日本周辺海域における透明度の季節変動と臨界深度との関係

抄録

Aim of this paper is to reveal major causes of seasonal variability of phytoplankton biomass in the seas adjacent to Japan by relating changes of transparency to those of environmental factors. Generally, phytoplankton biomass begins to increase when the Sverdrup's critical depth exceeds the upper mixed layer depth in spring, and decreases in summer with development of the seasonal pycnocline. After the summer minimum, it increases temporally in early autumn when an upper mixed layer becomes deeper and yet shallower than the critical depth. Critical depth model is applicable to waters in the transitional/subarctic Pacific and the Japan Sea. On the other hand, in the Pacific off the southern midcoast of Japan and the East China Sea, seasonal variability of phytoplankton biomass is not fully explained by the critical depth model. These waters are characterized by low concentration and no seasonality of nutrients. Significant negative correlations were observed between surface nutrient concentrations and transparency. These facts indicate that the physical conditions such as critical depths, mixed layer depths and solar irradiance as well and the chemical condition like nutrients availability are responsible for the seasonal variability and annual level of phytoplankton biomass in the seas adjacent to Japan.