Using Semi-Permeable Membrane Devices and Stable Nitrogen Isotopes to Detect Anthropogenic Influences on the Truckee River,USA

Stable nitrogen isotopes (δ¹⁵N) and semipermeable membrane devices (SPMDs) were used together to provide evidence of potential anthropogenic connections to aquatic organisms in the Truckee River, which flows through the Reno/Sparks metropolitan area in Nevada. Crayfish, snail, and periphyton δ¹⁵N values, and SPMD toxicity data collected during high and low flow periods at seven primary sites on the river were used with water quality and flow data for the assessment. All biota showed an increase of δ¹⁵N on both dates at sites downstream of inflows of a water-quality impaired tributary and urban drain relative to upstream. In addition, most of the lowest δ¹⁵N values on each date occurred at the most downstream site on the river. SPMDs sample lipophilic organic contaminants and can be used to assess organic contaminant toxicity to aquatic organisms because they use a membrane that mimics organic contaminant uptake by fish. In this study, results from a fluoroscan test [pyrene index (PI)] of SPMD extracts that responds to higher molecular weight polycyclic aromatic hydrocarbons (PAHs) showed patterns similar to stable isotope data, although observed peaks in PI values occurred in the urban area upstream of where peak δ¹⁵N values occurred. The CYP1A biomarker test, which responds to PAHs, certain polychlorinated biphenyls (PCBs), and organochlorines, showed peak toxic equivalents (TEQ) values farther downstream of the urban area. Thus, it is likely that PAHs were contributing to toxicity in the urban area, whereas other nonurban sources of organic carbon may have been present farther downstream. The combined use of stable isotope measurements and SPMDs provided a means of simultaneously examining whether aquatic biota are incorporating constituents from potential food sources (via stable isotopes) or exposure through water (via SPMDs).