Solar ultraviolet radiation (UVR, 280-400 nm) in aquatic environments significantly affects photosynthetic carbon fixation by marine phytoplankton. To document such UV-related effects in Chinese waters, we measured in-situ photosynthetic carbon fixation in the coastal surface seawater of the South China Sea under solar radiation in the presence and in the absence of UVR during the winter monsoon period (15 October to 16 December 2005). Phytoplankton biomass (Chl a) ranged from 1.40 to 3.79 mu g l(-1), 75-98% of which was accounted for by piconanoplankton cells ( 20 or <20 mu m) insignificantly interacted to influence the UV-induced inhibition of carbon fixation.