In order to investigate the migration pattern of the pond smelt, Hypomesus nipponensis, the life history trajectories of populations from various localities in Japan were estimated using the otolith microanalysis technique. The strontium:calcium (Sr : Ca) ratio of otoliths was determined for pond smelt reared in fresh, brackish and sea water and was found to show a linear increase with increasing salinity. Salinity trajectories were constructed for populations based on a regression of Sr : Ca ratio to salinity. Some individuals in Lakes Abashiri and Ogawara, and all fish in Lakes Takahoko and Shinji showed values which were consistently low, indicating residence in these lakes throughout the life history. High values were also absent in individuals from the Ishikari River, so it was concluded that the populations migrated between fresh and brackish waters. However, pond smelt with high Sr : Ca ratio profiles indicating seawater residency were found in the Hei River, off Hachinohe, off Misawa, Lake Obuchi, Oppa Bay, and also some individuals from Lakes Abashiri and Ogawara. In these cases, residence in the sea appeared to have been short, except for Hei River and Lake Ogawara. These results indicate a wide flexibility in habitat selection by pond smelt, with brackish environments possibly playing an important role in the life history of the species.

Abundance and distribution of resting cysts of toxic dinoflagellates, Alexandrium spp. (A. tamarense and/or A. catenella), were investigated in the surface sediments of Ise Bay and Mikawa Bay in June 2002, using primuline-staining direct count method. The cysts were abundant at innermost part and west of central part of Ise Bay, and eastern part of Mikawa Bay. The cyst density varied from 22 to 2,254 cysts cm^-3 wet sediment (mean=365 cysts cm^-3) and from 31 to 7,311 cysts cm^-3 wet sediment (mean=1,831 cysts cm^-3) in Ise Bay and Mikawa Bay, respectively, showing an increasing trend of cyst abundance from 1980’s when previous monitorings were conducted. It is also evident that, among bays reported so far in Japan, Ise Bay and Mikawa Bay are the ones having, respectively, the high and the highest cyst abundances. The cyst density in both bays tended to increase drastically where mud content exceeds ca. 70%, although this is not always the case. This may indicate that the selective mechanisms of sediment particles by physical factors, such as hydrographic and sedimentation processes, act on cysts strongly but the magnitude of cyst formation in the water column above the sediment also affects the cyst abundance simultaneously.

To elucidate the spatial and temporal dynamics of sources of particulate organic matter (POM) in the estuarine turbidity maximum, surveys (12 cruises) were conducted along the Chikugo River in the innermost Ariake Sea, Japan, from 24 February to 24 April 2005. To elucidate the influence of semi-lunar periodicity, surveys were conducted around the high slack tide in the daytime and were divided into three categories according to the tidal range; spring (>400 cm), medium (200–400 cm), and neap tides (<200 cm). At spring tides, the vertical mixing was strong and the turbidity maximum (>100 NTU) was formed at lower salinity (<5), synchronized approximately with both the maximum of particulate organic carbon and pheophytin concentrations (>5 mgl^-1 and >10μgl^-1, respectively). In contrast, the turbidity was low and higher chlorophyll a concentrations indicated active production of phytoplankton at neap tides. The POM in the surface water at spring tides had stable carbon isotope ratios (δ¹³C, mean±standard deviation: -25.3±0.9‰) and C/N atomic ratios (C/N, 11.6±2.0) significantly higher than those at neap tides (δ¹³C: -26.0±1.0‰, C/N: 7.9±1.4). The results indicate that the POM maximum accompanied by the estuarine turbidity maximum consists of resuspended detritus of both phytoplankton and terrestrial C₃ plants.