Research Article Penetration of ultraviolet radiation in streams of eastern Pennsylvania: Topographic controls and the role of suspended particulates Patrick Belmont 1,2, * , Donald P. Morris 1 , Frank J. Pazzaglia 1 and Stephen C. Peters 1 1 Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18105, USA 2 Current address: University of Minnesota, National Center for Earth-surface Dynamics, Minneapolis, MN 55414, USA Received: 13 April 2008; revised manuscript accepted: 4 February 2009 Abstract. Penetration of ultraviolet radiation (UVR) in stream ecosystems is determined by the concen- tration 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 attenu- ation 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 ,r 2 = 0.68, P < 0.0001), DOC concentration (MWS:DOC, r 2 = 0.65, P < 0.0001), and DOC optical quality (MWS:Fluores- cence Index, r 2 = 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, r 2 = 0.39, P = 0.012) and optical quality (% Agriculture:Fluores- cence Index, r 2 = 0.32, P = 0.036) in watersheds devoid of wetlands. Key words. Ultraviolet radiation; dissolved organic carbon; geomorphology; land use. Introduction Quantifying penetration of ultraviolet radiation (UVR) in streams is the first, necessary step towards understanding organismal responses and adaptations to UVR exposure in lotic environments. Exposure to damaging ultraviolet-B radiation (UV-B; 280 – 320 nm) in stream ecosystems is an important concern due to recent and pervasive changes in atmospheric, climatic, and riparian zone factors that strongly influence stream optical environments (Madronich et al., 1998). Two fundamental questions in aquatic ecology that are yet to be comprehensively addressed include: a) what environmental factors control the * Corresponding author phone: 612-625-7037; fax: 612-624-4398; e-mail: belmont@umn.edu Published Online First: March 12, 2009 Aquat. Sci. 71 (2009) 189 – 201 1015-1621/09/020189-13 DOI 10.1007/s00027-009-9120-7 # Birkhäuser Verlag, Basel, 2009 Aquatic Sciences