Phytoplankton are more tolerant to UV than bacteria and viruses in the northern South China Sea

In late summer of 2005, the effects of ultraviolet radiation (UVR) on primary production (PP), bacterial production (BP) and viral decay rates (VDR) were investigated along a salinity gradient in the northern South China Sea. The freshwater input increased the UVA diffuse attenuation coefficient (up to 2.7 m(-1)) near the Pearl River estuary and consequently influenced the UVR inhibitory effects, as VDR was significantly correlated with the UVA diffuse attenuation coefficient. UVR inhibition of PP was significantly higher in upwelled waters than downwelled waters, suggesting that the vertical mixing (i.e. upwelling and downwelling) was an important factor regulating microbial sensitivity to UVR. UVR inhibited PP and BP by similar to 15 and 25%, respectively, and the UVR inhibition of BP was significantly higher than that of PP in most of the samples; hence there was a competitive advantage for phytoplankton over bacteria under the relatively high UVR exposure. UVR increased VDR by similar to 30%, which decreased bacterial mortality by 0.12 to 0.3% h(-1) and mitigated the inhibitory effects of UVR on bacteria. In general, phytoplankton were more tolerant than bacteria and viruses to UVR in the sub-tropical northern South China Sea.