Abstract
Size-fractionated chlorophyll a and primary productivity were measured in the coastal area of the Shiretoko Peninsula in May to June（late spring to early summer）and in August and Septembe（r late summer to early autumn），２００７－２００９．In the late spring to early summer, large size phytoplankton（＞１０μm）contributed to total chlorophyll a concentration in the Okhotsk Sea side of the Shiretoko Peninsula（western side），whereas small size phytoplankton（＜２μm）were more common in the Nemuro Straight side of the peninsula（eastern side）．In contrast, maximum chlorophyll a-specific primary productivity（an index of phytoplankton
growth rate）was observed in the large size phytoplankton on the western side, and large or middle（２－１０μm）sized phytoplankton were found on the eastern side, implying that they are important primary producers．In the late summer and early autumn, the large size phytoplankton contributed to total chlorophyll a concentration on the western side, whereas large or small size phytoplankton were found on the eastern side．However, maximum chlorophyll a-specific primary productivity was observed in the middle size phytoplankton on the western and eastern sides, implying that they are important primary producers．The results suggest that important primary producers are not different between both sides of the peninsula, but change seasonally．
