Variation characteristics of algae density and water environment factors in Lake Nansi before and after division of the water transfer

Abstract

为探究大规模调水输入背景下南四湖生态环境演变特征，采用多种非参数统计方法综合分析2010—2020年南四湖藻密度、总氮（TN）、总磷（TP）、氨氮、叶绿素a（Chl.a）、五日生化需氧量（BOD₅）、高锰酸盐指数（COD_Mn）年际年内变化规律和影响因素.结果表明：2010—2020年南四湖上、下级湖水质综合污染指数年均值范围分别为0.759~0.945、0.719~0.926，水质总体逐渐改善，但TN单项污染指数存在超标现象，主要可能与入湖河流中高浓度氮输入有关.受南水北调东线工程运行初期与严峻旱情的影响，南四湖藻密度、TP、氨氮、Chl.a在2013—2015年发生突变，随后均呈增加趋势.南水北调东线工程的运行对南四湖藻密度的年内波动影响强烈，调水工程运行后每年10月份出现峰值，其中2016年上级湖藻密度高达520.0×10⁴ cells/L.湖区藻密度不仅与氮磷质量比（TN/TP）、TP、COD_Mn、Chl.a呈显著相关，还与降雨、水温密切相关.在南四湖生态环境保护治理过程中，应强化关键水质指标的污染监管与治理，完善调水方案和水生态危机应急管理办法.;In order to explore the characteristics of the algae density and its water environment factors in Lake Nansi under the context of water transfer, non-parametric statistical methods are used to comprehensively analyze the inter-annual and intra-annual distribution trends of algae density and its key water environment factors (total nitrogen(TN), total phosphorus(TP), ammonia nitrogen, chlorophyll-a, BOD₅, COD_Mn). The results show the annual average of comprehensive pollution index of water quality fluctuates in the range of 0.759-0.945, 0.719-0.926 in upper and lower lakes from 2010 to 2020, but the TN in Lake Nansi still exceeds the standard, which was mainly related to the high input of nitrogen into the lake. TP, ammonia nitrogen, chlorophyll-a and algae density had a sharp turning during 2013-2015 because of drought and the early operation of the South-to-North Water Transfer Project. The water quality indicators all increased afterward. The east route of the South-to-North Water Transfer Project has a strong impact on the intra-annual fluctuation of algae density. The peak value occurs in October when the algae density was 520.0×10⁴ cells/L in upper lake in 2016. Algae density was not only significantly related to ratio of nitrogen to phosphorus, TP, COD_Mn, chorophyll-a, but also closely related to rainfall and water temperature. In the process of environmental protection and management of Lake Nansi, the supervision of key water quality needs to be strengthened. The management of water diversion and ecological crisis should be improved.