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 determine the principal factors influencing ultraviolet radiation (UVR) attenuation. Water clarity ranged widely, with the diffuse vertical attenuation coefficient for photosynthetically active radiation (K(dI)PAR) varying from 0.26 to 17.2 m(-1), and UVR attenuation varied positively and significantly with K(dI)PAR. Particle concentration (total suspended solids, TSS) was the single most useful water quality characteristic for predicting broadband UVB (305-320 nm) and UVA (320-400 nm) attenuation.. Dissolved organic carbon 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 be explained by phytoplankton or other absorbing organic matter in the particulate phase, suggesting instead an important role for scattering. Attenuation 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 previously reported for other lakes. Attenuation at 310 and 320 nm confirmed the conclusion from spectrally integrated measurements that particles are the dominant agents of variable UVR attenuation in Lake Erie.