『Abstract
　In this study, the sediment profiles of total phosphorus (TP), inorganic phosphorus (Pi), organic phosphorus (Po), C/P and N/P were used to investigate time-dependent P distribution changes in Lake Chaohu. The characteristics of Pi and Po fractions in the surface sediments were studied and the difference between east and west lake region was also discussed. The Pi and Po contents displayed a clear gradient from east to west in sediments of Lake Chaohu, and the Po/P ratios were lower in sediments with industrial and urban pollution sources input in west lake region. The study indicated that different sediments area had diverse concentrations and distributions of Po fractions due to their different drainage basin and pollution sources. The profile distribution of the C/P and N/P ratios decreased with increasing depths and stayed relatively constant ratios at the depths of 15-30 cm. The C/P and N/P ratios were always below Redfield ratios in sediment profile, indicating P enrichment but likely due to the preferential loss of carbon in respect to phosphorus. The rank order of Pi-fractions extracted was HCl-Pi＞NaOH-Pi＞NaHCO3-Pi in surface sediments. The relative distribution of NaHCO3-Pi and HCl-Pi was agreement with the trophic conditions of the regions studied. Among the sequentially extracted Po forms, the rank order of Po fractions was residual Po＞HCl-Po＞fulvic acid-P＞humic acid-P＞NaHCO3-Po, with mean relative proportion of 5.4:3.4:2.2:1.1:1.0.

Keywords: Organic phosphorus; Phosphorus distribution; Lake sediment; Eutrophication』

Introduction
Materials and methods
　Characteristics of water quality in study area
　Sediment samples and analysis
　Phosphorus fractionation
Results and discussion
　Surface sediment characteristics
　Phosphorus distribution in vertical sediment profiles
　Inorganic phosphorus fractionation in surface sediments
　Organic phosphorus fractionation in surface sediments
Conclusions
Acknowledgments
References