Modern ostracod assemblages in the Kii Channel delineate three biofacies. These biofacies correspond to sedimentary and hydrographic gradients ranging from low-salinity coastal water to warm, saline offshore water intruding from the Kuroshio Current. Species diversity remains consistently high across the Kii Channel, with no sites exhibiting unusually low values. In marked contrast, adjacent Osaka Bay shows strongly depressed diversity due to anthropogenic eutrophication and seasonal hypoxia. These patterns indicate that the Kii Channel represents a relatively undisturbed natural system in which the ecological imprint of Kuroshio-driven hydrographic contrasts is clearly expressed. A noteworthy finding is a cluster of Neomonoceratina delicata valves at station K7, representing one of the northernmost occurrences of this subtropical species in surface sediments of the Japanese archipelago. More broadly, our results demonstrate that ostracod biofacies provide a sensitive indicator of spatial environmental variability in marginal seas influenced by western boundary currents. • Three modern ostracod biofacies are identified across the Kii Channel. • Biofacies track gradients from low-salinity coastal to Kuroshio-derived offshore waters. • Salinity and mud content best explain spatial turnover of dominant ostracod groups. • High diversity across the Kii Channel contrasts with diversity loss in eutrophic Osaka Bay. • The Kii Channel provides a natural reference for benthic biofacies under boundary currents.
Iwatani et al. (Fri,) studied this question.