Contribution of absorption and scattering to the attenuation of UV and photosynthetically available radiation in Lake Biwa

Abstract

Underwater ultraviolet (UV) attenuation is typically modeled using correlative relationships between UV attenuation and dissolved organic carbon (DOC) concentration or colored dissolved organic matter (CDOM) absorption. Our objective was to validate and extend a biogeooptical model of UV penetration that would take into account total absorption as well as scattering by all components of the water column. Diffuse attenuation coefficients, Kd(λ), for photosynthetically available radiation (PAR) and UV irradiance were measured using a PUV500 radiometer at 13 stations in Lake Biwa, Japan, in late summer. Absorption, a, and scattering, b, were measured at nine PAR wavelengths using an AC‐9 absorption‐attenuation meter. The average Kd(PAR) through the euphotic zone, Kdav(PAR), was estimated from the measured inherent optical properties using Kirk's equations derived from Monte Carlo simulations. The modeled Kdav(PAR) was strongly correlated with measured Kd(PAR) values (r2 = 0.998), although the model systematically underestimated measured values by ~15%. The relative contribution of absorption and scattering to UV attenuation was evaluated from measurements of the absorption coefficients, an extrapolation of b into the UV, and Kirk's equations. The UV‐absorption coefficients for CDOM and particulate matter were measured spectrophotometrically. There was a close agreement between modeled and measured Kd(UV) values (r2 ~ 0.990). The Lake Biwa data showed that particulate absorption and scattering play significant roles in UV attenuation; on average, absorption by water, CDOM, and particles contributed 1, 66, and 21% respectively to Kdav(340), while scattering by particles contributed 11%. At a turbid site (NS9) absorption by water, CDOM, and particles contributed 0.3, 40, and 36% respectively to Kdav(340), while scattering contributed the remaining 24%. The model described here integrates the effects of all absorption as well as scattering components and provides a more accurate estimate of underwater UV exposure than previous approaches based on CDOM absorption only.