Stable Isotopes in Sedimentary Organic Matter from Lake Dianchi and their Indication of Eutrophication History

Sediment cores were collected from the central and northern parts of Lake Dianchi, a large and shallow eutrophic lake situated in southwest China. Total organic carbon, total nitrogen (TN), and total phosphorus (TP) as well as the delta C-13 and delta N-15 were analyzed in the sediment cores. Age model of the sediment cores were established according to Cs-137 geochronology, which reveals that the sedimentary record covers a period of the last 50 years. During that time, Dianchi had been undergoing a distinct conversion from oligotrophic to eutrophic, as a result of increasing nutrient loadings. The two cores displayed similar increases for values of TN and delta N-15, and the variations of the both parameters matched well with that of TP, which presumably suggested that delta N-15 is a reliable proxy for anthropogenic nutrient input. Also, dynamics of delta N-15 and TP showed that anthropogenic nutrients input seemed to start in the 1970s. The upward elevation of delta N-15 might be ascribed to the increasing input of isotopically heavier dissolved inorganic nitrogen and the accelerated denitrification process when the lake water was oxygen-depleted. The less variation of delta N-15 in the uppermost several centimeters of both cores were probably the result of pollution controls carried out by the local government in the recent decade. The upward increasing of delta C-13 in the two cores seemed to be induced by the enhanced productivity since 1980, which was in accordance with limnological observation. Therefore, delta C-13 values were believed to be an effective proxy for reconstructing the history of eutrophication in Lake Dianchi. In addition, this study also suggested that carbon and nitrogen isotopes are applicable to large, shallow lakes in interpreting the past environmental change.