Summer variability of UVR effects on phytoplankton primary production in a tropical coastal area of the Southern China Sea

KUNSHAN GAO,; GANG LI,; WALTER HELBLING, E; VIRGINIA ESTELA VILLAFAÑE

Puerto Rico

Congreso; 33rd Annual Meeting of the American Society for Photobiology; 2006

During summer (June to September) 2005 we carried out simulated in situ experiments to determinethe effects of solar UV radiation (UVR, 280-400 nm) on primary productivity of natural phytoplanktonassemblages in the coastal water (23o 24 ,117o 07 ) of the Southern China Sea. Surface sea waterwas collected and exposed to solar radiation for 2-3 hours under 3 radiation treatments (i.e. PAB,280-700 nm; PA, 320-700 nm and P, 400-700 nm) at 7 levels of irradiance (using neutral densityscreens) to obtain photosynthesis versus irradiance (P-E) curves. Periods of high phytoplanktonbiomass were associated with the presence of tropical storms (i.e., heavy rain), during whichstratification in the water column allowed for the development of a bloom (>20 g chl-a L-1)dominated by the diatom Skeletonema costatum; the rest of the time, natural assemblages weredominated by monads / flagellates. During the study period UVR inhibition was variable, but itreached values as high as 50%. Carbon fixation during cloudy days was higher in samples receivingUVR than in those under PAR-only treatment, indicating the use of short wavelength energy forphotosynthesis. On the other hand, primary production was significantly reduced by UVR duringsunny days. We also found differences in responses depending on the size structure of theassemblages: Photosynthetic inhibition of piconanoplankton cells (<20 m) was higher than that inmicroplankton (> 20 m) suggesting that the smaller cells are more sensitive to solar UVR thanlarger ones.