Catchment Influences on Carbon Stable Isotope Variation in Trout; Might It Be Methane?

Stable isotope analysis is widely used in ecology to trace spatial origins and trophic interactions for many species. Salmonid fish, including brown trout (Salmo trutta), are often a focal species for stable isotope analysis due to the high ecological importance and anadromous life cycles. Variations in δ¹³C values of trout have been linked to catchment characteristics, such as land use. These variations reflect carbon processes at the base of the food web, with chemosynthetic processes such as methane oxidation producing lower values than those of the photosynthetic carbon processing. We examined δ¹³C values in brown trout fry across 51 sites in two tributaries of the River Tweed, Scotland to investigate catchment drivers of trout δ¹³C variation. Soil drainage and pasture cover were identified as the strongest predictors of fry δ¹³C values, with lower drainage and higher pasture correlating with lower δ¹³C. A subset (26) of the sites was sampled for mayflies (Baetis spp.), finding a strong correlation between the δ¹³C values of mayflies and trout fry, suggesting trout δ¹³C is indicative of broader carbon processes at the site. Increases in cover of pasture and low drainage soils are known to result in elevated CH₄ concentrations in streams, while low δ¹³C values are an indicator of methane-derived carbon in the food web. The observed link between δ¹³C variation and catchment features associated with methane production, such as poorly drained soils and pasture, points to a potential role for methane-derived carbon in structuring upland stream food webs.