Biological production in meso-scale eddies caused by frontal disturbances of the Kuroshio Extension

Abstract

Frontal disturbances of the Kuroshio and the Kuroshio Extension in the Pacific Ocean cause meso-scale eddies which contribute to accelerating local biological production. Temporal and spatial changes in nutrient and phytoplankton biomass observed in the vicinity of the meso-scale eddies in several recent research cruises revealed upwelling high primary production offshore, and suggested an uptake process by phytoplankton. Here we propose a newly developed “Upwelling–Production Diagram” for evaluating nutrient consumption in relation to primary production, and show that this diagram would be one of the more promising ways of estimating temporal changes in the biological production process from Eulerian observational data. In the Kuroshio Extension, according to observations and numerical simulations, maximum chlorophyll-a concentration in the eddy reached 3–4 μg l−1 in 3–4 d, which is of the same order as the estimates found in the Kuroshio itself. This indicates that primary production caused by frontal eddies is a phenomenon occuring in both the Kuroshio and the Kuroshio Extension. In addition, the density of copepod nauplii increased with chlorophyll-a concentration in the eddy, and large numbers of eggs and larvae of anchovy were recognized at stations where the copepods were highly concentrated. It is difficult to explain how this physical phenomenon directly affects larval survival in the short period of one week, but this correspondence strongly indicates that frontal disturbances contribute not only to primary production, but also to fish reproduction.