Vertical attenuation of underwater ultraviolet radiation [UVR; UVA (320-400 nm) and UVB (280-315 nm) together with photo-synthetically available radiation (PAR, 400-700 nm)] was measured at 22 sites from 5 freshwater bodies of various trophic states. The factors controlling the attenuation of UVR were examined. At all the sites, underwater irradiances decreased alsmot exponentially with depth and were more attenuated in dimension order from UVB, UVA, to PAR. Substantial variation in attenuation was observed among water bodies. Attenuation depths (Z1% for UVB ranged from 0.3 m to 1 m in the eutrophic sites, while exceeding 2 m in oligo-mesotrophic ones. Z1% of UVB and that of UVA were 0.5 and 0.9 times the Secchi disk depth, respectively. Attenuation coefficients (m-1) of UVA and UVB showed significantly positive linear correlations with both chlorophyll-a and particulate organic carbon concentrations. Dissolved organic carbon (DOC) was weakly correlated with attenuation coefficients of UVA and UVB. This disagrees with previous reports that stated DOC is the major factor controlling underwater UVR. Phytoplankton is considered to be the more important factor controlling the attenuation of underwater UVR in water bodies with high phytoplankton content and low DOC.
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