Tracking Of Phosphorus Sources In The Sediments Of Tongyang River Along Chaohu Lake By Using The Oxygen Isotope Compositions

Phosphorus is an essential nutrient for life.It is also the limiting factor for aquatic primary production,and thus the critical nutrient pollutant that evoking eutrophication in natural lakes and reservoirs.Chaohu Lake is one of the three major eutrophic lakes in China,and it is also a typical phosphorus–limited eutrophic lake.Nutrient pollutants are transported mainly into lake through rivers,during this transport,a part of the nutrients would be deposited into the river sediments,which thus become one of the important materials for tracking the sources of pollutants in lake ecosystems.Diverse forms of phosphorus in river sediments have different bioavailability and therefore could be used to indicate changes in environmental conditions.The oxygen isotope compositions(δ¹⁸O_P)in form of phosphorus have been used to identify and trace the sources,migrations and transformations of phosphorus in aquatic sediments.Therefore,it is of great significance to study the source,migration,transformation,and circulation of phosphorus in the sediments of inflow rivers along Chaohu Lake through the characteristics of phosphorus occurrence and phosphate oxygen isotope composition.Here we collected sediments from rivers of Fengle,Tongyang and Qiyang,analyzed the concentration through SMT phosphorus fraction extraction method and the oxygen isotope ratios of phosphorus forms,and tracked sources and transformations of phosphorus in these inflow rivers.Then selected the Tongyang River as the representative area,using SEDEX graded extraction to subdivide the Ca–bound phosphorus into authigenic phosphorus and detrital phosphorus,determined the oxygen isotope composition of iron–bound phosphorus,authigenic phosphorus and detrital phosphorus in the sediments of Tongyang River,and the source and its conversion cycle of phosphorus was furtherly analyzed.At the same time,the vertical distribution of different phosphorus forms and their indication of source change in estuarine columnar sediments of Tongyang River were studied by SEDEX extraction.The main results and conclusions are as follows.（1）The oxygen isotope values(δ¹⁸O)of different forms of phosphorus in the surface sediments of inflow rivers deviated far from the phosphate oxygen isotope equilibrium（19.4‰–21.5‰）,and thus can be used to trace the source and the cycling of phosphorus.The oxygen isotope values of iron oxide–bound phosphorus in Tongyang River sediments indicated a main phosphorus source of domestic sewage.In addition,the depletedδ¹⁸O_Fe/Al-Piandδ¹⁸O_Fe-Piand significant negative correlations among phosphorus forms suggested a source from the hydrolysis of organic phosphorus.The oxygen isotope value of inorganic phosphorus released by hydrolysis of organic phosphorus was calculated as 12.2‰–14.6‰,close to or depleted than the values ofδ¹⁸O_Fe/Al–Pi（15.4±2.1‰）andδ¹⁸O_Fe–Pi（17.0±1.3‰）in river sediments,indicating that the iron oxide–bound phosphorus in Tongyang River sediments adsorbed the inorganic phosphorus released from the hydrolysis of organic phosphorus.Theδ¹⁸O_Ca–Pioverlapped to those of chemical fertilizers and WWTP effluents in Tongyang River sediments,implied occurrence of new formed Ca–bound phosphates originated from the phosphates of non–point agricultural discharge and domestic runoff.Well correlations between detrital phosphorus and the authigenic phosphorus in Tongyang River sediments indicated similar phosphorus sources.Similarδ¹⁸O_Detr–Piandδ¹⁸O_Auth–Piwere observed and they were enriched than theδ¹⁸O of weathered phosphate,indicating further that newly formed secondary Ca–bound phosphate in both authigenic and detrital phosphorus.（2）The total phosphorus,inorganic phosphorus,organic phosphorus,loosely adsorbed phosphorus and iron–bound phosphorus in Tongyang River sediment core showed a decreasing trend up and down,while the detrital phosphorus showed a slight increase with depth.Inorganic phosphorus is the main form of phosphorus in sediment core,with an average proportion of 71.6±6.6%for the total phosphorus;iron–bound phosphorus is the main form of inorganic phosphorus,with an average proportion of 33.9±11.2%for the total phosphorus.High iron–bound phosphorus and organic phosphorus in the sediment core indicated main sources of domestic sewage and agricultural non–point emissions from human activities.In addition,the vertical distributions of different forms of phosphorus in sediments indicate a main source of natural weathering below the layer of 17cm in Tongyang River,while a dominant source of domestic sewage and agricultural runoff in the upper layer.