Spatiotemporal variability in sources of suspended sediments from a Scottish river using compound-specific isotope analysis (CSIA): a nested catchment approach

The River Dee is one of the major river systems in Scotland, renowned for its economically important Atlantic salmon (Salmo salar) population. The Tarland Burn (70 km²), an intensively managed catchment, is a significant source of nutrients and suspended sediments (SS) to the River Dee, causing degradation of its water quality. To trace the SS sources in the Tarland Burn catchment, we used compound-specific isotope analysis (CSIA) fingerprinting technique. The CSIA fingerprinting technique applied in this study involved (i) carbon isotope ratio (δ¹³C) measurements of plant derived long-chain fatty acids (LCFAs) extracted from source soils and from river SS as a mixture signal as input tracer values, and (ii) computation of source proportions in the mixture using an end member mixing model ‘MixSIAR’ which is based on the Bayesian approach.

Source soils were sampled from the land-use types observed in a headwater catchment (10 km²) i.e. arable, temporary grassland under arable rotation, permanent grassland, coniferous forest, heather moorland and riparian zone. SS samples were collected from the headwaters, second order streams, and also from the outlet of the Tarland Burn catchment, representing a nested sampling approach. A comparison of the two common suspended sediment collection techniques to understand the role of sampling technique and associated particle sizes resulted in no substantial difference in the tracer values. SS were sampled once every two months over a period of 14 months between May 2017 and June 2018. δ¹³C values of LCFAs (even homologues between C_22:0 - C_30:0) of the SS (mixture) were within the range of source soils corresponding tracer values, confirming their conservative behaviour during transport.

Quantification of source proportions using mixing model suggested that headwater streams SS originated predominantly from permanent grasslands. They are largely located on steep topography, leading to higher hydrological connectivity and possible increased pressure from livestock. Although plantation forestry and heather moorland are prominent land-uses in the catchment, their contribution as SS sources is marginal. More intensive arable land use in the lowland areas of Tarland catchment was reflected by their high contribution to SS at the downstream locations. More intensive rainfall events during winter likely led to higher sediment fluxes from the normally less connected permanent grasslands at the catchment outlets during high flow.

Our attempt of gathering source soil information from a headwater region of a catchment and upscaling this information to model the source proportions in downstream mixtures integrating the whole catchment was successful, however uncertainties increased for the downstream results