Abstract: Through the determinations of δ¹³C_org, δ¹⁵N, total organic C, total N and C/N ratio in Longgan Lake sediments, this paper analyzed the sources of sedimentary organic matters in the Lake, variations of lake productivity, and evolution process of sediment since 1948. The results showed that the sedimentary organic matters mainly derived from native aquatic organic matters. The aquatic plants were float grasses accompaning with algae. Land matters and city pollutants had little influence, while large amounts of applied chemical fertilizers had great effects on the lake sediments. With the input of nutrients, the primary productivity of the lake increased, and algae began to flourish. In the sedimentary history, algae started to grow earlier in site L₂ than in site L₁, but the sediment in site L₁ could fix more nutrients.

Key words: Age structure, Life table, Survival curve, Environmental factor, Time sequence