Response of natural phytoplankton assemblages to solar ultraviolet radiation (UV-B) in the coastal water, Japan

Variability in the effect of solar ultraviolet radiation (UV-B) on primary productivity of natural phytoplankton assemblages was examined in coastal water off Manazuru Harbor, Sagami Bay, central Japan for two full years during the period from September 1996 to September 1998. Solar UV-B, UV-A, and PAR were determined in air and water. Surface water was exposed to UV-B+UV-A+PAR and UV-A+PAR with Mylar film and primary productivity under those two light regimes was determined with C-13 method. Size distribution of chlorophyll a concentration was also examined. When the occurrence of > 10 mum size fraction was lower than 80% in a warm season, the UV-B effect was significant. Whenever red tide species occurred, they did not respond to UV-B radiation. Although winter population was also dominated by > 10 mum size fraction, a ratio of UV-B to UV-A+PAR was already so low that the UV-B effect was insignificant. The occurrence of > 10 mum size fraction, which might be enhanced by a supply of nitrate and consequently could be related to high supply of dissolved organic matter, seemed to play a significant role in controlling the effect of UV-B on primary productivity in the coastal water. Reduction rate of primary productivity ranged from <10% from November to April to 41% at maximum from May to October with annual reduction rate of 10% in the present study.