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

Abstract

Temporal and spatial changes in nutrient and chlorophyll distributions caused by frontal disturbances of the Kuroshio, a western boundary current in the Pacific Ocean, are presented. The regions of high nitrate and phosphate concentrations are associated with an eddy of positive vorticity which separates from the Kuroshio front within a few days of generation. The nutrients supplied to the euphotic layer by upwelling and mixing in the cyclonic eddy accelerate primary production in the frontal region. The specific growth rate calculated from temporal changes in chlorophyll and nitrate concentrations, of 0.8 d-1 is considerably larger in the offshore region. According to a numerical simulation made using a turbulent closure model a chlorophyll maximum occurs during the fifth day after the generation of the eddy and the growth rate in the euphotic layer is in good agreement with that estimated from observations. After the peak production the growth becomes nutrient limited. Since the frontal disturbances occur in associated with short-term fluctuations in the Kuroshio frontal meander, which has a period of a few weeks, the biological production enhancement by this kind of eddy is expected to occur with similar frequency. It is estimated that the total annual nitrogen input to the region via the eddies could result in a carbon production rate of 40 gC m-2 y-1.