STRONG GEOMORPHIC CONTROLS ON STREAM OPTICAL ENVIRONMENTS IN EASTERN PENNSYLVANIA

Ultraviolet radiation (UVR) has profound implications for stream ecosystems, influencing processes that range from microbial metabolism to fish reproduction and development. Penetration of UVR is determined by the concentration and optical properties of dissolved organic carbon (DOC) and suspended sediment, both of which are ultimately sourced from contributory hillslopes. Here, we document the optical environment of 37 first and second order tributaries distributed throughout the Lehigh River watershed, eastern Pennsylvania, over a four year study. Our results are among the first to demonstrate that basin geomorphology, particularly mean watershed slope (MWS) is highly correlated with measured UVR penetration (MWS:K_d320 , r ² = 0.68, P < 0.0001). MWS is correlated with the concentration (MWS:DOC, r² = 0.65, P < 0.0001) as well as the optical quality (MWS:Fluorescence Index, r² = 0.71, P < 0.0001) of DOC. MWS is also correlated to the proportion of UVR attenuation that is accounted for by particles (MWS:a_p320 /a_t320 , r² = 0.21, P < 0.0048). The relationships are robust across a variety of watersheds that differ in land use, forest coverage, and wetland coverage, indicating that diverse hillslope form and processes, set by the degree of fluvial incision and underlying rock type, combine to establish the template for stream optical environments. Agricultural land use exerts a secondary, but discernable effect 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.