Abstract: In the purpose of revealing and moreover improving the water quality of Caohai Lake, eleven water quality parameters were monitored seasonally in 2017 at 12 sampling sites, and in subsequence, the nutritional status in the lake was assessed and divided into different partitions by employing comprehensive trophic status index (TLI) and ArcGIS. According to national standard Quality Standard for Surface Water Environment(GB 3838-2002), the water quality in the lake was classified. Results unveiled that: (1) except for pH and dissolved oxygen, all parameters showed significant seasonal differences. The concentration of suspended solid material (SPM) was highest in autumn, and the concentrations of TN and TP were highest in spring, while those of COD_Mn, NH₄⁺-N, and Chl.a were highest in summer; concentrations of TN, TP, NH₄⁺-N and SPM decreased gradually from the upstream to the downstream. (2) In a holistic sense, the water body in Caohai Lake was mesotrophic to slightly eutrophic (45.3≤ TLI(∑)≤57.7). According to the TLI(∑) values, the lake was divided into four sectors, namely, the inlet area of the lake (slightly eutrophic in general), the areas located near the urban area of Weining county and river inlet (slightly eutrophic), the middle area of the lake (mesotrophic to slightly eutrophic), and the middle-lower reach to the outlet of the lake (mesotrophic). (3) The water quality classification of Caohai Lake was mainly Ⅱ-Ⅳ, dominantly affected by TN and COD_Mn. In conclusion, controlling the discharge of waste water in surrounding areas, in particular, the upstream of the lake area and the water inlets, should be the key of improving the water quality of Caohai Lake.

Key words: water quality protection, Caohai Lake, spatial-temporal fluctuation, nutritional status partition, comprehensive trophic status index (TLI)