7. CONCLUSIONS
1) For the overall DIC isotopic composition in the Genggahai Basin, we found that δ13CDIC-I was the most negative, with an average value of –11.1 ‰, followed by δ13CDIC-R, with an average value of –10.8 ‰; δ13CDIC-L was the most positive, with an average value of –6.91 ‰. This was caused by isotope fractionation resulting from the photosynthesis of aquatic plants after spring water inflow into the lake.
2) Owing to variations in the photosynthetic activity intensity of different aquatic plants, there were also significant variations in the δ13CDIC-L values in areas with different aquatic plants. We found that areas with M. spicatumand P. pectinatus communities had similar δ13CDIC-L values, with an average of –6.8 ‰, whereas the δ13CDIC-L value of the Chara spp. community was more positive, at –4.0 ‰. This likely occurred because Chara spp. plants have a higher photosynthetic rate and are more capable of using CO2for photosynthetic activity, converting them into plant organism.
3) Based on hydrochemical observations, we found that the δ13CDIC of the lake water was primarily affected by the δ13CDIC-Iand aquatic plant photosynthesis. The change in δ13CDIC-I to a more positive value resulted from carbon isotope equilibration between 13C from carbonate weathering in the watershed and12CO2 from soil respiration.
4) The changes in the δ13CDIC-Lcomposition of the Genggahai Lake indicated that the DIC from lake inflow and the photosynthesis of aquatic plants were the key components in the carbon cycle of the lake.