Seasonal Variations Of Thermal Stratification And Its Environmental Effects In Lake Lugu

To study the seasonal thermal stratification and its environmental effects in Lake Lugu,vertical water temperatures profile were monitored from August2011to October2013, lakestability, mixing depth and thermocline depth etc were calculated using “Lake Analyzer”accordingly and vertical profiles of water temperature, DO, pH, Chl a and total phosphorus wereinvestigated once every two months. Results showed that Lake Lugu is a monomictic lake,holomixing were only found in winter, with average lake stability less than20J.m-2. Seasonalstratification usually formed at mid to late March, and were unstable until late May, with lakestability slowly increasing from50J.m-2to hundreds, and thermocline depth varied between13m and18m and occasionally happen in the first2m of the surface layer. Then, lake stabilitywould raise rapidly from only a few hundreds J.m-2to more than2000J.m-2and would peaked atmid and late August (maximum value2874J.m-2were found at14Aug2012), and thermoclinebecame stable, firstly occurred at around10m then gradually decreased to about12m, the samething happened to mixing depth, from9m to10m; Since late August, as lake stability, mixingdepth and thermocline depth all decreased rapidly, seasonal stratification began to disintegrateand then disappeared by the end of December to start turnover. Vertical distribution of DO, pH,Chl a, TP were influenced by thermal stratification status of the lake. They were very well mixedduring winter mixing period. The changes in chemical profiles were affected by the formation ofseasonal thermal stratification because mixing process between surface layer and bottom layerwere inhibited by thermocline. In April, Chl a in the epilimnion were higher than that of thehypolimnion. DO and pH, distributed evenly in the epilimnion, gradually decreased downwards.Strengthened thermal stratification persisted in summer, fall and the early winter, enlarged thestritification of DO and pH. DO peaked in the metalimnion and declined steeply downwards.Lowest DO concentration (3mg/L) in the upper60m water body were found in August. DO andpH partly increased in late winter after the great decrease of lake stability. Stratification inhibitedvertical transport of TP. Mean concentration of TP in the upper60m were highest during mixingperiod and gradually decease with the establishment and development of seasonal thermalstratification, then increased a little in the late winter. This article elaborated the seasonalvariations of thermal stratification and its control on the space-time distribution of DO, pH, Chl a and TP in Lake Lugu, which will contribute to assessment for the influence of future externalpollutions and environmental changes on Lake Lugu.