Effects of the long-term ecological restoration in the eutrophic plateau shallow lake—A case study of Dabokou, Lake Dianchi

大泊口位于滇池草海南部,水域面积0.52 km²,平均水深约2 m,作为滇池草海重富营养化水域生态修复示范区,大泊口分别于2015和2019年开展了两期生态修复工程,经过近年来的系统治理,大泊口水生态治理效果初步显现。为分析探究成功修复湖区水质改善、生态系统企稳向好的原因,本研究选择2015年2月-2021年12月共7年的连续监测数据,根据工程开展情况以及水生态状况将大泊口水域划分为4个部分(A1~A4水域),首先分析4个区域内主要的水质指标(悬浮物(SS)、化学需氧量(COD_Cr)、总磷(TP)、总氮(TN)和叶绿素a(Chl.a))的变化趋势和相关性,其次探究不同类型生态工程的修复效果,最后与草海和外海水域进行对比,分析大泊口的治理效果。结果表明,治理后大泊口A1~A4水域的COD_Cr、TP和Chl.a稳定下降,COD_Cr分别降低18.65、27.96、25.26、40.92 mg/L,TP分别降低0.11、0.10、0.11、0.14 mg/L,Chl.a分别降低0.037、0.068、0.067、0.106 mg/L,SS具有较强的季节性波动,TN仅有东南部A4水域表现出持续下降的趋势,平均下降0.68 mg/L。通过相关性分析得出,COD_Cr、TP与Chl.a的相关性最强,COD_Cr和TP可能是影响大泊口藻类生长的关键环境因子;大泊口开展的引水换水工程和微滤净化(除藻)工程,降低了营养盐浓度、加速了水体交换,进而降低了藻类生物量以及发生藻源性污染的风险,再通过沉水植物修复工程进一步净化水质,为形成稳定的草型湖泊提供条件,并且认为沉水植物修复工程是上述工程中最为直接、经济和长效的手段措施;最后将大泊口典型水域的水质和草海、外海水质进行比较,可以认为在相同的地理及气候条件下,大泊口东南水域的生态修复取得了一定的成功。大泊口部分湖区可作为重度富营养化藻型湖区成功修复的案例,其生态修复经验对于持续推进大泊口、草海甚至整个滇池的生态环境修复具有重要的借鉴和参考价值。;Dabokou is located in the south of Caohai, Lake Dianchi, with an area of 0.52 km² and an average water depth of about 2 m. As a typical severely eutrophic lake, Dabokou has undergone two phases of ecological restoration in 2015 and 2019. In recent years, the effects of the restoration have begun to be seen. In order to analyse and explore the reasons for the improvement of water quality and ecosystem in the successful restoration waters, this study selected the continuous monitoring data from February 2015 to December 2021 and divided Dabokou into four parts from A1 to A4 according to the engineering situation and water ecological situation. Firstly, the change trends and correlations of suspended solids (SS), chemical oxygen demand (COD_Cr), total phosphorus (TP), total nitrogen (TN) and chlorophyll-a (Chl.a) in the four areas were analysed. Secondly, the restoration effects of different ecological projects were investigated. Finally, the restoration effects of Dabokou were analysed by comparing with Caohai and Waihai. The results showed that after treatment, COD_Cr, TP and Chl.a decreased stably from A1 to A4 area. COD_Cr decreased by 18.65, 27.96, 25.26, 40.92 mg/L; TP decreased by 0.11, 0.10, 0.11, 0.14 mg/L; Chl.a decreased by 0.037, 0.068, 0.067, 0.106 mg/L, respectively. SS showed strong seasonal variations and TN showed a continuous decreasing trend only in area A4, the south-eastern part of Dabokou, with a decrease of 0.68 mg/L. Correlation analysis showed that COD_Cr and TP had the strongest correlation with Chl.a, which could be the key factors affecting algal growth. The water diversion and exchange project reduced nutrient concentrations and accelerated water circulation, and the microfilter cleaning and algae removal project reduced algal biomass and the risk of algal pollution. And through the purification functions of submerged macrophytes, the plant restoration project provided conditions for the formation of the macrophyte-dominated state lake. It is considered that the plant restoration project is the most direct, economical and long-term measure among the above projects. Comparing Dabokou with Caohai and Waihai, it can be concluded that under the same geographical and climatic conditions, the ecological restoration of the south-eastern water of Dabokou has achieved some success. Overall, the Dabokou area, as a case of successful restoration of the severely eutrophic, algae-stable state lake, its experience has important reference value for the further ecological environment restoration of Dabokou itself, Caohai and even the whole of Lake Dianchi.