Cell Size-Dependent Effects of Solar UV Radiation on Primary Production in Coastal Waters

Abstract In order to examine the effects of solar ultraviolet radiation (UVR, 280–400 nm) on photosynthesis of differ-ently cell-sized phytoplankton, natural phytoplankton assemblages from the coastal waters of the South China Sea were separated into three groups (>20, 5–20, and <5 μm) and exposed to four different solar UV spectral regimes, i.e., 280–700 nm (PAR+UVR), 400–700 nm (PAR), 280–400 nm (UV-A+B), and 315–400 nm (UV-A). In situ carbon fixation measurements revealed that micro-plankton (>20 μm) efficiently utilized UV-A for photosyn-thetic carbon fixation, with assimilation number of up to 1.01 μg C (μg chl a)−1h−1 under 21.4 Wm−2 UV-A alone (about half of noontime irradiance at the surface), about 40 % higher than nanoplankton (5–20 μm). UV-B (280– 315 nm) of 0.95 Wm−2 reduced the carbon fixation by approximately 20 and 57 % in microplankton and nano-plankton assemblages, respectively. In contrast, smaller picoplankton (<5 μm) was unable to utilize UV-A for the photosynthetic carbon fixation. In addition, only micro-sized assemblages demonstrated the UV enhancement on their primary productivity in the presence of PAR, by about 8 % under moderate intensities of solar radiation