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- Title
Occurrence of Surface Phytoplankton Bloom as the Kuroshio Current Passes an Island.
- Authors
Gao, Jie; Guo, Xinyu; Yoshie, Naoki; Ding, Xiaokun
- Abstract
When 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. Plain Language Summary: Kuroshio Current is a western boundary current in the northwestern Pacific Ocean. It has a surface current speed of ∼1 m/s and a width of 100–150 km. There are many islands located in its pathway. When the Kuroshio Current passes around an island, a surface phytoplankton bloom often occurs, and the depth of the subsurface maximum (a layer with the maximum phytoplankton concentration) rises in the wake of the island. The processes causing this uplift explained 50% of the increase in the surface phytoplankton. Biogeochemical processes, highly intensified by the nutrient supply in the upwelling region, contributed the other 50%. The growth of pico‐phytoplankton had the most sensitive response to the nutrient supply in the wake. Surface phytoplankton blooms were weaker in winter than in summer because of the absence of the subsurface maximum. The larger the island, the more notable the phytoplankton bloom. Key Points: A 3D biogeochemical model was used to examine the mechanisms causing biogeochemical enhancement in the island wake of the Kuroshio RegionHydrodynamic processes caused 50% of the surface phytoplankton bloom, and the remainder resulted from nutrient supply to the euphotic zonePico‐phytoplankton responses were the most sensitive to the nutrient supply in the island wake
- Subjects
ALGAL blooms; PACIFIC Ocean currents; KUROSHIO; MARINE zooplankton; GEOSTROPHIC currents; EUPHOTIC zone; ISLANDS
- Publication
Journal of Geophysical Research. Oceans, 2022, Vol 127, Issue 9, p1
- ISSN
2169-9275
- Publication type
Article
- DOI
10.1029/2021JC018242