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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.
Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark
Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark
Acknowledgements
The authors would like to thank Dr David J.S. Montagnes and two anonymous referees for handling this manuscript and the insightful comments. This work was supported by the National Basic Research Program of China (No. 2009CB421207), the Changjiang Scholars and Innovative Research Team Program (IRT0941), the Special Research Fund for the National Non-profit Institutes (No. 2008M15), the National Natural Science Foundation (No. 40930846, No. 40876058) and the MEL Visiting Fellowship Program (MELRS1006). We are also grateful to Weizhou Chen of Nan'Ao Marine Biology Station of Shantou University for his helpful assistance and to Professor John Hodgkiss for polishing the English in this manuscript.
Notes
Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark