Abstract
This study was conducted as a first step for using chlorophyll a (chl a) as an environmentalindicator of water quality. The variability of phytoplankton distribution from bi-weekly to seasonal scales in the ocean south of Honshu Island was analyzed in relation to temporal changes of the Kuroshio Current. Sets of 5-year data (1998 to 2002) including chl a data from the ocean color sensor, SeaWiFS, sea surface temperature (SST) from the NOAA/Advanced Very High Resolution Radiometer (AVHRR), and digitized Kuroshio axis data from the Japan Hydrographic Association's Marine Information Research Center were used in the study area (30-39°N, 130-142°E). It was found that the Kuroshio axis was the strongest factor in altering chl a distribution. To the north of the Kuroshio axis there is a high chl a concentration region, with a low chl a region to the south. High chl a immediately north of the Kuroshio axis continues from 2 weeks to 1 month, and its horizontal scale is 150-450km. The high chl a region associated with the meandering S-shaped path of the Kuroshio, where it overlaps the region of low SST, is centered around 33°N, 138°E ; it is 140 km in diameter and lasts 2 weeks. Around the Izu Islands the so-called 'island mass effect' creates a high chl a region which lasts for 2 weeks to 3 months ; its scale is 5 to 30km in diameter. On the longer time-scale variability of chl a, two different patterns of seasonal variability were observed in the regions north and south of the Kuroshio axis.
