Heavy rainfall alters the underwater light-environment characteristics in typical urban lakes: evidence from Qingshan Lake in the middle Yangtze River

Heavy rainfall significantly affects aquatic environments in urban lakes. To investigate the changes in the underwater light environment of shallow urban lakes under heavy rainfall disturbances, this study examined two heavy rainfall events in May 2023 at Qingshan Lake, a typical urban lake in the middle reaches of the Yangtze River. Spatiotemporal patterns of the water-quality indicators and light-environment parameters were analyzed before and after rainfall. Results indicate that: (1) Heavy rainfall significantly altered the water quality and eutrophication status. Near combined sewer outfalls, water pollution inputs were more direct, with higher total nitrogen, total phosphorus (TP) and chlorophyll a (Chl-a), permanganate index (CODMn), suspended solid (SS) and the comprehensive trophic-level index (TLI(Σ)) values, but lower water transparency (SD). All indicators showed unimodal variations, with faster responses near discharge points. (2) Underwater light environment near shoreline discharge points responded more sensitively. After rainfall, light conditions deteriorated significantly, while the lake center showed stronger lag effects. (3) Submerged light radiation exhibited exponential attenuation with band selectivity: UV-A and near-infrared light decayed the fastest, while green-to-red (500–700 nm) decayed slowest. The euphotic-zone depth (Zeu) and the photosynthetically active radiation attenuation coefficient (Kd [PAR]) were significantly affected by the eutrophication status. (4) Influencing factors varied spatially. Near discharge points, Chl-a and CODMn dominated, while at the lake center, TP and SS dominated. Most factors showed significant positive correlation with Kd and negative correlation with Zeu. (5) In the study area, light environment gradually returned to pre-rain levels 24 to 36 hours after rainfall. However, external pollutant input from combined storm-sewage systems near the shoreline has markedly accelerated the degradation of the light environment. This study reveals patterns and driving mechanisms of urban lake light environments under heavy rainfall, providing theoretical support for ecological restoration and precise light-environment regulation in shallow urban lakes.