Changes in water quality in Dianchi Lake over a long temporal scale have been closely related to climate change and the issuance of governance policies. In this study, the Mann–Kendall test and the Theil–Sen estimation were utilized to examine the average trends of water quality changes at ten monitoring sites within Dianchi Lake over the past 30 years (1989–2018) and the corresponding impacts of policies (including point-source, nonpoint-source (NPS) pollution control, ecological restoration (ER), and enforcement and supervision) and climatic conditions. Analysis of regression curves indicates that NH4+-N and total nitrogen (TN) in Dianchi Lake slightly increased, while total phosphorus (TP), biochemical oxygen demand (BOD5), permanganate index (CODMn), and Chl-a decrease when compared to baseline concentrations at the initial study period. During the medium phase of the study period, policy impacts on water quality were inconspicuous, with certain pollutants exhibiting no relationship with policy issuance, evidenced instead by water temperature-induced fluctuations in Chl-a, which inversely correlated with nutrient trends. During the 11th Five-Year Plan, pollutants including NH4+-N, TN, TP, and CODMn, which reached peak levels, subsequently underwent a marked reduction between 2009 and 2015. This pronounced decrease is likely attributable to the implementation of comprehensive NPS and ER policies during this period.

  • The water quality of Dianchi Lake improved during the 13th Five-Year Plan (FYP) compared with the 7th FYP.

  • Water quality responses to policy issuance can be categorized into rapid or negligible/delayed patterns.

  • Water temperature-driven changes in water quality were found in the middle of the study period.

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