Observation of thermohaline structure and phytoplankton biomass in the shelf front of East China Sea during early summer

東シナ海陸棚域の横断面観測を1987年6月に北緯30°40'線に沿って実施した結果、黄海冷水系の低温沿岸水塊と対馬暖流系の高温高塩分水塊が形成する顕著な不連続構造を見出した。この不連続構造は温度躍層と同時に密度躍層を伴っており、躍層の深度は中国陸岸寄りで浅くなった。また、この躍層近傍では周辺よりも高いクロロフィルa濃度と植物プランクトン、特に鞭毛藻類の高いバイオマスが観測された。さらに、そこでは溶存酸素が過飽和であったことから活発な光合成作用が示唆された。溶存無機態リンの濃度は表屠で非常に低く、一方底層では高かった。不連続面付近で植物プランクトンバイオマスが高い理由として、不速続面下部からの連続的な栄養塩の供給と、上層からの水中日射エネルギーの供給が同時に成立することおよび密度成層の存在があげられた。

Observations were carried out on oceanographic conditions and phytoplankton biomass in the shelf front region of the East China Sea during early summer in 1987 along the 31°40' N latitude line. As a result of water mass analysis on T-S diagram, two characteristic water masses were identified. Those were cold coastal water mass originated from Yellow Sea and warm water mass originated from Kuroshio water, between them internal discontinuous layer (thermocline and pycnocline) was formed at intermediate depth. Phytoplankton biomass, phytoflagellates biomass in particular, and concentration of chlorophyll a were high in the vicinity of the discontinuous layer. Dissolved oxygen was supersaturated in the chlorophyll rich water which indicated active photosynthesis. Nutrient in terms of dissolved inorganic phosphorus was almost completely depleted in the surface water but still remained in the deeper water below the high chlorophyll layer. The possible reason for the high phytoplankton biomass present in the layer may be due to continuous supply of nutrients from bottom water, availability of underwater irradiance and density gradient formed in the layer.