Deep current structure in the Toyama Deep-Sea Channel in the Japan Sea

The flow field in the Toyama Deep-Sea Channel (TDSC) in the Japan Sea was investigated based on mooring observations. An asymmetric current system accompanying offshore and onshore currents over the east- and west-side slopes in the channel, respectively, is suggested. A bottom intensified flow characteristic was observed at the offshore stations in the Yamato Basin. The asymmetric current system in the channel is also suggested by the asymmetric distribution of water characteristics across the TDSC in Toyama Bay; a cold dense water mass with higher dissolved oxygen (DO) and higher transmittance (Tr) was found over the west-side slope of the channel, whereas a water mass with lower DO and lower Tr was distributed over the east-side slope, suggesting a turbidity current from the head of Toyama Bay. The currents facing the shallower depth on their right-hand-side, along with the density distribution in the TDSC, suggest a density current system under the influence of the earth’s rotation. The dissolved oxygen concentration in the TDSC was significantly lower than that in the offshore region of the same temperature range. This suggests that the water mass over the west-side slope in the TDSC is a modified offshore water mass which experienced significant mixing with the low DO water mass over the east-side slope in the TDSC, probably due to strong shear between the offshore and onshore currents in the narrow channel.

Spatial variations in zooplankton community structure along the Japanese coastline in the Japan Sea: influence of the coastal current

This study evaluates spatial variations in zooplankton community structure and potential controlling factors along the Japanese coast under the influence of the coastal branch of the Tsushima Warm Current (CBTWC). Variations in the density of morphologically identified zooplankton in the surface layer in May were investigated for a 15-year period. The density of zooplankton (individuals per cubic meter) varied between sampling stations, but there was no consistent west–east trend. Instead, there were different zooplankton community structures in the west and east, with that in Toyama Bay particularly distinct: Corycaeus affinis and Calanus sinicus were dominant in the west and Oithona atlantica was dominant in Toyama Bay. Distance-based redundancy analysis (db-RDA) was used to characterize the variation in zooplankton community structure, and four axes (RD1–4) provided significant explanation. RD2–4 only explained < 4.8 % of variation in the zooplankton community and did not show significant spatial difference; however, RD1, which explained 89.9 % of variation, did vary spatially. Positive and negative species scores on RD1 represent warm- and cold-water species, respectively, and their variation was mainly explained by water column mean temperature, and it is considered to vary spatially with the CBTWC. The CBTWC intrusion to the cold Toyama Bay is weak and occasional due to the submarine canyon structure of the bay. Therefore, the varying bathymetric characteristics along the Japanese coast of the Japan Sea generate the spatial variation in zooplankton community structure, and dominance of warm-water species can be considered an indicator of the CBTWC.

Spatial variations in zooplankton community structure along the Japanese coastline in the Japan Sea: influence of the coastal current

This study evaluates spatial variations in zooplankton community structure and potential controlling factors along the Japanese coast under the influence of the coastal branch of the Tsushima Warm Current (CBTWC). Variations in the density of morphologically-identified zooplankton in the surface layer in May were investigated for a 15-year period. The density of zooplankton (individuals per cubic meter) varied between sampling stations, but there was no consistent west–east trend. Instead, there were different zooplankton community structures in the west and east, with that in Toyama Bay particularly distinct: Corycaeus affinis and Calanus sinicus were dominant in the west and Oithona atlantica was dominant in Toyama Bay. Distance-based redundancy analysis (db-RDA) was used to characterize the variation in zooplankton community structure, and four axes (RD1–4) provided significant explanation. RD2–4 only explained

Habitat of the deep-sea anemone, Cribrinopsis japonica, with shrimp: Does Cribrinopsis japonica establish a symbiotic relationship with shrimp?

We recently found Cribrinopsis japonica Tsutsui & Tsuruwaka, 2014 (Shinkai-hakutou-ginchaku in Japanese name) at the depth between 384 and 800 m in Toyama Bay, Sea of Japan. Since then, C. japonica has been reared under atmospheric pressure in the laboratory for seven years. C. japonica may use a fluorescent protein carried in its tentacles to lure shrimp (Tsutsui et al., 2016*1). However, the ecology of C. japonica in the deep-sea is hardly known. To elucidate the unknown ecology, we coupled one of the first long-term in situ studies of deep-sea organisms with complementary laboratory experiments. Our exploration of deep-sea benthos revealed that C. japonica inhabits the deepest areas of the sea floor at 1,960 m. Moreover, 80% of C. japonica in the deep-sea stayed together with the deep-sea shrimp. In the laboratory environment, when we added the same shrimp species which was observed in situ to the rearing tank with C. japonica, C. japonica stayed closer with the shrimp without attacking using the tentacles. It is rare to observe different animals together at one place or space since there are very few animals in the ocean floor at > 1,000 m depth in the Sea of Japan (Motokawa & Kajihara, 2017*2). In such depopulated environment, it is conceivable that C. japonica and the shrimp may receive benefit mutually or one side by establishing a ‘symbiotic relationship.’ We will elucidate their relationship in more details by studying the possible ‘symbiosis’ in the laboratory.