The variation of water quality from 2012 to 2018 in Lake Chaohu and the mitigating strategy on cyanobacterial blooms

巢湖自1990s中期至2012年间水质明显改善，但是近年来水质改善效果变缓，2018年蓝藻水华面积显著增加，为有效评估巢湖水体环境的变化，通过对2012-2018年巢湖17个点位的逐月调查数据分析阐述了近年来巢湖水质和藻情的变化特征，并在流域空间尺度上分析了巢湖流域水污染治理的进展和不足，为后续治理方向的调整和确定提供支撑.2012-2018年湖区调查数据显示：巢湖湖体总磷和总氮浓度显著升高，铵态氮浓度显著下降，水华蓝藻总量显著升高.在空间上，各污染指标水平呈现由西向东呈逐渐降低的趋势，但是各指标在不同湖区随时间的变化趋势差异明显，西部湖区的总磷、总氮和水华蓝藻指标近年来略有下降或持平，中部和东部湖区则显著升高，所以巢湖湖体总氮和总磷浓度的升高主要源于中、东部湖区的升高，这也是这两个湖区水华蓝藻变动的主要驱动因素.主要入湖河口数据显示：西部4条主要入湖污染河流（南淝河、十五里河、塘西河和派河）水质明显改善，但仍处于较高污染水平，中东部入湖河流（兆河、双桥河和柘皋河）总磷浓度明显升高，是中东部湖区水体营养盐升高的主要原因.中东部河流入湖污染的增加加剧了该区域湖体的富营养化水平，尤其是总磷浓度明显提升，导致中东部湖区夏季水华蓝藻的优势种从鱼腥藻种类演替为微囊藻种类.夏季微囊藻的大量繁殖，使得2018年巢湖中东部湖区部分月份水华面积异常增高.因此，巢湖流域的治理应该在持续强化流域西部合肥市污染治理的同时，增加对流域中部和东部治理的关注和投入.;The water quality of Lake Chaohu has improved significantly from the mid-1990s to 2012. But in recent years, water quality improvement has slowed down. In particular, cyanobacterial blooms area increased significantly in 2018. In order to evaluate the variation of water environment, we analysed the change characteristics of water quality and cyanobacterial blooms situation in Lake Chaohu using the investigation data covering 17 sites from 2012 to 2018. We also elaborated the progress and shortcomings of water pollution control in Lake Chaohu in the spatial scale of the basin, which will provide support to the modification and confirmation of pollution control policy. Our results showed that the mean values of total phosphorus concentration, total nitrogen concentration and bloom-forming cyanobacterial biomass (chlorophyll-a and phycocyanin concentration) increased significantly from 2012 to 2018, and the ammonia nitrogen concentration decreased significantly during this period. In spatial, the eutrophication presented slight decreasing trend in the western region, but it significantly increased in the central and eastern regions, especially for the total phosphorus concentration. Even though, the eutrophication level in the western region was still higher than those in the central and eastern regions. The relief of eutrophication was mainly attributed to the decrease in lake-inlet pollution from Nanfei River, Shiwuli-Tangxi River, and Paihe River. The increasing lake-inlet phosphorus pollution from Zhaohe River (in the central region), Shuangqiao River and Zhegao River (in the eastern region) directly contributed to the worsening trend of water quality in the two regions. In addition, with the increasing of total phosphorus level in the eastern region, the dominant bloom-forming cyanobacterial species in the region had shifted from Dolichospermum to Microcystis in summer, which might be one of the reasons why cyanobacterial blooms area in Lake Chaohu increased abnormally in 2018. Therefore, the pollution control of Lake Chaohu Basin should continue to strengthen the control in the western part of the basin, while increasing the attention and investment in the central and eastern part of the basin.