湖南省大通湖百余年环境演化历史及营养物基准的建立

科学有效地治理退化湖泊需要知晓湖泊的演化历史, 并设立合理的参照目标(即环境基准)来及时评判治理效果。湖南省大通湖位于经济发达的长江中下游地区, 发挥着重要的湖泊水生态系统服务功能。在强烈的人类活动干扰下, 该湖近年来生态系统退化严重, 但其水环境演变的历史缺少详细的记录。研究对大通湖沉积钻孔的年代、烧失量、化学元素、沉积物总磷(TP)和沉积硅藻等沉积指标进行了分析, 重建了大通湖百余年来的环境变化历史。结果表明: 在人类干扰加强和气候变化的共同作用下, 大通湖生态系统及环境发生了显著的变化, 硅藻群落由中营养属种Aulacoseira granulata占优过渡到以富营养浮游类型Stephanodiscus hantzschii、S. minutulus占优的过程, 揭示了该湖自1980s以来显著的富营养化过程; 对应的, 其他沉积指标亦发生了显著的变化。冗余分析揭示出沉积物总磷(TP)和铅(Pb)含量是影响湖泊环境演化的2个显著变量, 这表明工农业发展带来的营养和重金属输入对大通湖环境演化影响巨大。利用区域-总磷转换函数重建了大通湖过去百余年湖水总磷变化历史, 选择1850s人类活动相对较弱时期的湖水总磷(50—60 μg/L)及沉积物磷的浓度(600 mg/kg)值作为该湖的基准环境, 为湖泊的富营养化治理提供修复目标。

ENVIRONMENTAL EVOLUTION OF DATONG LAKE OVER THE PAST 160 YEARS AND ITS NUTRIENT BASELINE

Shallow lakes are essential part of aquatic ecosystem and play irreplaceable roles in regional water supply,shipping transportation,maintaining regional ecosystem balance and other aspect.However,most of Chinese shallow lakes are facing environmental problems,especially the eutrophication.The restoration and management of eutrophic lakes require knowledge of their environmental evolution and nutrient baseline.Such knowledge is crucial for sustainable environment management and decision-making.Datong Lake is the biggest inland aquaculture lake in Hunan Province which locates in one of the developed regions of China(in the middle and lower reaches of the Yangtze River).Under the strong disturbance of human activities(i.e.agriculture,industrialization,urbanization),the lake ecosystem has deteriorated during recent years.Due to the limited information on the history of water environment,we lack of the information of the Datong Lake’s nutrient baseline,leading to a restoration dilemma that could not provide a scientific target for the system governance.Paleoliminological technique is important for assessing the lake’s historical evolution,nutrient baseline and degradation mechanism.Among the multiple paleolimnological proxies,diatom plays a primary product role in lacustrine ecosystems and responds sensitively to external and internal drivers.Diatom remain can be well preserved and becomes one of potential proxy indicators of lake ecosystem changes.Based on the multiple proxy analysis including 210Pb/137Cs,loss-on-ignition,metal elements,sediment total phosphorus(TP),and diatoms from Datong Lake,we reconstructed the history of local environment over the past 160 years.With the increasing pressure from human disturbance and climate change,diatom communities had experienced a significant shift from mesotrophic specie Aulacoseira granulata to eutrophic planktonic species Stephanodiscus hantzschii,S.minutulus,underlin-ing the process of eutrophication.Diatom inferred-total phosphorous(DI-TP)started around 1980 with TP concentration increase from 66 to 202μg/L and prior to 1930 DI-TP was constant at 50—60μg/L.Four diatom assemblage zones were identified according to changing abundance of dominant taxa.The zone I(39—25 cm;pre-1930AD)was dominated by the planktonic A.granulata,which had the maximum abundances(60%—80%)and the lake in s state of meso-trophic.ZoneⅡ(25—10 cm;1930—1980AD)was characterized by the gradual decline of A.granulata and a slightly increase of eutrophic planktonic species indicating that Datong Lake appeared a nutrient enrichment event(DITP fluctuated between 66—83μg/L).ZoneⅢ(10—4 cm;1980—2010AD)was dominated by the eutrophic planktonic species S.hantzschii,S.minutulus,and a sudden decrease in A.granulata(<10%).This period showed that Datong Lake had enter a eutrophic stage.ZoneⅣ(the top 4 cm;post-2010AD)was characterized by slightly decrease in percentage of S.hantzschii,S.minutulus and the DI-TP had fallen back.Redundancy Analysis revealed that the sedimentary TP and Pb were the two significant variables influencing the diatom evolution.Simultaneously,eutrophication process of Datong Lake shows the coupling trends with heavy mental pollution,exhibiting that the input of sewage from industry and agriculture in Datong Lake catchment in recent decades may be the common cause of eutrophication and heavy mental pollution in Datong Lake Basin.Hence,nutrient and heavy metal input from industry and agriculture are crucial factors on environmental evolution of Datong Lake.In the meantime,we also should pay attention to the influence of climate changes to the shallow lake ecosystem,the decrease of wind speed and the raise of annual average temperature were conducive to the bloom of the harmful algae,which will prolong the span of algae growing period and strengthen the release of internal nutrition.We reconstructed the historical epilimnetic TP over the past 160 years,and chose the TP concentration of 50—60μg/L,sedimentary TP(600 mg/kg)in 1850s when the anthropogenic activities were relatively weak as reliable targets(nutrient baseline)for the management.This study showed that Datong Lake has a high nutrient baseline and the heavily polluted water is difficult to recover.Therefore,banning aquaculture,controlling agricultural point and non-point pollution,cultivating submerge aquatic plants and reducing anthropogenic disturbance in Datong Lake catchment might be effective approach for its eutrophication control.This study can provide a nutrient baseline of scientific information for better management strategies of lake ecosystem and water resource of present lake and other similar lakes in Yangtze floodplain.