Chlorophyll a and photosynthetic carbon fixation in a tropical coral reef bay (Sanya Bay) of the northern South China Sea, with special reference to the effects of solar ultraviolet radiation B.

Sanya Bay is a typical tropical coral reef bay with high water transparency. Solar Ultraviolet Radiation B (UVB) radiation exposure affects phytoplankton primary production, but the understanding of its underlying mechanism is still limited. This study investigated the responses of spatial and seasonal changes of chlorophyll a (Chl a) and phytoplankton photosynthetic carbon fixation to solar UVB in Sanya Bay in 2011. Results showed that Chl a was the highest in autumn and lowest in spring. Surface phytoplankton photosynthetic carbon fixation (SPP) was higher in winter and lower in summer, while water column integrated primary production (IPP) showed little variation among seasons except spring. Overall, Chl a, SPP and IPP all decreased with increasing distance from the shore. UVB inhibited both SPP and IPP throughout the year, with the inhibition ratios (IR) ranging from 32.01 % to 38.88 %. Chl a, SPP and IPP could be affected by the interaction between UVB, nutrient levels, water column stability and light availability. The study highlighted the importance of long-term observation to establish a more comprehensive understanding of the driving mechanism of Chl a and primary productivity in Sanya Bay.