This study aims to understand the variability of net ecosystem metabolism in a tropical sea. The contrasting pattern of metabolic state between wet and dry seasons was caused by the pronounced difference of river exports in the Kaoping coastal sea. The depth-integrated gross primary production (IGPP) through the euphotic zone ranged from 2451 to 16 230 mg C-2 day(-1) in summer, and from 844 to 5549 mg C m(-2) day(-1) in winter, and was apparently regulated by oceanic temperature, nutrients and organic carbon. The depth-integrated dark community respiration (IDCR), attributed largely to bacterial respiration (BR, similar to 69%), ranged from 861 to 12 418 mg C m(-2) day(-1) in summer, and from 997 to 5781 mg C m(-2) day(-1) in winter. GPP and DCR correlated inversely with salinity but positively with nutrients, Chlorophyll a, dissolved organic carbon (DOC) and particulate organic carbon (POC). The autotrophic state (IGPP : IDCR > 1) prevailed in most stations in summer, whereas the heterotrophic state (IGPP : IDCR < 1) occurred in all but Station B1 in winter. Bacterial production (BP) and bacterial respiration (BR) also correlated inversely with salinity but positively with nutrients and DOC. Bacterial carbon demand (BCD) was 0.15 GPP in summer and 0.64 GPP in winter, supporting the autotrophic and heterotrophic conditions in summer and winter, respectively. The metabolic state is apparently determined by seasonal variation of temperature and river exports.