A case study of system and planktonic responses in a subtropical warm plume receiving thermal effluents from a power plant

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 H. 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 degrees 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 degrees C, which was similar to 12 degrees 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) addition, 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.