Occurrence of Surface Phytoplankton Bloom as the Kuroshio Current Passes an Island

Abstract

AbstractWhen the Kuroshio Current passes by islands, a surface phytoplankton bloom is often observed in the wake of the island. To investigate its causes, a typical example from the Kuroshio region, in which a geostrophic current (1 m/s) passes around a cylindrical island, was simulated using a three‐dimensional biogeochemical model. At the surface, concentrations of dissolved inorganic nitrogen, phytoplankton, and zooplankton increased in the wake. In the euphotic zone, phytoplankton mainly grew in cyclonic eddies, where nutrients originated from the deep ocean. The sum of biogeochemical processes related to the phytoplankton increased immediately behind the island and had a maximum of 0.045 mmol N m−3 day−1 at 50 km downstream, which gradually decreased to a low value further from the island. Among four types of phytoplankton, the pico‐phytoplankton‐dominated the bloom behind the island, which was also observed in the Tokara Strait. A tracer run indicated that the hydrodynamic processes directly contributed to 50% of the increase in the phytoplankton concentration on the surface in summer and a decrease in winter, depending on the appearance of a subsurface chlorophyll‐a maximum. With an increase in island size, the size of eddies, distance between eddies, and surface phytoplankton concentration also increased. For the first 300 km behind the island, no obvious zooplankton growth was observed, but a box eNEMURO result indicated that trophic transfer from phytoplankton to zooplankton emerged further away, which provides a possible explanation of Kuroshio Paradox.