Study on the Effect of Water System Connection on the Improvement of Water Quality of Inner Lakes in Town—Taking Seven Lakes in Yangshuo Urban Area of Guilin as an Example

Urban lake degradation caused by intensive urbanization necessitates systematic solutions, with water connectivity being a crucial ecological restoration strategy. This study evaluates the two-year effects (2020–2022) of connectivity interventions on seven lakes in Yangshuo, Guilin, classified by connectivity: multi-channel (Mc), single-channel (Sc), and non-connected (Nc). Regular monitoring of the physicochemical parameters and microbial communities revealed significant patterns: multi-channel connected lakes exhibited superior water quality improvement, with trophic state downgrading (weak eutrophic → mesotrophic), but the water quality of Sc-BQ was deteriorating. Seasonal variations showed wet season peaks in pH, DO, CODMn, and Chl-a, versus dry season elevations in NH3-N, NO3-N, TN, and TP. Correlation analysis identified organic matter as the primary driver of eutrophication, with TN strongly linked to NH3-N, indicating persistent domestic sewage contamination. Microbial community restructuring was accompanied by changes in water quality, and the abundance and diversity of OTUs decreased after restoration. Notably, Limnohabitans dominated Mc lakes (31.82–35.1%), while Pleurocapsa prevailed (37.85%) in Nc-LH under weak eutrophic conditions. These findings demonstrate that multi-channel connectivity effectively enhances hydrodynamic conditions and pollutant dispersion, whereas inadequate connectivity exacerbates nutrient accumulation. The study provides critical empirical evidence for optimizing urban lake management, emphasizing the necessity of multi-dimensional connectivity designs and targeted control of untreated sewage inputs in water system rehabilitation projects.