The distribution of the size-fractionated chlorophyll a in the subtropical circle area of the northwest Pacific Ocean during the autumn 2002

The size-fractionated chlorophyll a concentration was surveyed in the subtropical circle area of the northwest Pacific Ocean, which is in the area east of Ryukyu Islands chain, Taiwan Island and Luzon Island (15°00′~27°40′N, 121°30′~131°00′E) during the autumn 2002. The results showed that chlorophyll a concentration in the surface water was 0.039~0.271μg/dm~3,with an average value of 0.090±0.047μg/dm~3 in autumn;the chlorophyll a concentration in the northern area east of Islands Chain was higher than that in the southern surveyed area;the vertical distribution of chlorophyll a concentration was rather uniform; the chlorophyll a concentration in the surface water was lower,increasing with depths upon the euphotic layer and decreasing with depth under the euphotic zone until 200 m depth. The size-fractionated chlorophyll a concentration showed that the contribution of picoplankton was predominated (86%), the contributions of nanoplankton and netplankton accounted for 11% and 3% respectively;the abundance of phytoplankton in the surface water was 10~3cell/dm~3, and the small grain size diatoms were predominant.It suggested that the picoplankton was the most important group contributed to the chlorophyll a concentration in the surveyed sea area.