Carbon Storage Estimation And Temporal And Spatial Distribution Of Organic Carbon Sources In Yilong Lake

The lake has a higher carbon source sink intensity than the ocean.The organic carbon and inorganic carbon in the lake water body are the intermediate links of water-mud interface,water-air interface and water-land interface,and play a crucial role in lake carbon cycle.In order to explore the spatio-temporal variation patterns of carbon abundance in shallow eutrophic lakes in karst area and understand the source composition of organic matter in eutrophic lakes,seasonal sample collection work was carried out at 20 sampling sites in Yilong Lake for two years.The physical and chemical properties of water,carbon concentration of different forms,carbon and nitrogen content of organic matter from different sources and their isotope signals were systematically analyzed.The temporal and spatial distribution patterns of carbon concentrations in different forms were discussed before and after hydrological regulation,and the dynamic change of water carbon storage in the water body of Yilong Lake were estimated.A Bayesian mixed model was constructed by combining signals from different organic carbon sources to quantitatively calculate the composition of organic carbon sources in the water body of Yilong Lake.Furthermore,carbon and nitrogen isotope signals of phytoplankton,which was the main source of particulate organic carbon,were analyzed to identify the impact of changes in carbon and nitrogen sources on lake organisms.The results were as follows:（1）The carbon in Yilong Lake water was mainly in dissolved state,among which the concentration of dissolved inorganic carbon（DIC）was the highest,with an annual average of 42.03±4.45 mg/L,accounting for 60.50%of the total carbon in the water,and the concentration of dissolved organic carbon（DOC）was 16.57±1.95 mg/L,accounting for 23.85%,while the particulate carbon concentration was relatively low.The particulate organic carbon（POC）and the particulate inorganic carbon（PIC）concentration were 10.05±1.81 mg/L and 0.82±0.56 mg/L respectively,accounting for14.47%and only 1.18%respectively.Analysis suggested that algae biomass and water replenishment intensity were the main controlling factors affecting the temporal and spatial distribution of carbon concentration in different forms in Yilong Lake water.The concentrations of DOC and POC in Yilong Lake water were significantly higher than those in oligotrophic lakes,and slightly higher than those in other eutrophic lakes.However,the concentrations of DIC and PIC were significantly higher than those in non-karst lakes,which reflected that the karst lakes had stronger carbon source sink intensity.The seasonal change of carbon storage in Yilong Lake water was estimated according to the water volume,showing the average annual carbon storage in the water body was 5.30 Gg.Seasonally,the carbon storage of September and March was the lowest（4.94 Gg and 4.95 Gg,respectively）,while June was the highest（6.02 Gg）,and that of December was 5.29 Gg,which showed a characteristic of increasing with the increase of water volume.Reflecting water volume was the main reason for the seasonal variation of total carbon storage in water bodies.（2）Qualitative analysis showed that the phytoplankton biomass represented by chlorophyll a concentration was closely related to the concentration of organic carbon（WOC）in Yilong Lake water,and was the main source of WOC in Yilong Lake.Further quantitative results using Mix SIAR model showed that the contribution of phytoplankton to POC in wet years（90.7%）was higher than that in dry years（68.9%）,and carbon sources such as terrestrial and aquatic vascular plants decreased significantly in wet years（high water level）.The hydrodynamic change of Yilong Lake,which reflected the influence of water level fluctuation,altered the migration mode of suspended particulate matter,leading to significant changes in the source composition of POC.The contribution rate of phytoplankton in DOC sources was 62.0%,which was lower than that of phytoplankton to POC,indicating that DOC might be related to the yield of organic matter from different sources and the rate of mineralization and decomposition of organic residues.In addition,the contribution of phytoplankton to WOC showed a seasonal change in dry season（December and March）higher than that in rainy season（September and June）,and the spatial characteristics of the central lake area was higher than the western and eastern lake areas.Other carbon sources showed the opposite trend,which may be affected by the change of seasonal input from the terrestrial source,the spatial distribution pattern of rivers entering the lake,and the growth cycle of aquatic plants.The composition of organic carbon sources in water clarified the important impact of algae-derived carbon sources,and it is necessary to focus on controlling the biomass of phytoplankton in Yilong Lake.（3）The results ofδ¹³C of phytoplankton in Yilong Lake showed thatδ¹³C signal of phytoplankton showed lower values in wet year（-26.89±1.34‰）than in dry year（-25.30±0.97‰）,and higher values in September than in other months.Meanwhile,the spatial distribution ofδ¹³C signal showed higher values in western lake basin than those in central and eastern lake basins.With increasing water volume in Yilong Lake,there was an increase in inorganic carbon input and a decrease in nutrient levels,resulting in a significant decline in phytoplanktonδ¹³C signal.This may reflect the rapid response of phytoplanktonδ¹³C signal linked to human activities such as hydrologic control.Theδ¹⁵N signal of phytoplankton varied from-0.28‰to 10.25‰,and was significantly higher in wet year（6.28±1.07‰）than in dry year（2.78±1.55‰）.Meanwhile,the values of phytoplanktonδ¹⁵N were lower in fertilization season（June and December）compared to other seasons,and also higher in rainy season（September）and early dry season（December）than those in central and eastern lake basins.The lowerδ¹⁵N values of phytoplankton are closely related to lake-water NH₃-N concentration,reflecting the role of nitrogen pollution from agricultural non-point sources（mainly ammonia fertilizer）in affecting phytoplankton absorption.The results of Mix SIAR model showed that nutrient loading derived from agricultural non-point sources decreased from 88.4%in the dry year to 79.0%in the wet year,with the contribution of point source pollution increasing from 11.6%to 21.0%.Specifically,the relative contribution of chemical fertilizers decreased from 41.2%to 14.0%,the proportion of organic fertilizers increased from 47.2%to 65.0%.The results showed that the water diversion project and the replacement of chemical fertilizer with organic fertilizer effectively reduced the concentration of nutrient salts,changed the composition of carbon and nitrogen sources available to phytoplankton in Yilong Lake,and reduced theδ¹³C and increased theδ¹⁵N of phytoplankton,indicating that human activities had a significant impact on the carbon and nitrogen cycle of lake organisms.