Synergistic effects of metal pollution and habitat degradation from artisanal gold mining drive species-specific oxidative stress and biodiversity loss in a semi-arid river catchment

Abstract

Artisanal and small-scale gold mining (ASGM) is a critical economic activity in developing nations but poses a severe threat to aquatic ecosystems through the release of heavy metals and habitat destruction. This study provides an integrated ecotoxicological and ecological assessment of ASGM impacts in the Upper uMzingwane catchment, Zimbabwe. We combined analysis of metal bioaccumulation (Cd, Cr, Pb, Hg species) and oxidative stress biomarkers in 12 fish species with a historical comparison of biodiversity changes using museum records (1947–1994). Sediment and fish tissue metal concentrations were significantly elevated at ASGM sites. We identified two key bioaccumulation patterns: benthic Mochokidae catfish (Chiloglanis spp.) as hyper-accumulators of sediment-bound Cr and Pb (Bioaccumulation Factor > 10,000), and the pelagic zooplanktivore Mesobola brevianalis as a high accumulator of Cd and Hg. Linear regression revealed strong, species-specific oxidative stress responses. The summation of metal concentrations (ΣMetals) had a significant additive effect on glutathione (GSH) induction, particularly in Labeobarbus mariequensis (p < 0.01, r2 = 0.54), Enteromius spp., and M. brevianalis (p < 0.01, r2 = 0.42). Contemporary surveys recorded only 17 species compared to 21 historically, confirming the local extinction of the Gobiidae family and several Enteromius species. Our results demonstrate that ASGM drives biodiversity loss through synergistic mechanisms: direct metal toxicity—mediated by oxidative stress—and indirect habitat degradation, which exacerbates predation by invasive species. This study underscores the necessity of regulating ASGM operations and advocates for the use of specific fish species as bioindicators in monitoring programs. The synergistic mechanisms of toxicity and habitat degradation identified here are representative of ASGM impacts worldwide. Our findings provide a globally relevant framework for using multi-species bioindicator approaches to diagnose and monitor the ecological consequences of mixed-pollution in river systems.