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Variation in UV irradiance related to stratospheric ozone levels affects photosynthetic carbon fixation of winter phytoplankton assemblages from surface coastal water of the South China Sea

Abstract Solar ultraviolet radiation (UVR, 280–400 nm) in aquatic environments significantly affects photosynthetic carbon fixation by marine phytoplankton. To document such UV-related effects in Chinese waters, we measured in-situ photosynthetic carbon fixation in the coastal surface seawater of the South China Sea under solar radiation in the presence and in the absence of UVR during the winter monsoon period (15 October to 16 December 2005). Phytoplankton biomass (Chl a) ranged from 1.40 to 3.79 µg l−1, 75–98% of which was accounted for by piconanoplankton cells (<20 µm). The photosynthetic carbon fixation obtained under photosynthetically active radiation (PAR) at noon varied from 5.0 to 37.8 µg C l−1 h−1, whereas the carbon fixation rate ranged from 2.36 to 9.98 µg C (µg Chl a)−1 h−1. UV-A and UV-B significantly lowered the photosynthetic rates by up to 38% and 29%, respectively. Reduced levels of UV-A on cloudy days resulted in enhanced photosynthetic carbon fixation by up to 6.6%. The UV-A-induced inhibition of photosynthetic carbon fixation by phytoplankton assemblages in the coastal water was negatively correlated with the temperature levels but positively related to pH values. Stratospheric ozone variation affected the UV-B/PAR ratio, which then influenced the UV-B-induced inhibition of phytoplankton photosynthesis: a higher UV-B/PAR ratio led to higher UV-B-related inhibition. Changes in salinity, PAR and cell-fractions (>20 or <20 µm) insignificantly interacted to influence the UV-induced inhibition of carbon fixation.