2.2 Sample collection
The water samples in this study were tested from January 2010 to April 2022, and the frequency of testing was three times per month, at the beginning, middle, and end of the month. The study selected the average of the three data as the water sampling data for the month to avoid chance. And most of the water sampling data were collected under clear or cloudy weather, reducing the effect of rainfall. Water samples were collected from three reservoirs in Chuzhou City during the study period, and when the sampling was completed, the water samples were transported to the laboratory in an insulated box with ice packs to test the required indicators. A total of 10 water quality parameters were chosen for reflecting reservoir water quality in the study, which were Water Temperature (WT), pH, Secchi Depth (SD), Dissolved Oxygen (DO), Chemical Oxygen Demand (CODMn), Biological Oxygen Demand (BOD), Ammonia Nitrogen (NH3-N), Total Nitrogen (TN), Total Phosphorus (TP), and Chlorophyll a (Chl-a). Among them, WT, pH, and DO were measured in situ using a multi-parameter water quality analyzer (YSI 6600V2, USA)., while the remaining parameters were measured by the (Editorial Committee, Methods of Analysis for Water and Wastewater Monitoring , State Environmental Protection Administration, 2002). However, when the data were finally selected, it was found that there were large areas of missing data or some key indicators that were not measured in the previous years. Thus the indicator data from 2019 to 2021 were selected as the determining factors for judging the conditions of water quality and the trophic status of the water bodies in three reservoirs in Chuzhou City.