Temporal And Spatial Patterns Of Dissolved Carbon And Its Budget In Poyang Lake

Wetland ecosystem is one of the main ecosystems on earth,and its organic carbon storage and carbon cycle are the important basis of global carbon cycle research,and also one of the important contents of global climate change research.Dissolved carbon in water is an important part of wetland carbon pool.It is of great significance to understand its composition and source characteristics and analyze its change of budget and expenditure for further understanding of wetland carbon cycle.Taking Poyang Lake,the largest shallow-water lake in China,as the object of this study,the dissolved carbon and stable isotope contents in water of Poyang Lake and lakes entrance in wet and dry seasons were measured.The spatial and temporal distribution characteristics of dissolved inorganic carbon（DIC）,dissolved organic carbon（DOC）and their carbon isotope values in Poyang Lake and the influence factors influencing their variation were analyzed.This paper discussed the sources of dissolved carbon in Poyang Lake and the relationship of dissolved carbon budget in Poyang Lake was preliminarily analyzed by estimating carbon flux.The main results are as follows:（1）The DIC concentration in Poyang Lake ranged from 3.53 mg/L to 8.28mg/L,and the DOC concentration in lake area and lake entrances ranged from 1.18 mg/L to 3.62mg/L,which was significantly higher than that in the lower reaches of the Yangtze River and the estuary,while significantly lower than that in the inland lakes.The seasonal characteristics were higher in wet season than those in normal season and dry season,and the spatial characteristics are higher in the entrance of the five rivers than those in the lake area.Correlation analysis shows that:DIC was positively affected by water temperature,p H,EC and TDS.The DIC concentration in lake water increased with the increase of solutes in lake water.Although there was no significant correlation among DIC,N and P nutrients,N and P nutrients were important factors controlling the growth of phytoplankton.High phosphate input in dry season led to a large number of phytoplankton growth,resulting in a lower DIC concentration in dry season than in wet season.DOC concentration was significantly positively correlated with T,Chla and COD_Mn,and significantly negatively correlated with DO,but not significantly correlated with TN and TP.Water temperature and Chla were important factors affecting DOC content in Poyang Lake.N and P were not the limiting factors of DOC content in Poyang Lake.（2）δ¹³C can effectively identify the sources of various carbon in water.Theδ¹³C_DICvalues of Poyang Lake water ranged from-15.89‰to-3.14‰,with an average of-10.63‰.The seasonal variation was obvious,with a positive trend in dry season and a negative trend in wet season.In space,the value in lake area was larger than that in lake entrance.Theδ¹³C_DOC values ranged from-28.94‰to-24.66‰,and were significantly higher in normal season than in wet season and dry season.The mean values ofδ¹³C_DOC in wet season were close to those in dry season.Theδ¹³C_DOC values of Poyang Lake indicated great spatial variation.Correlation analysis showed that:δ¹³C_DIC was significantly correlated with T and p H,whereas significantly positively correlated with TDS,DO and Turbidity.The physical and chemical characteristics of water body were the main factors affecting the spatial and temporal distribution ofδ¹³C_DIC values in Poyang Lake.There was no significant correlation betweenδ¹³C_DOC and environmental factors,beside a significant positive correlation betweenδ¹³C_DOC and DO in dry season.Theδ¹³C_DOCvalue of Poyang Lake was negatively correlated with DOC concentration,and the endogenous substances in Poyang Lake contribute to DOC concentration to a certain extent.（3）DIC in the inflow rivers of Poyang Lake was mainly derived from soil CO₂and carbonate weathering.In wet season,the mean value of DIC from soil CO₂ was69.95%,while that from carbonate minerals was about 30.05%.In dry season,the mean DIC from soil CO₂ was 60.56%,whereas the DIC from carbonate minerals itself was 39.44%.In general,DIC in the inflow rivers of Poyang Lake mainly came from soil CO₂.DIC in lake water mainly came from CO₂ dissolution at water-gas interface,river input and organic matter mineralization decomposition.In wet season,the mean values of DIC from CO₂ dissolution at water-gas interface,river input and organic matter mineralization decomposition were 42.48%,30.25%and27.27%,respectively.During the dry season,the mean value of DIC from CO₂dissolution at water-gas interface,river input and mineralization decomposition of organic matter were separately 49.94%,35.51%and 14.56%.In general,CO₂dissolution at water-gas interface and river input were the main sources of DIC in Poyang Lake.（4）The DOC in the inflow rivers of Poyang Lake mainly came from soil organic matter（53.58%）and phytoplankton photosynthesis（46.42%）,and the DOC from soil organic matter was larger than that from phytoplankton photosynthesis.In wet season,the contribution rate of organic carbon from soil organic matter was significantly greater than that from algae and other plankton.In dry season,the contribution rate of organic carbon in lake and river from algae and other plankton photosynthesis was significantly greater than that from soil organic matter.The DOC in Poyang Lake mainly came from soil organic matter（42.46%）,phytoplankton（47.28%）and photosynthesis of aquatic vascular plants（10.27%）.The DOC in Poyang Lake mainly came from phytoplankton photosynthesis and then from the river input into the lake,while the lake sediment resuspended DOC was the smallest.（5）Phytoplankton and its primary production process play a key role in regulating the carbon budget in lake water.The phytoplankton species composition in Poyang Lake was relatively stable,and the dominant species were single,mainly including chlorella and Tympanoides.The seasonal variation trend of primary productivity in Poyang Lake was consistent with the distribution trend of phytoplankton abundance.The primary productivity in wet season was significantly higher than that in dry season,and the southern lake was significantly higher than that in the northern lake.（6）The carbon sink potential of Poyang Lake was 5308.2 t/a,of which the DIC input flux was 3817.26 t/a,and the DOC input flux was 1490.94 t/a.As a whole,Poyang Lake was a net carbon sink,and the dissolved carbon flux from external sources accounts for 66%of all input fluxes,of which 32%came from atmospheric CO₂ input and 34%from river input.The dissolved flux of internal input mainly came from the mineralization and decomposition of organic matter,photosynthesis of plankton and sediment re-suspension,accounting for 34%of all input flux.The dissolved carbon flux of Poyang Lake was 3364.5 t/a,where about 55%of dissolved carbon from Poyang Lake was transported to the Yangtze River,and DIC was the main component（59%）.