To explore planktonic and ecosystem responses to thermal effluents of a power plant, three transect surveys were conducted in Nov-01', May-02' and Jun-02' at the bay adjacent to the outlet of Taiwan Nuclear Power Plant II. At the given station, seasonal trends were evident with most maximal measurements observed in Jun-02'. Physical mixing between background seawater and thermal effluents played an important role in determining planktonic biomass since chlorophyll (Chl, < 0.15 - 1.27 mgChl m^-3 ) and bacterial biomass (BB, 11 - 48 mgC m^-3 ) increased almost linearly seaward. Temperature (20 - 45°C) manipulation experiments suggested that phytoplankton were more vulnerable than heterotrophs to thermal stress. Differential temperature responses of auto- and heterotrophs result in primary production (PP, < 1 - 100 mgC m^-3 d^-1 ) increasing seaward, while community respiration (CR, 15 - 68 mgC m^-3 d^-1 ) and bacterial growth rate (BGR, 0.03 - 0.9 d^-1 ) showed opposite trends. The plume system was heterotrophic (PP/CR ratio < 1) in areas with bottom depths ca. < 10 m, and then switched to autotrophic status (PP/CR ratio > 1 - 3.7) in deeper regions. High observed dissolved organic carbon (DOC) anomaly (23 – 34 gC m^-3 ) implied that heterotrophic metabolism was seldom limited by bottom- up control processes. Short-term manipulation experiments showing that BGR and CR increased with rising temperature up to ca. 37°C, which was ~12°C higher than frequently reported values from most coastal and estuarine ecosystems. We ascribed this to the effects of temperature-substrate interaction. The results of organic carbon (zooplankton extract) ad dition experiments suggested a certain fraction of the in situ DOC was as labile as animal tissue since the increasing trends of BGR in the enriched and control treatments behaved similarly. From a carbon cycling perspective, the positive temperature responses of heterotrophic activities imply that in coastal systems with a high loading of anthropogenic DOC, the biogenic emission rate of CO₂ might increase exponentially as global temperatures rise.