. Spawning of this species was most intense from April to August, although it also occurred in other months, except for from December to February.

Distributions of planktonic larvae of the burrowing mud shrimp Laomedia astacina and the ghost shrimp Niphonotripaea japonica were observed in the estuary of Tama River in Tokyo Bay by longitudinal section samplings in August, 2010 and 2011 and by a 24-hour-sampling at the river mouth in the August, 2006. Larvae of L. astacina were abundant in the higher salinity layer (S > 20), with the exception of the hypoxic layer (DO < 1 mg L^-1), in the estuarine waters. The high proportion of zoea 3–4 stages in the total larvae of L. astacina and the low outflow flux at the river mouth suggest that the planktonic phase of this species is completed within estuarine waters. Larvae of N. japonica, including developed stages, were distributed consistently in the layer of the bay waters deeper than 3 m, excluding the hypoxic layer, in both 2010 and 2011. Larvae of this species occurred in the estuarine waters as well as L. astacina in 2011, but disappeared mostly from estuarine waters in 2010 when the hypoxic layer was well-developed near the bottom. The absence of stages past the zoea 2 stage of N. japonica in the estuarine waters and the high outflow flux of the larvae to the bay at the river mouth suggest that the main larval habitat is not estuarine waters but bay waters. The lower limits of the vertical distribution of planktonic larvae were influenced by the development of bottom hypoxia in both species. Especially in the estuarine waters, bottom hypoxic water may be an important factor controlling the retention and the outflow of these larvae.