Variability of UVR effects on photosynthesis of summer phytoplankton assemblages from a tropical coastal area of the South China Sea.

Abstract

From June to September 2005, we carried out experiments todetermine the ultraviolet radiation (UVR) -induced photoinhibitionof summer phytoplankton assemblages from a coastal site ofthe South China Sea. Variability in taxonomic composition wasdetermined throughout the summer, with a peak chlorophyll a(chl a20 lg chl a L)1) dominated by the diatom Skeletonemacostatum that was detected early in the study period; the rest ofthe time samples were characterized by monads and flagellates,with low chl a values (1–5 chl a lg L)1). Surface water sampleswere placed in quartz tubes, inoculated with radiocarbon andexposed to solar radiation for 2–3 h to determine photosyntheticrates under three quality radiation treatments (i.e. PAB, 280–700 nm; PA, 320–700 nm and P, 400–700 nm) using differentfilters and under seven levels of ambient irradiance using neutraldensity screens (P vs E curves). UVR inhibition of samplesexposed to maximum irradiance (i.e. at the surface) varied from)12.2% to 50%, while the daytime-integrated UVR-relatedphotoinhibition in surface seawater varied from )62% to 7%.The effects of UVR on the photosynthetic parameters PBmax andEk were also variable, but UV-B accounted for most of theobserved variability. During sunny days, photosynthesis ofmicroplankton (>20 lm) and piconanoplankton (<20 lm) weresignificantly inhibited by UVR (mostly by UV-B). However,during cloudy days, while piconanoplankton cells were stillinhibited by UVR, microplankton cells used UVR (mostlyUV-A) as the source of energy for photosynthesis, resulting inhigher carbon fixation in samples exposed to UVR than the onesexposed only to photosynthetically active radiation (PAR). Ourresults indicate that size structure and cloudiness clearlycondition the overall impact of UVR on phytoplankton photosynthesisin this tropical site of South China. In addition, modelpredictions for this area considering only PAR for primaryproduction might have underestimated carbon fixation due toUVR contribution.