ABSTRACT: We present a simple method for describing the influence of variable attenuation of spectral irradiance, K_d(λ), on water-column photosynthesis and its inhibition by ultraviolet radiation (UV). Theapproach is based on weighted water transparency, a calculation introduced by Vincent et al. 1998 (Ann Glaciol 27:691-696). Key results of a depth- and spectrally-resolved model of photosynthesis can be reproduced for a broad range of water types bysimple parameterizations using a reference solar irradiance spectrum at the surface and water transparency [1/K_d(λ)] weighted spectrally for biological effectiveness. Transparency that has been weightedspectrally by the normalized product of irradiance and photosynthetic absorption (PUR-weighted transparency, TW_PUR) describes spectral effects on photosynthesis in the water column. An empirical parameterization oftransparency weighted by the product of surface irradiance and the biological weighting function for inhibition of photosynthesis (TW_PIR), along with TW_PUR, describes the inhibition of water-columnphotosynthesis relative to the uninhibited rate. Our approach is directly compared with an analysis that used weighted transparency as an indicator of the potential for inhibition of photosynthesis by UV as influenced by variations in chromophoricdissolved organic matter associated with climate change over the past 6000 yr (Pienitz & Vincent 2000, Nature 404:484-487). Results demonstrate how weighted transparency, used as an indicator of potential inhibition, can be transformed into a predictor ofbiological effects.