Evaluating and improving the assessment of compound-specific stable isotope derived sediment fingerprinting results in an agricultural watershed in British Columbia, Canada

Abstract

Agricultural fields are a known contributor of sediment to streams and rivers, but determining specific sources of sediment in agricultural watersheds characterized primarily by C3 plants has proven difficult with traditional sediment fingerprinting methods. This study aimed to use compound-specific stable isotopes of long-chain fatty acids (LCFAs) to determine the sediment contribution from multiple sources – cropped, grazed, forage, riparian zones, banks, and forested soils – to Murray Creek, a tributary to the Nechako River in British Columbia, Canada. Source and sediment samples were collected in 2019 and analysed for LCFA concentrations and δ¹³C FA values (C20:0-C30:0, C32:0). Statistical analyses were undertaken to determine the discrimination capabilities of the LCFAs. Results showed that discrimination was poor across the agricultural land uses, though forested samples were clearly identified. For mixing in Murray Creek, just three sources – agriculture (including riparian areas), forested, and banks – were used. The results found agriculture and banks to be the primary sources of sediment. This is important because Murray Creek delivers sediment to important fish spawning habitat, which has been identified as one of multiple causes of fish population declines. The difficulty in discriminating between the agricultural land use types reflects multiple confounding factors including the multi-use nature of agricultural land in Murray Creek (i.e., land can be used as harvested forage and unmanaged grazing in the same year), the similarities in isotopic signatures across C3 plants, and the temporal insensitivity of the analysis, which may pick up the vegetation signatures of previous years. While the LCFAs were not able to identify specific fields of importance in the timeframe of this study, this technique would be valuable if the sources were more unique, if more samples of each source were taken for better characterization, and if previous land use in the agricultural fields was incorporated.