Submarine groundwater discharge (SGD) is an important nutrient source for the coastal ecosystems of Obama Bay as well as river water. However, the response of phytoplankton to nutrients supplied from freshwater is unknown. In this study, we evaluated the spatial and temporal variability of primary production from phytoplankton in Obama Bay. From May 2011 to March 2012, we made monthly field observations of Chlorophyll-a (Chl-a) concentration and calculated potential primary productivity (PP) using a simple mathematical model. Chl-a concentrations and PP were each higher in the upper layer from November to February, when the river discharge was high. During the months from March to October, when the nutrients in the upper layer were almost depleted, Chl-a concentrations and PP were higher in the lower layer. In May 2011, we observed significantly higher concentrations of Chl-a (51.8 μg l⁻¹) in the bottom layer at an area 2 km offshore from the river mouth. We also observed low-salinity water in the bottom layer at around the same area in June 2011, one week after a notable flood. Together these observations strongly suggest that SGD led to the phytoplankton bloom observed in the bottom layer during the spring.

To clarify the effects of the marine environment on the adductor muscle wet weight of bottom-cultured Japanese scallop Mizuhopecten yessoensis in the coastal area of the northeastern part of Hokkaido, Japan, monthly surveys were conducted from March 1992 to December 2012. Spearman’s rank correlation coefficients were calculated between mean adductor muscle wet weights from August to October in four-year-olds and monthly marine environments such as water temperature, Chlorophyll a concentration as an indicator of food, and the sea level difference between Wakkanai and Abashiri as an indicator of Soya warm current from April to July. Significant positive correlations were found between mean adductor muscle wet weights and all marine environments in April. We used a generalized linear model (GLM) to statistically analyze these field survey data. The model used water temperature, Chlorophyll a concentration, and sea level difference in April as explanatory variables was selected by AIC. These results suggest that marine environments in April are important for changes in adductor muscle wet weight of bottom-cultured Japanese scallop in the coastal area of the northeastern part of Hokkaido.

Two Nematalosa species, N. japonica and N. come, are valuable target species for local fisheries in the Okinawan region of Japan. We examined the occurrence patterns, growth, osteological development, and feeding habits of two Nematalosa species collected from Nakagusuku Bay, Okinawa Island, Ryukyu Archipelago. Preflexion larvae first occurred in offshore areas and thereafter recruited to sandy tidal flats as they developed swimming abilities. The timing of recruitment to the sandy tidal flats occurred 10 days (10 mm) after the hatching of both species. Postflexion larvae and juveniles occurred mainly at sandy tidal flats and tidal mudflats, respectively. Metamorphosing larvae occurred in both habitats. These findings suggest that the larvae moved from sandy tidal flats to tidal mudflats during the metamorphosis phase. This phase is recognized as a semi-complete period in which swimming ability is attained, thus active movement can be assumed. The timing of this directed movement occurred ca. 30 days (15 mm) after hatching. The main food of larvae and juveniles (<22 mm) were calanoid copepods, and juveniles (>22 mm) began to feed on detritus. Juveniles inhabited and grew up in the tidal mudflats and adjacent shallow areas, and reached to ca. 100 mm one year after hatching. Shallow areas, especially tidal flats, are considered to be an important nursery area for larval and juvenile stage of these two Nematalosa species. Recent artificial environmental changes in Okinawa Island could reduce potential nursery areas for these species. Therefore, shallow habitats should be conserved to preserve the populations of the two species.