Entrainment of coastal water into a frontal eddy of the Kuroshio and its biological significance

Abstract

The Pacific coastal areas of Japanese Island are major spawning grounds of various fishes. It is considered that large amount of eggs and larvae are dragged into the Kuroshio front so that the survival of fish larvae at the front is important for their recruitment. From this viewpoint, a low-salinity water mass, which was withdrawn from the coastal area to the Kuroshio front, was investigated by drifters, in addition to fine-scale hydrographic observations and water sampling in and around the Kuroshio frontal area off Enshu-nada. The drifters were transported to the east within the low-salinity water along the Kuroshio front in the first stage, and were thereafter entrained into an eddy, which was caused by the frontal meander. They moved closely to each other along the front, but diverged in the eddy. This movement of the drifters coincided with the deformation of low-salinity water mass; the low-salinity water concentrated at the Kuroshio front surrounded by strong salinity gradients at first, while it spread out horizontally and became vague in the shallow surface layer in the frontal eddy. Comparing temperature sections across the front, the strong upwelling was detected in the eddy. Limiting factors for primary production and growth rates were calculated in six sections using the observed temperatures and concentrations of nutrients. In the frontal area of the Kuroshio, low concentration of nutrients limited the primary production shallower than 50 m. Due to the low productivity, concentration of chlorophyll a in the low-salinity water tended to decrease, although the initial concentration was high. Once the coastal water mass was entrained into the frontal eddy, on the contrary, the concentration recovered due to the enhanced primary production in the subsurface layer supported by the upwelling of nutrient-rich water. Fish larvae in the low-salinity water are assumed to use the new production in the eddy; otherwise, they would starve. The entrainment process, which was probably caused by offshoreward movement of the Kuroshio, holds the key to successive survival and recruitment of fish larvae in the Kuroshio system.