Transfer Processes Of Phosphorus And Their Mechanisms Under The Exposure-immersion Pattern In The Yangtze Estuarine And Tidal Flats

The estuarine and coastal tidal flat, as a typical transitional zone between land and ocean, is a multifunctional and complex ecosystem with special ecological values and potential resources. Due to the interaction of land and ocean, the estuarine and coastal tidal flat is characterized by intense hydrodynamic conditions, frequent sediment transport and material exchange, steep physiochemical gradients and high biodiversity, especially the periodical change process of "exposure-immersion" in the tidal flat that is raised by tidal ebb and flood. Taking the Yangtze estuarine and coastal ecosystem as an example, the temporal and spatial distribution of phosphorus in the overlying water columns, sediments and its pore waters have been discussed, and the biogeochemical processes of phosphorus speciation in the rhizospheres of macrophytes involving accumulation and transfer, and its key effects have been analyzed. Meanwhile, the transfer processes and transformation mechanisms under the "exposure-immersion" pattern have also been discussed through the field sampling and the laboratory simulation.The results had shown that the different input sources and the hydrodynamic conditions were the main factors controlling the spatio-temporal variations of total phosphorus. By the controls of physico-chemical parameters, there were inter-transformations among the different forms of phosphorous near the sediment-water interface. However, there was an unbalance of sedimentary phosphorus distribution due to the impact of various pollutants input. The coastal distribution of Fe-P in the sediments was similar to the distributional pattern of salinity in the overlying waters, which indicated that salinity could influence the biogeochemical behaviors of phosphorus. On the contrary, the influence of grain size was relatively weak because the particle compositions and crystal structures of sediments had been changed by periodical "exposure-immersion" in the tidal flat. In addition, the reactive iron also affected the transfer and transformation of phosphorus in the sediment-water interface. Based on the quantitative analysis for the contents ofphosphorus species, the potentially bioavailable phosphorus was assessed by introducing the notion of bioavailability. The assessed results showed that about 32.75% of total phosphorus could be utilized by organisms.By comparison of phosphorus species in the sediments where had plats(Sp) with them in the rhizospheric sediments(SR), the relationships among different forms of phosphorus in SR was weaker than that in Sp, which demonstrated the rhizospheric effects on phosphorus cycle, as well as the relationships among physico-chemical parameters. Moreover, Fe-P in SR, as one of indicators for sediment quality, could not accumulate in the severe polluted areas, maybe indicating that the root system was able to degrade the pollutants and had uptake ability. On the other hand, Fe3+ to Fe2+ ratios which was used to represent the redox status suggested that the rhizosphere of tidal flat was under the reduction-less oxidation status, which was almost equal to the measured redox potentials. In this redox status, the phosphate adsorbed by iron oxyhydroxides could exchange with anther phosphate in the pore waters, which built a good relationship between sediments (pore waters) and overlying waters, and influence the bioavailability of phosphate. As for Org-P, the distribution between Sp and SR was controlled by phosphorus assimilation and remineralization, which constructed a biological cycle through food chain.There were quite intricate relationships among sedimentary phosphorus, reactive iron, organic matters, water contents and grain sizes of core sediments, which indicated that the physico-chemical parameters were important for phosphorus, iron and organic matters to enhance their activities in the process of sedimentation. Through the assessment of difference in accumulation rates between surficial and deep sediments, two retention mechanisms of phosphorus were discovered. One was the sequestr...