Effect of sewerage development on water quality and invertebrate assemblages in a Japanese river over the long term

River water degradation arising from wastewater effluents is a major factor contributing to the deterioration of river ecosystems; yet direct evidence of the beneficial influence of long-term sewerage development on such ecosystems is scarce at a basin scale. The present study, over 12 years, examined changes in sewerage connection rate, water quality, and invertebrate assemblages at 10 study sites in the Shigenobu River, southwestern Japan, and investigated the relationships between them. Sewerage connection rates increased in the three main municipalities in the basin during the study period (range: 8.6%–31.4%). At the lowland sites, the concentration of total inorganic nitrogen (TIN) decreased by 38.9% on average and the richness metrics of invertebrate assemblages significantly increased at several sites. Furthermore, the richness metrics negatively related with TIN concentration, implying that invertebrate diversity recovery was the result of water quality improvement. Our findings strongly suggest that sewerage system improvement is an effective measure for restoring river ecosystems.