Landscape Influences on Stream Habitats and Biological Assemblages

<em>Abstract.</em>—Rapid urbanization in the southwest United States has increased concern about water quality of streams and its impact on aquatic biota. One way to estimate potential toxicity impacts is to deploy passive samplers that accumulate many organic contaminants associated with anthropogenic landscapes. Semipermeable membrane devices (SPMDs) use a lipid to mimic bioaccumulation of hydrophobic organic contaminants (HOCs), including polycyclic aromatic hydrocarbons (PAHs), pesticides, and industrial compounds. We investigated effects of land use and streamflow on the presence of HOCs and potential toxicity in the Truckee River and Lake Tahoe watersheds of Nevada and California. We used SPMDs during August/ September 2002 and 2003 base flows and during March 2003 high flows. We employed two complementary toxicity tests to assess potential toxicity to aquatic organisms. The fluoroscan (pyrene index) is designed to assess PAH concentrations. The CYP1A test measures the toxicity of aryl hydrocarbon receptor type compounds, which include PAHs, polychlorinated biphenyls (PCBs), and dioxins. A relatively strong correlation (<em>r </em>= 0.79) between the pyrene index and CYP1A in our data indicated that PAHs were the dominant group of organic contaminants sequestered by our SPMDs. Due to its low-density urbanization, SPMD extracts from the Lake Tahoe watershed generally had less toxicity than those deployed in the Truckee River watershed. Samples from the Truckee River and its tributaries near Reno/Sparks had the highest toxicity, owing to dense residential development and light industry. Higher percentages of urban and agricultural land use correlated with increased toxicity of SPMD extracts, although urban land use had a much greater influence. Streamflow was a less important factor than land use, and only flows greater than 5 m³/s correlated with toxicity. Toxicity decreased with higher flows, probably due to dilution. Toxicity of SPMD extracts at all sites during both high and low flows significantly correlated with percent urban land use (<EM>R</EM>² = 0.32). Generally, toxicity did not correlate with dissolved oxygen, pH, or temperature, but did with specific conductance (<EM>R</EM>² = 0.27) when samples influenced by geothermal water were excluded.