Effects of Typhoon Kaemi on coastal phytoplankton assemblages in the South China Sea, with special reference to the effects of solar UV radiation

In order to see how physical and chemical changes in coastal environment caused by tropical cyclones would affect primary productivity of phytoplankton, we investigated the photosynthetic carbon fixation, the chlorophyll a concentration, and the species composition of phytoplankton assemblages in the surface coastal waters of the South China Sea, during 18 July to 7 August 2006, before and after Typhoon Kaemi's landing (on 25 July, 122 km away from the experimental site). Chlorophyll a concentration started to decrease from 12.3 mu g L-1 30 h before to 1.5 mu g L-1 40 h after its landing, reducing the capacity of photosynthetic carbon fixation in the seawater to one eighth. Prior to the typhoon's arrival, microplankton (> 20 mu m) accounted for 50% of the total chl a, being mainly dominated by the diatom Thalassiosira sp.; during the period 30 h before and 6 days after the typhoon, however, pico-nanoplankton (< 20 mu m) comprised 98% of the total chl a. By 2 weeks after the typhoon, microcells (> 20 mu m) dominated the species again, and occupied 60% of the total chl a, with the diatom Nitzschia sp. being the dominant group. Since the typhoon induced changes in taxonomic structure and environmental factors, the apparent photosynthetic efficiency (alpha) of phytoplankton assemblages was markedly enhanced after the typhoon, while both the light-saturating point (E-k) and noontime photosynthetic rates were significantly reduced. Solar ultraviolet radiation (UVR, > 55 W m(-2)) significantly inhibited the photosynthetic carbon fixation by 17% and 28% before and after the typhoon, respectively, reflecting a higher UVR sensitivity of the posttyphoon phytoplankton assemblages.