Attenuation of ultraviolet radiation in a large lake with low dissolved organic matter concentrations

Solar radiation spectra were measured in mesotrophic Lake Erie in 1997 to d etermine the principal factors influencing ultraviolet radiation (UVR) atte nuation. Water clarity ranged widely, with the diffuse vertical attenuation coefficient for photosynthetically active radiation (K(dI)PAR) varying fro m 0.26 to 17.2 m(-1), and UVR attenuation varied positively and significant ly with K(dI)PAR. Particle concentration (total suspended solids, TSS) was the single most useful water quality characteristic for predicting broadban d UVB (305-320 nm) and UVA (320-400 nm) attenuation.. Dissolved organic car bon concentration (DOC) and fluorescence were much less important than TSS in predicting UVR attenuation over the typical range of water clarity (K(dI )PAR < 2 m(-1)). The predominant role of TSS in UVR attenuation could not b e explained by phytoplankton or other absorbing organic matter in the parti culate phase, suggesting instead an important role for scattering. Attenuat ion spectra through the UV region were not monotonic at the higher particle concentrations and could not be modelled solely as a function of DOC as pr eviously reported for other lakes. Attenuation at 310 and 320 nm confirmed the conclusion from spectrally integrated measurements that particles are t he dominant agents of variable UVR attenuation in Lake Erie.