Heavy metal accumulation and limited trophic transfer in the Xiangjiang River food web

Heavy metal contamination has long been a major environmental concern in the Xiangjiang River, a principal tributary of the Yangtze River that flows through one of China's most important nonferrous-metal-producing regions. However, trophic transfer of heavy metals within the river's fish assemblages remains poorly understood. In this study, fish were collected from four sections of the Xiangjiang River during the wet season. Concentrations of Cu, Zn, Pb, Cd, Mn, and As in muscle tissue were quantified together with carbon and nitrogen stable isotope ratios. Stable isotope analysis revealed differences in trophic position among feeding guilds, with mean trophic levels decreasing in the following order: carnivorous > omnivorous > filter-feeding > herbivorous. Element accumulation also varied among guilds. Herbivorous fish generally exhibited the lowest concentrations, whereas filter-feeding fish had relatively high concentrations of Cu and Mn, carnivorous fish of Zn, Cd, and As, and omnivorous fish of Pb. No significant relationships between element concentrations and trophic position were detected in the headwaters or the upper and middle reaches. In the lower reach, however, Cu and Mn concentrations decreased significantly with increasing δ¹⁵N, indicating trophic biodilution. These findings suggest that heavy metal accumulation in Xiangjiang River fish reflects the combined effects of feeding ecology, spatial variation in exposure, and element-specific physiological regulation rather than trophic position alone. This study advances understanding of contaminant transfer in mining-impacted riverine food webs and provides a scientific basis for fish-based biomonitoring and ecological risk assessment.