Organic Carbon Dynamic Transport And Flux Changes Within Tidal Cycles In Salt Marsh Marginal Zone

Carbon cycle in marsh areas is playing an important role in global carbon cycle. The tidal flat of Yangtze Estuary is facing the open sea, and is influenced by profound special hydrodynamic factors such as tidal effect, wave effect and rapid silting, which make organic carbon concentration, flux and other characteristics different from those in other estuarine marsh areas of the world. Therefore, the study of dynamic transport and flux of organic carbon in salt marsh areas is the key role in ecosystem of salt marsh areas, which is also a neglecting part of global carbon cycle research in coastal zone. The correct understanding of organic carbon concentration changes and flux changes under special dynamic forces like tides and waves in salt marsh areas is meaningful in unveiling the vital significance of costal carbon cycle in global carbon cycle.This article took the upsurge beach salt marsh marginal zone of Chongming Dongtan along Yangtze River as the main study area. The main purposes of this article were to study the short-term organic carbon dynamic transport and flux changes in salt marsh areas, to reveal the changes of short-term organic carbon concentration, to calculate organic carbon flux in every single tidal cycle and to analysis the impact factors to the short-term organic carbon, thus, unveiling the organic carbon flux changes in tidal flats. The main conclusions could be listed as follows.1. The average value of organic carbon concentration in salt marsh area was 18.2 mg g -1, and the average value of organic carbon concentration in bald field was 10.8 mg g -1. The average organic carbon concentration value in salt marsh area during neap and spring tidal cycles was 1.7 times as that of bald field. The average organic carbon concentration value in summer was 1.5 times than that in spring in salt marsh area. The sediment organic carbon concentration in spring showed a steady trend with an average value of 1.6 mg g -1. The sediment organic carbon concentration value in internal salt marsh was higher than that of marsh edge, which was higher than that of bald field.2. The basic rule of tidal-cycle sediment organic carbon concentration changes in suspended sediments in spring and summer seasons could be roughly divided into two types. Peak value occurred during young flood and latter half ebb flood respectively with fairly large changes in organic carbon concentration during the whole tide. Only one peak value occurred in the latter half ebb flood during the whole tide. In this type, organic carbon concentration showed a gradual increase in the young flood and concussion of increasing and decreasing in latter half ebb flood.3. The range of total organic carbon flux values in salt marsh area were between 1676 g/m and 15114 g/m. Most net carbon flux values in salt marsh area were positive, indicating organic carbon flux in salt marsh area was in a stable state of onshore accumulation. The range of total organic carbon flux values in bald field were between 1787 g/m and 50391 g/m. Frequent onshore and offshore alternating changes were obvious in bald field, but organic carbon flux in bald field was accumulating onshore overall. The process line of organic carbon transportation rate ( fPOCx) in these two measuring points were both shaped sine curves, which presented onshore transportation in high tides and offshore transportation in ebb tides. Onshore transport was dominant.4. Under calm weather conditions, tidal effects mainly controlled the organic carbon transport capacity and quantity in both bald field and salt marsh measuring points. Under calm weather conditions, there was significant positive correlation of power function between the maximum water depth (hmax) and total organic carbon flux (FPOCx) in each cross-section, with coefficient of 0.94. There was remarkable linear positive correlation between the maximum water depth and the mean water velocity, and between particle organic carbon concentration and the maximum water depth, with coefficient of 0.74 and 0.61 respectively.5. By means of mechanism analyses method, during the three tides experimented in salt marsh area, the contribution rate of advective sediment flux was between 45﹪and 81﹪.The contribution rate of tidal pumping effect was between 18﹪and 54﹪. During the four tides experimented in bald field, the contribution rate of advective sediment flux was between 22﹪and 61﹪.The contribution rate of tidal pumping effect was between 38﹪and 77﹪.Advective sediment flux and tidal pumping effect were the most important items to the total organic carbon discharge.