Winter mixed layer depth and spring bloom along the Kuroshio front: implications for the Japanese sardine stock

Recruitment of Japanese sardine Sardinops melanostictus is related to interannual variability in the winter mixed layer depth (MLD) near the Kuroshio axis (line of maximum current), possible because MLD may influence the feeding environment of sardine larvae through phytoplankton productivity. However, a relationship between the winter MLD and phytoplankton productivity has not been shown. Particle release experiments with quasi-observed current fields from ocean reanalysis products and satellite-observed phytoplankton (chlorophyll a) density from 1998 to 2006 showed that deeper waters of the winter mixed layer flowing 0 degrees to 0.5 degrees north of the Kuroshio axis led to a greater bloom in the subsequent spring, although such a relationship was not detected south of the Kuroshio axis. By using the output of a 3-dimensional, high-resolution lower trophic level ecosystem model that reproduced the MLD-phytoplankton relationship, we found that entrainment of deeper nutrient-rich subsurface water leads to abundant nutrients in the early spring and enhances the subsequent spring bloom along the northern side of the Kuroshio axis. On the southern side, where mode water develops in winter, the deeper winter mixed layer does not necessarily contain higher nutrient contents, because nutrient vertical profiles often have inversions. These results support the hypothesis that sardine larvae that are distributed in the deeper winter mixed layer north of the Kuroshio axis (called the Kuroshio frontal zone) encounter a higher phytoplankton density, which yields favorable feeding conditions, resulting in recruitment success.