[A-OS13] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS13-02] Characteristics of Water Masses and Nutrient Properties in the southern Sea of Okhotsk

*西岡 純¹、豊田 威信¹、山下 洋平²、鈴木 光次²、村山 愛子¹、今井 望百花³、中村 知裕¹、大島 慶一郎¹、三寺 史夫¹ (1.北海道大学低温科学研究所、2.北海道大学地球環境科学研究院、3.北海道大学環境科学院)

キーワード：オホーツク海、栄養塩、季節サイクル、水塊構造

Although several previous studies reported the observed results of nutrient concentrations in the southern Sea of Okhotsk, a seasonal sea-ice area, the seasonal variation of nutrient concentration throughout the year and its controlling mechanisms have little been elucidated yet. To understand the relationship between the seasonal variations of water mass structure and nutrient conditions in the seasonal sea-ice area, a series of shipboard-monitoring campaigns was established in the southern Sea of Okhotsk in spring (April), late summer (September), late fall (December), and mid-winter (February). We also analyzed biogeochemical parameters, such as dissolved oxygen concentration, oxygen isotopes in water, and chlorophyll-a, with water mass structure in the study area. We compiled all data from a series of shipboard-monitoring campaigns in this study. The biogeochemical data analyses from these observations reveal controlling mechanisms of the seasonal variation of nutrient concentration in the southern Sea of Okhotsk. Nutrient concentrations in surface seawater under sea ice in mid-winter were the highest throughout the year. During the spring phytoplankton bloom, the lowest nitrate concentrations were observed in the Surface Low Salinity Water (SLSW) (i.e., nearly depleted), whereas silicate was abundant in the surface layer. The deep mixing of water masses before the arrival of sea ice could be a critical process that increases nutrient concentration in offshore waters near the Kuril Basin. The processes of constructing an Intermediate Cold Water (ICW), which originated from the East Sakhalin Current and mixed with deeper waters in winter and the SLSW in spring, have a crucial role in the setup of high nutrient conditions for producing massive spring phytoplankton bloom in this area.