Diatom to dinoflagellate shift in the summer phytoplankton community in a bay impacted by nuclear power plant thermal effluent

Understanding how nuclear power plant thermal effluents influence the phytoplankton community may provide insights into the potential ecological consequences of global warming. In the present study, long-term trends in the phytoplankton community structure under the influence of nuclear power plant thermal effluent in the subtropical Daya Bay (DYB) in China were investigated in the summer season from 1982 to 2005. Water temperature at the outfall station was significantly higher than in the surrounding water, by as much as 5.6 degrees C, and increased by 6.8 degrees C during the 23 yr study period. The contribution of diatoms and dinoflagellates to the total phytoplankton showed significant correlation with temperature (R-2 > 0.65), negative for diatoms, while positive for dinoflagellates. Although dinoflagellate abundance increased over time at both the outfall and adjacent (control) stations, the increase at the outfall station was much more dramatic and accelerated over time. No clear relationship between the phytoplankton shift and stratification was evident. When water temperature reached 35 degrees C or > d 3.7 degrees C above that at the control station, dinoflagellates, such as Ceratium furca, C. fusus, C. trichoceros, Dinophysis caudate and Protoperidinium depressum, grew to prominence, accounting for about 50% of the total phytoplankton abundance. On the contrary, the diatom contribution decreased during the study period, from 82.0% in 1982 to 53.1% in 2005. These results suggest that the rise in temperature caused by power plant thermal discharge has imposed strong influences on the phytoplankton community, favoring dinoflagellates over diatoms, with a remarkable diatom to dinoflagellate shift when temperature increases to a threshold level of 35 degrees C or reaches a threshold differential of 3.7 degrees C relative to the normal ambient temperature in DYB.