We measured the attenuation of ultraviolet B (UVB) and ultraviolet A (UVA) radiation in 32 streams located within the Ontonagon River watershed on the Upper Peninsula of Michigan, USA. Attenuation coefficients (Kd) of UVB and UVA ranged widely among these streams, but generally translated into relatively shallow 1% transmission depths into the water column (2–45 cm for UVB and 6–103 cm for UVA). Both Kd UVB and Kd UVA were positively correlated with stream dissolved organic C concentration (DOC, range 2–35 mg C/L). Absorbance coefficients of dissolved matter (ad) of UVB and UVA also were strongly correlated with DOC. Kd UVA (but not Kd UVB) was weakly related to the concentration of particulate organic C and DOC molar absorptivity. DOC-specific Kd UVB was, on average, higher in streams of our study compared to previously published values from lakes and wetlands. We developed a statistical model that predicts UVB flux to benthic organisms. The model incorporates information on water depth, DOC concentration, surface reflectance, and forest canopy cover. This stream-UVB model (SUM) predicts very low UVB flux to the benthic areas of most wetland and forested streams of this region during cloudless, midsummer days. Overall, our results suggest a low likelihood that stream organisms in this region are normally exposed to high levels of ultraviolet radiation because shading is provided by both stream DOC and forest canopy.
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