Study on the water quality evolution mechanism of typical shallow lake based on in-depth mining of water quality indicators

• Over 60 % of BYDL’ WQ has met Class III standards or better in the past six years. • Nutrient load decreased significantly due to ecological dredging and other measures. • T water , DO, and pH are influenced by natural-anthropogenic coupling effects. • BYDL has successfully concluded the initial stage of aquatic ecosystem governance. • Ecological remediation was the key focus for BYDL ’s subsequent management stage. The water quality (WQ) of Baiyangdian Lake (BYDL), known as the ‘kidney of North China’, declined continuously for about four decades from the 1980s because of socioeconomic development. Since the Xiong’an New Area was established in 2017, remediation efforts have been implemented and the WQ has improved. In this study, we analyzed six years of WQ data from BYDL using ordinary least squares regression, time series analysis, and the Mann-Kendall trend test. The results reveal a marked improvement in WQ since 2018, with Class III accounting for over 60 % of observations. There were significant decreasing trends in chemical oxygen demand, total nitrogen, total phosphorus, ammonia nitrogen, and Chlorophyll-a ( p < 0.05) and an overall increasing trend in dissolved oxygen ( p < 0.05). The WQ indicators varied seasonally. The WQ improvements generally aligned with the timing and patterns of the major engineering interventions but short-term fluctuations in the WQ indicators may have been caused by climatic variability. The findings highlight the complexity of the WQ pattens in BYDL and illustrate that the management strategy shifted from an initial phase that focused on pollution control to a second phase of ecological restoration integrated with environmental governance. This information can be used to refine future restoration strategies for BYDL and to guide sustainable management practices in similar shallow lake systems.