Seasonal variations of chlorophyll a and environmental factors in the coastal area of the Okhotsk Sea, Hokkaido

Abstract

　In order to elucidate the mechanism supporting the high productivity of the Okhotsk Sea, seasonal variations of chlorophyll a concentration and environmental factors were investigated off Lake Notoro, a coastal area of the Okhotsk Sea, Hokkaido, from 2011 to 2015. Every year the study area was composed of water influenced by the Soya Warm Current Water in the late spring to late autumn, and by the East Sakhalin Current Water, a cold current, in the late autumn to spring. Nutrient concentrations were low in the warm-current period and high in the cold-current period. Large (>10 μm) phytoplankton blooms were observed from April to May every year. The maximum chlorophyll a concentrations in the blooms were generally 10-30 μg/L; the concentrations in the blooms were up to thirty times higher than the values in non-spring blooms. 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 the late autumn in low-nutrient conditions, whereas phytoplankton were dominated by smaller (<10 μm) ones during the late autumn to pre-bloom in high-nutrient conditions.