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

Abstract

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) 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.