[Sources of Organic Carbon in the Surface Water of Lijiang River].

This study selected a larger typical karst basin, that of Lijiang River, as the research subject and studied the sources of organic carbon in the Lijiang basin in July, August, and September in 2016 by an isotope tracer method, a correlational analysis method, and endmember mixture model, using a stable isotope of carbon and the organic carbon to nitrogen ratio. The results showed the following. ① The spatial distribution characteristics of DIC concentration were as follows: karst area> karst and non-karst mixed area> non-karst area. The content of DIC downstream was higher than upstream. The spatial distribution of DIC concentration was mainly controlled by the distribution of carbonate rocks in the Lijiang basin. ② DOC was the main part of TOC in the Lijiang basin. TOC was mainly derived from the primary productivity of aquatic organisms. The spatial distribution characteristics of autochthonous organic carbon content were as follows: karst area> karst and non-karst mixed area> non-karst area, which may be related to more lush aquatic plants in the karst area than in the non-karst area and the carbonic anhydrase activity in the karst area being higher than in the non-karst area. The content of autochthonous organic carbon in the TOC ranged from 1.02 to 5.14 mg·L, with an average of 2.54 mg·L. There was no significant spatial difference of the proportion of autochthonous organic carbon in the TOC; it ranged from 51.68% to 85.99%, with an average of 73.07%. ③ The POC concentration, the content of autochthonous organic carbon in the POC, and the proportion of autochthonous organic carbon in the POC had no significant spatial differences. The main source of POC was allochthonous organic carbon, and the content of autochthonous organic carbon in the POC ranged from 0.01 to 0.16 mg·L, with an average of 0.05 mg·L. The proportion of autochthonous organic carbon in the POC ranged from 3.69% to 41.94%, with an average of 17.31%. ④ The spatial distribution of the content of DOC and autochthonous organic carbon in the DOC are as follows: karst area>karst and non-karst mixed area>non-karst area. DOC mainly came from the primary productivity of aquatic organisms. The content of autochthonous organic carbon of the DOC ranged from 0.97 to 5.10 mg·L, with an average of 2.48 mg·L. The spatial distribution of the proportion of autochthonous organic carbon in the DOC had no obvious difference and ranged from 54.43% to 94.69%, with an average of 79.51%. Studying the influence of aquatic photosynthetic organisms on organic carbon in rivers can provide basis for scientific problem solution of the stability of karst carbon sinks.