Effects of seston on ultraviolet attenuation in Lake Biwa

We examined the attenuation of underwater ultraviolet (UV) radiation and photosynthetically available radiation (PAR) in Lake Biwa, Japan, at offshore and inshore sites and under contrasting stratification and mixing regimes. There were large spatial differences in the water column transparency to both wavebands, despite little change in concentrations of dissolved organic carbon (DOC). The 1% of surface irradiance depth varied from 0.3 to 2.7 m at 305 nm, from 0.8 to 6.3 m at 380 nm, and from 2.3 to 12.8 m for PAR. Both PAR and UV transparency declined abruptly in the South Basin of the lake when a typhoon caused the resuspension of sediments. The water column ratio of UV to PAR increased by 30% at all stations over the course of a 3-week sampling period associated with the general increase in phytoplankton concentrations. At several sites, the diffuse attenuation coefficient for UV radiation deviated substantially from that predicted from UV-DOC models. A significantly positive linear relationship was found between UV attenuation (K _d determined with a profiling UV radiometer) and the beam attenuation coefficient at 660 nm as measured by transmissometer. These results indicate that scattering and absorption by particulate matter can reduce UV transparency to below that inferred from DOC concentrations, and that current UV-exposure models should be modified to incorporate this effect. Received: March 21, 2001 / Accepted: August 17, 2001