VARIABILITY IN TIMING AND MAGNITUDE OF SPRING BLOOM IN THE OYASHIO REGION, THE WESTERN SUB-ARCTIC PACIFIC OFF HOKKAIDO, JAPAN

The spring bloom of phytoplankton is a well-established, regular, seas onal event in the western subarctic Pacific and is considered one of t he most important conditions of massive production of pelagic fishes. A series of 12 cruises was conducted from 1990 to 1992 to examine the timing and magnitude of the spring phytoplankton bloom in the Oyashio region, the western subarctic Pacific off Hokkaido, Japan. An interann ual variability in the bloom events was also analysed. On the basis of hydrographical characteristics, the study area was divided into three water masses: the Oyashio Water Mass, the Mixed Water Mass, and the C oastal Water Mass. Spring blooms were observed first in April in the O yashio and the Coastal Water Masses, and continued to May in 1991 and 1992. However, no bloom was recorded in the Mixed Water Mass. High nut rient supply into the surface mixed layer during winter is likely to b e one of the factors supporting an intense spring bloom in the Oyashio Water Mass. A significant positive relationship between log-transform ed surface chlorophyll a concentration and maximum density gradient (M DG) within the euphotic layer was obtained in April, indicating the im portance of vertical stability of the water column in the initiation o f spring blooms in the Oyashio and the Coastal Water Masses. The sprin g blooms in 1991 were much more extensive and lasted longer than in 19 90. It is suggested that meteorological conditions and abundance of gr azers were responsible for this interannual difference.