| We carried out experiments to evaluate short-term impact of UVR on photosynthesis of phytoplankton assemblages from coastal waters off SE China. Sea surface samples were taken and exposed to different radiation treatments with the presence or absence of PAR or UVR using sharp cut-off filters. Three different solar radiation treatments (i.e. PAB, 280-700 nm; PA, 320-700 nm; P, 400-700 nm) or combined with three nutrient concentrations (CO2 or nitrate) were implemented during March 2004 - Sep 2005. In addition, during Jul to Sep 2005, phytoplankton was exposed to 1) UVR only (UG-11 filter) and UV-A only (UG-11 + Folex320 filter), 2) solar radiation above 280, 295, 305, 320, 350 and 395nm (i.e. determine wavelength dependent inhibition), 3). Attenuated solar radiation with or without UVR to determine P-E curves. Solar UVR significantly inhibited photosynthetic rates of phytoplankton assemblages during spring and winter, with UV-A accounting for most of the observed inhibition; very little or no UVR-induced inhibition was observed during summer. Assimilation numbers during summer were significantly higher than those in spring and winter, with mean values of samples exposed to PAR of 7.62 (summer), 2.83 (spring), and 4.35μg C (ug chl a)-1 h-1 (winter), respectively. Experiments conducted with samples receiving only UVR or UV-A radiation (i.e., no PAR) and attenuated solar radiation confirmed the previous findings that tropical phytoplankton assemblages can use UV-A as source of energy for photosynthesis. In fact, the amount of carbon fixed under UV-A-only treatment was as much as 0.50μg C (ug chl a)-1 h-1. The response of phytoplankton assemblages to UVR was wavelength dependent, the shorter wavelength, the higher inhibition on photosynthesis. The seasonal variation on the impact of UVR on phytoplankton carbon fixation could be attributed to differences in vertical mixing and stratification and in species composition.
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