Persistent spatial heterogeneity in community structure of benthic macroinvertebrates in rivers around an abandoned mine

Assessing decadal-scale ecological recovery in mine-impacted rivers is crucial for effective ecosystem conservation. This study evaluated changes in the ecological status of the Watarase River system, Japan, approximately 50 years after the closure of the Ashio Copper Mine (1973). We conducted seasonal field surveys (2022–2023) of benthic macroinvertebrates and water quality in the Watarase River and its four tributaries. We also analyzed historical field survey data on macroinvertebrate community structure collected before mine closure, immediately prior to closure, and shortly after closure. We found that trace metal concentrations in the mainstream of the Watarase River have decreased remarkably since the 1960s. In contrast, no statistically significant associations were observed between typical structural measures of benthic macroinvertebrate communities (total abundance and taxon richness) and the extent of trace metal pollution in the 2020s. Non-metric multidimensional scaling analysis revealed that, from 50 years ago, the community structure of benthic macroinvertebrates in the mainstream remains distinct from that of the uncontaminated reference rivers. Metal-sensitive taxa (e.g., Ephemerellidae and Heptageniidae) remain scarce in the mainstream despite recovery from metal contamination, whereas the community continues to be dominated by metal-tolerant taxa (e.g., Baetidae and Chironomidae). These results suggest that potential factors hindering biological recovery include the indirect effects of wastewater treatment, such as biofilm formation on riverbed sediments resulting from the use of calcium hydroxide in flocculation–sedimentation processes; increased flood susceptibility associated with historical forest degradation; and a reduction in interstitial spaces within riverbed sediments. This study highlights the necessity of a comprehensive management approach that addresses stressors beyond the simple reduction of metal concentrations to achieve the ecological restoration of historically metal-polluted rivers.