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ABSTRACT Changes in water quality in Dianchi Lake over a long temporal scale have been significantly influenced by climate change and policy issuance. This study utilized the Mann -Kendall test and Theil -Sen estimation to examine water quality trends at 10 monitoring sites within Dianchi Lake from 1989 to 2018. The corresponding impacts of policies, including point source (PS) and non-point source (NPS) pollution control, ecological restoration (ER), and enforcement and supervision (ES), along with climatic conditions, were evaluated. Analysis of regression curves of water quality indexes indicates a slight increase in ammonia (NH4+-N) and total nitrogen (TN), while total phosphorus (TP), biochemical oxygen demand (BOD5), permanganate index (CODMn), and chlorophyll-a (Chl-a) decreased from the baseline time (year 1989). During the mid-phase (2001 -2006) of the study period, the impacts of policies on water quality were inconspicuous, with water temperature affecting Chl-a concentration variations. During the 11th Five-Year Plan, pollutants like NH4+-N, TN, TP, and CODMn peaked and then significantly declined between 2009 and 2015, likely due to comprehensive issuance of NPS and ER policies. This study can provide a reference for water quality management in eutrophic lakes.
Shen et al. (Thu,) studied this question.