| Tidal flat is one of the most active ecosystems in surface sphere of the earth, and plays an important role in maintaining high primary production in coastal ecosystem. The uptake, adsorption, retention and other processes of tidal marsh delay the release of nutrient and contamination from marsh into estuary purifying water quality of estuarine ecosystem. Whereas, eutrophication in coastal water and the exchange between coast and marsh speed nutrient enrichment in tidal flat, and lead to the function shift from "sink" to "source" of marsh. Consequently, the variations occur in habitat and ecological function of marsh ecosystems. It is urgent to carry out the researches on the nutrient biogeochemical cycles and isotopes indicators in tidal flat of the Yangtze Estuary, one of the most important drivers of marine environment.On the basis of water quality survey of the Yangtze Estuary and the adjacent sea and seasonal sampling of tidal flats (including Chongming Dongtan, Hengsha Island and Fengxian flat), our objects were to determine 1) plant uptake, nutrient sediment pool and the nutrient flux between pore water and coastal water; 2) the response of those processes to eutrophication in coast; 3) nutrient cycle and ecological indexes of tidal flat ecosystem using carbon and nitrogen contents and stable isotopes of marsh plant and sediment. These results would aid in the sensitivity of environmental management on nutrient enrichment and the development of protection measures in tidal flat ecosystems. The results are as follows:1. Marsh vegetation was one of the carbon and nitrogen repositories in the Yangtze estuary. Carbon and nitrogen net annual production of Spartina alterniflora, Phragmites australis and Scripus mariqueter were 17.44×104,4428.73t in the Yangtze estuary respectively (this result was underestimated). Nitrogen annual production was 35.45,30.19 and 9.18 g N·m-2·yr-1 for these three marsh plant respectively. The aboveground production was larger than the belowground, meaning that carbon and nitrogen would be removed from atmosphere and marsh system by the harvest of the aboveground organism to purify water quality of the Yangtze estuary. The bellowground/aboveground biomass ratio in S. mariqueter was higher than that in other plant, and it was higher in Fengxian flat than other two flats. The results showed that the belowground biomass was not only a survival strategy for marsh plant to unstable environment, but also a factor to abiotic environment and nutrient of marsh sediment.2. Sediment pool was carbon source and nitrogen sink for coast ecosystem. Sedimentation rates of organic carbon and nitrogen were 45-86g C·m-2·yr-1,4.74-6.17 g N·m-2·yr-1 respetively. Except that net belowground carbon production of S. mariqueter was mainly deposited in sediment,74%-82% net belowground production of S. alterniflora and P. australis was exported from marshes. Meanwhile, nitrogen sedimentation rate was larger than the belowground production, and 45%-55% nitrogen of sediment under the S. mariqueter and S. alterniflora was imported from external source. TOC and TN showed significant linear relationship in three flats, but the slope of Fengxian flat, namely C/N, obviously differed from other two marshes, indicating the impact of coastal eutrophication and sewage discharge on Fengxian flat. The growth of marsh plant, especially the invasion of S. alterniflora, accelerated nutrient sedimentation, and nitrogen retention in sediment exceeded carbon sedimentation.3. The nutrient diffusive flux in sediment-water interface showed that the export flux of flux NH4+-N (-24.17μmol·m-2·h-1) was higher than the import of (NO3-+NO2-)-N (13.8μmol·m-2·h-1), resulting in the purification function on water quality. The export of SiO32--Si, NH4+-N and PO43--P from tidal marshes regulated nutrient level and composition, and specially lifted the silicon beyond potentidal element limitation in the coastal system in summer. Ammonium as the main form of DIN was 1-2 orders of magnitude higher than (nitrate+nitrite)-N (0.38-36.13μ.mol·L-1) in pore water. In Chongming Dongtan, ammonium concentrations of pore water in macrophyte (S. alterniflora and P. australis) zones were over one order of magnitude less than bare flat and S. mariqueter zone. Moreover, the linear regression analysis showed significant relationships between biomass in spring and summer growing seasons and nutrient concentrations (except SiO32--Si) in CM. The mechanisms could interpret the impact of marsh plants on pore water nutrient:1) the uptake directly decreases nutrient concentrations of marshes; 2) the variations of the redox environment in sediment by vegetation growth affect nutrient form in pore water.4. Marsh plants were binned into three groups in the Yangtze estuary:C3 plant (δ13C,δ15N were-24.3～-30.0%。,1.6-8.3%。respectively), C4 plant (δ13C,δ15N were-12.3～-14.6%。,2.5～7.2%o respectively) and marsh algae including phytoplankton and benthic algae (813C,δ15N were-15.7～-22.8%。,4.0～7.2%。respectively) by stable isotopes. Theδ15N values of marsh plant showed evident difference between Fengxian flat and other two flats, and had negative correlation with nutrient of coastal water and pore water. However, the organic of sediment was a mix of C3, C4 and suspended particulate matter. The contribution of C3 and C4 plants to organic matter of surface sediments were 42.97%,19.21% respectively, higher than that in shallow sediment (42.04%,14.85% respectively). This result indicates the increase impact of S. alterniflora on marsh sediment. The responses ofδ13C,815N values of sediment were overlapped by intense physical condition and export-save balance of organic pool to reflect nutrient enrichment in the estuaries. 5. Based on water quality survey of the Yangtze estuary and Hangzhou Bay, the comparisons of nutrient dynamic and stable isotopes showed that C/N ratio andδ15N of sediment effectively reflected nutrient dynamic in marshes, while the vegetation indexes included belowground/aboveground biomass, N content,δ15N and the distribution of invasive species. Therefore, the ecological signal of nutrient dynamic progressively enlargeed along pore water, sediment and marsh plant. In addition, the results indicated that isotope variations were effected by nutrient source, nutritional status of coast and plant physiology in tidal flat ecosystem. Specially, eutrophication might influence the application of carbon and nitrogen content and isotopes as ecological indexes to the biogeochemical cycle of wetland ecosystems.In summary, it is proved by our survey that tidal flats act as carbon source and nitrogen sink of the Yangtze estuary, and play important roles in maintaining high production and purifying water quality by plant growth, nutrient sedimentation and the exchange at sediment-water interface. Carbon and nitrogen stable isotopes are the effective and sensitive ecological indexes to identify organic sources and nutrient dynamic of estuarine ecosystem. Long-term data of nutrient dynamic and sensitive index of nutrient enrichment would be established in biogeochemistry of tidal flat environments.
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