Estimating attenuation of ultraviolet radiation in streams: field and laboratory methods.

We adapted and tested a laboratory quantitative filter pad method and field-based microcosm method for estimating diffuse attenuation coefficients (K(d)) of ultraviolet radiation (UVR) for a wide range of stream optical environments (K(d320) = 3-44 m(-1)). Logistical difficulties of direct measurements of UVR attenuation have inhibited widespread monitoring of this important parameter in streams. Suspended sediment concentrations were manipulated in a microcosm, which was used to obtain direct measurements of diffuse attenuation. Dissolved and particulate absorption measurements of samples from the microcosm experiments were used to calibrate the laboratory method. Conditions sampled cover a range of suspended sediment (0-50 mg L(-1)) and dissolved organic carbon concentrations (1-4 mg L(-1)). We evaluated four models for precision and reproducibility in calculating particulate absorption and the optimal model was used in an empirical approach to estimate diffuse attenuation coefficients from total absorption coefficients. We field-tested the laboratory method by comparing laboratory-estimated and field-measured diffuse attenuation coefficients for seven sites on the main stem and 10 tributaries of the Lehigh River, eastern Pennsylvania, USA. The laboratory-based method described here affords widespread application, which will further our understanding of how stream optical environments vary spatially and temporally and consequently influence ecological processes in streams.