Abstract
This study was carried out off Shibetsu in the Nemuro Strait from 2009 to 2013, to elucidate the seasonal variations of temperature, salinity, nutrients, and chlorophyll a. Every year, the study area was composed of water influenced by the Soya Warm Current Water in late spring to late fall, and by the East Sakhalin Current Water and Oyashio Water, cold currents, in late fall to spring. Nutrient concentrations were low in the warm-current period and high in the cold-current period. Large（＞10 μ m）diatom blooms were observed from February to May every year. The maximum chlorophyll a concentration in the blooms over the five years was 27μg L－1, although the maximum chlorophyll a concentrations in the blooms differed among various
locations and years. Remarkable decreases were found in nutrient concentrations with the progress of the spring bloom ; nitrate was considered to be a limiting nutrient in the spring bloom. Large phytoplankton were dominant during the post-bloom to late fall in low-nutrient conditions, whereas phytoplankton were smaller（＜10μ m）during late fall to pre-bloom in high-nutrient en phytoplankton and nutrients.
