[Seasonal stratification and eutrophication characteristics of a deep reservoir, Longtan Reservoir in subtropical area of China].

In order to reveal the seasonal stratification and eutrophication characteristics of the subtropical large deep reservoir--Longtan Reservoir, the spatial and temporal distribution of environmental factors and eutrophic index were investigated during November (dry period) 2012, April (level period) and July (wet period) 2013. The results suggested that: (1) The stratification structure of Longtan Reservoir was meromictic lake, it had a single thermocline structure in the dry season, the surface layer to the 60 m was a mixomolimnion, 60-80 m was a thermocline, deeper more than 80 m was a monimolimnion. It had a double thermocline structure in flow period and wet period, the surface to 10 m was a mixed layer, 10-20 m was a thermocline, 20-40 m was a mixed layer, 40-60 m was a thermocline, deeper more than 60 m was a mixed layer. (2) The thermal stratification dominated the structure of other environmental factors, the stratification structure limited the water convection, especially the monimolimnion reduced the harm of the endogenous pollution. (3) The trophic level index (TLI) was 23.4-32.8 in the dry period, 27.1-38.6 in the flow period and 26.0-45.1 in the wet period, which were all Mesotropher. The trophic state index of total nitrogen was 60.3-72.5, which was eutropher to hyper eutropher, N: P was 107:1, which was phosphorus limited.