Penetration of ultraviolet radiation in streams of eastern Pennsylvania: Topographic controls and the role of suspended particulates
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Penetration of ultraviolet radiation (UVR) in stream ecosystems is determined by the concentration and optical properties of suspended sediment and dissolved organic carbon (DOC). This study documents the base-flow optical environment of 37 first- and second-order tributaries distributed throughout the Lehigh River watershed, eastern Pennsylvania, over a four year period. We measured a large range of attenuation coefficients (Kd380: 0.68 – 151.1 m−1, Kd320: 0.95 – 316.2 m−1) and 1 % transmission depths (2 cm – 147 cm). In addition, we quantified the significance of particulate material in UVR attenuation in streams, which generally accounted for 10–30 % of attenuation for the UV-B waveband. Our results indicated that basin morphology, particularly mean watershed slope (MWS), was highly correlated with UVR penetration (MWS:Kd320, r2 = 0.68, P < 0.0001),DOC concentration (MWS:DOC, r2=0.65, P < 0.0001), and DOC optical quality (MWS:Fluorescence Index, r2 = 0.71, P < 0.0001). The fact that these relationships are robust across a variety of watersheds that differ in land use, forest coverage, and wetland coverage, indicates that the geomorphic coevolution of hillslope form and process exert a strong control on stream optical environments via the establishment of hydrologic and edaphic conditions. Agricultural land use exerts secondary, but discernable effects on DOC concentration (% Agriculture:DOC, r2 = 0.39, P = 0.012) and optical quality (% Agriculture:Fluorescence Index, r2 = 0.32, P = 0.036) in watersheds devoid of wetlands.
Keywords.Ultraviolet radiation dissolved organic carbon geomorphology land use
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