Spatiotemporal heterogeneity of water quality and associated environmental factors in a shallow regulating lake of the south-to-north water diversion project: a case study of Nansi Lake, China.

Nansi Lake is a key regulating lake of China's South-to-North Water Diversion Project. We characterized survey-occasion and spatial variability in water quality and examined associations among physicochemical variables at 62 sites sampled in spring, summer, and autumn 2019 and winter 2021. Water-quality conditions were evaluated using the conventional Nemerow pollution index and the trophic level index; the legacy standard-value-ratio formulation was retained for sensitivity analysis. Total phosphorus (TP) and the permanganate index (CODMn) were elevated in summer and autumn 2019, coinciding with lightly eutrophic conditions, whereas total nitrogen (TN) peaked at 8.46 mg L⁻1 in winter 2021. Linear mixed-effects models showed significant survey-occasion effects on TN, TP, and CODMn (all p < 0.001) and significant survey occasion × lake zone interactions for TN and TP. The conventional and legacy pollution-index formulations were strongly rank-correlated (Spearman's ρ = 0.934), although sample-level pollution grades agreed for 79.4% of observations. Survey-specific Spearman analyses showed that neither temperature-TN nor temperature-TP correlations remained significant after Benjamini-Hochberg false-discovery-rate correction. Because the 2019 surveys and winter 2021 survey were conducted in different years, these contrasts may partly reflect interannual variability and should not be interpreted as pure seasonal effects. Together, these analyses distinguish robust survey-occasion patterns from associations that were not stable across surveys. The results support survey-occasion-specific water-quality monitoring, while hydrological mechanisms and operational measures require validation with synchronous hydrological observations.