Standard 10 d toxicity tests were conducted with freshwater benthic invertebrates using sediments containing a range of concentrations of polycylic aromatic hydrocarbons (PAHs) The assays were performed both under normal laboratory fluorescent light and ultraviolet (UV) light, which mimicked wavelengths present in sunlight, at about 10% of ambient solar intensity In sediments with elevated PAH concentrations, tests conducted with UV light resulted in significantly greater mortality of Hyalella azteca (amphipods) and Lumbriculus variegatus (oligochaetes) than tests performed under otherwise comparable conditions with fluorescent light There also was increased mortality of these two species, relative to controls, when surviving organisms from the 10‐d exposures to the PAH contaminated sediments were placed in clean water under UV light for 2 h These results suggest that the organisms accumulated PAHs from the test sediments, which were subsequently photoactivated by UV light to excited states more toxic than the ground state molecules The phenomenon of photoactivation has been examined for pelagic species exposed to PAHs, but not for benthic organisms exposed to sediment‐associated PAHs Our results suggest that failure to consider photoactivation of PAHs by sunlight could result in sediment toxicity test methods or criteria that are underprotective of benthic organisms

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Standard 10 d toxicity tests were conducted with freshwater benthic invertebrates using sediments containing a range of concentrations of polycylic aromatic hydrocarbons (PAHs) The assays were performed both under normal laboratory fluorescent light and ultraviolet (UV) light, which mimicked wavelengths present in sunlight, at about 10% of ambient solar intensity In sediments with elevated PAH concentrations, tests conducted with UV light resulted in significantly greater mortality of Hyalella azteca (amphipods) and Lumbriculus variegatus (oligochaetes) than tests performed under otherwise comparable conditions with fluorescent light There also was increased mortality of these two species, relative to controls, when surviving organisms from the 10‐d exposures to the PAH contaminated sediments were placed in clean water under UV light for 2 h These results suggest that the organisms accumulated PAHs from the test sediments, which were subsequently photoactivated by UV light to excited states more toxic than the ground state molecules The phenomenon of photoactivation has been examined for pelagic species exposed to PAHs, but not for benthic organisms exposed to sediment‐associated PAHs Our results suggest that failure to consider photoactivation of PAHs by sunlight could result in sediment toxicity test methods or criteria that are underprotective of benthic organisms

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