Consumer‐diet discrimination of <scp><i>δ</i>¹³C</scp> and <scp><i>δ</i>¹⁵N</scp>: Source‐ and feeding‐oriented patterns based on gut content analysis in a large subtropical river of China

Abstract

Abstract Understanding the trophic discrimination (∆ 13 C and ∆ 15 N) between consumers and diets in fluvial systems remains difficult because of the variable food sources and complex predator–prey interactions from headwaters to the estuaries. Here, stable carbon ( δ 13 C) and nitrogen ( δ 15 N) isotopes in fish and invertebrates from a large subtropical river in southern China were determined to explore trophic discrimination in conjunction with a gut content analysis. The ∆ 13 C values showed significant differences ( p &lt; .05) among functional feeding groups, with fish, shrimp, and insect scrapers presenting higher ∆ 13 C values (1.20 ± 0.23‰ to 1.51 ± 0.31‰) than other groups. The ∆ 15 N values varied significantly between invertebrates (0.64 ± 0.17‰ of insect collector‐gatherers to 1.63 ± 0.36‰ of shrimp predators) and fish (1.98 ± 0.19‰ of detritivores to 2.71 ± 0.43‰ of crustaceavores) and exhibited an increasing tendency from primary to secondary consumers. A linear regression analysis revealed that the longitudinal changes in ∆ 13 C and ∆ 15 N were closely associated with the δ 13 C of periphyton, the δ 15 N of particulate organic matter (POM) in water, and the relative contribution (%) of periphyton and organic detritus to the diet composition of consumers. These results indicated that discrimination factors might not only be influenced by the isotope signatures of basal food sources but also downstream shifts in dominant food items utilized by consumers. In particular, trophic discrimination between periphyton– and detritus–based food chains, such as “epilithic diatoms–shrimp scrapers–crustaceavorous fish” and “POM–bivalves–molluscivorous fish,” displayed regionally specific patterns. When back‐calculating to the diet assimilation and trophic position in subtropical streams and rivers, we suggest using the basin‐scale ∆ 13 C value of 0.96 ± 0.26‰ for all consumers and ∆ 15 N values of 1.07 ± 0.32‰ for invertebrates and 2.38 ± 0.37‰ for fish.