Scale-Dependent Effects of Riparian and Watershed Land Use on Water Quality and Fish Assemblages: Implications for Reservoir Ecosystem Management

Watershed land-use change is a major driver of freshwater ecosystem degradation, yet whether water quality and fish assemblages respond to land use at the same spatial scale remains poorly resolved. This study examined multi-scale land-use effects on both endpoints in Songhua Lake, a large reservoir in Northeast China. Water quality and fish assemblages were surveyed from 2020 to 2022 across seven inflowing sub-watersheds and five open-lake sites. Land-use composition was quantified at five spatial scales (200–2000 m riparian buffers and the sub-watershed), and redundancy analysis was used to assess scale-dependent associations. Water quality deteriorated along the disturbance gradient, with CODMn increasing by 33.9% and water quality index declining by 10.6% from low- to high-disturbance groups. RDA showed that water quality was best explained at the 200 m buffer (97.2%), with forest as the dominant predictor. In contrast, fish assemblage variation was best captured at the 2000 m buffer and sub-watershed scales (up to 99.96%), with urban land as the dominant predictor; β-diversity showed the strongest response (p < 0.001). These results reveal a scale mismatch between physicochemical and biological responses, indicating that effective reservoir management requires a dual-layer strategy combining riparian restoration for water quality with sub-watershed regulation of urbanization for fish conservation.