Penetration of ultraviolet radiation in streams of eastern Pennsylvania: Topographic controls and the role of suspended particulates

Abstract.

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 (K_d380: 0.68 – 151.1 m⁻¹, K_d320: 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:K_d320, 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.