Chlorophyll a (Chl a) concentration and primary productivity (PP), namely, carbon fixed strength of phytoplankton along four transects in the southern Huanghai Sea (SHS) were studied for their distribution features and controlling mechanisms based on the investigations from 17 October to 3 November 2005. The Chl a concentration in the study waters dynamically changed spatially. Surface Chl a concentrations ranged from 0.11 to 2.38 mg/m 3 with higher and lower values observed in the nutrient-laden inshore waters and central part of the SHS occupied by oligotrophic current, respectively. The vertical distribution of Chl a concentration showed a predominant pattern of subsurface concentration maximum profile. It followed the previous result of the deep dissolved oxygen concentration maximum profile, which was significantly correlated with phytoplankton and regional water mass. The primary productivity of carbon in autumn of the SHS, ranging from 95 to 1 634 mg/(m 2 ·d) mainly varied with nutrient condition, especially phosphate concentration in seawater and hydrological condition. Furthermore, associating the present study results together with previous studies, the annual value of carbon fixed production of phytoplankton in the entire marginal seas of East China (including the Bohai Sea, the Huanghai Sea and the East China Sea) was estimated to be 222 Mt, which accounted for 2% of that in the global margins. Besides, it was as 16.2 times as the annual value of apparent carbon sink strength (13.96 Mt) in the marginal seas of East China. This multiple was different in different sea areas (3.0 in the Bohai Sea, 6.7 in the Huanghai Sea and 81.6 in the East China Sea).
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