Spatial Distribution Characteristics Of Phosphorus Forms And Bioavailable Analysis In Surface Sediment Of Gufu River

Phosphorus is an important nutrient element and could be able to be absorbed and utilized by phytoplankton living in the water, then the phytoplankton would bloom which could lead to eutrophication of water body. Eutrophication has been attention at home and abroad, while the input of exogenous phosphorus pollutants have been greatly reduced, but the phosphorus in the water hasn’t significantly reduced as expected, then many scholars have suspected endogenous phosphorus was likely the main reason of leading to the phenomenon. The sediment plays an important role in the biogeochemical cycle of phosphorus, it is an important sink of phosphorus, and is also an important part of freshwater ecosystems. The sediment not only could release phosphorus to the water, but also could accept phosphorus from the water. Phosphorus in sediment would present different forms according to the changes of surrounding environment, and different forms of phosphorus have different chemical properties. The contents and distribution characteristics of phosphorus in sediment are important ways to reveal the mechanism and effect factors of eutrophication. So study about phosphorus forms in sediment has a very important significance.This study is supported by National Natural Science foundation project "A research on spatiotemporal evolution patterns of algae diversity and primary productivity in rich phosphorus river of geological origin of Three Gorge Reservoir"(40972218/D0218), and analyze the contents and distribution characteristics of phosphorus forms in surface sediment of Gufu River, which belongs to one of the three major tributaries of Three Gorges——Xiangxi River, and we have understood preliminarily the spatial distribution rules and mutual transformation of phosphorus forms in surface sediment of Gufu River. Then we have found out that the content and distribution rules of bioavailable phosphorus and release risk, which could provide scientific theoretical basis for preventing and controlling the eutrophication of Xiangxi River even Three Gorges Reservoir.There are all12sample points, and the sediment samples are taken once a month from August,2010to July,2011, except December,2011, adoptting the methods of sampling in the wild and determination indoor. The phosphorus forms in surface sediment are determined by chemical sequential extraction proposed by Jin Xiangcan, then we clarified the forms and contents of phosphorus in surface sediment of Gufu River. There were six phosphorus forms in the study, namely organic phosphorus (OP), dissolved phosphorus (DP), Iron bound phosphorus (Fe-P), aluminum bound phosphorus (Al-P), Calcium bound phosphorus (Ca-P) and occluded phosphorus (O-P).The analysis results about phosphorus in surface sediment of Gufu River show that the difference of contents of phosphorus forms is relatively large, and the rank order of all phosphorus forms from big to small was O-P> Ca-P> OP> Fe-P> DP> Al-P. The annual average contents of total phosphorus (TP) range from293.09to50595.15mg/kg; Inorganic phosphorus (IP) is the major phosphorus forms including Fe-P, Al-P, Ca-P and O-P, and the range is258.19.09～560.02mg/kg, accounting for about91.97%averagely. However, the annual average content of OP range from22.14to37.50mg/kg, and the OP/TP ratio account for8.03%only; The variation range of O-P is156.43～291.20mg/kg, and most annual mean values of O-P have exceeded50%; the annual average content of Ca-P was only less than O-P, and range from53.97to230.00mg/kg, accounting for28.10%of the TP; The changes of annual average contents of Al-P, Fe-P and DP are7.92～10.59mg/kg,14.98～22.79mg/kg,8.55～12.40mg/kg, accounting for2.23%,4.50%and2.54%of TP respectively.To study the possible mutual conversion process between various phosphorus forms in surface sediment of Gufu River, we have analyzed the correlation between various phosphorus forms and the results show that there is some certain transformation relationship between DP and Fe/Al-P, namely DP may transform into Fe/Al-P in the oxidative environment, but Fe/Al-P could occur reduction reaction in the reductive environment, and release phosphate and then transform to DP; Although the content of DP may increase after the mineralization of OP, OP would not convert into DP in some extent; Because OP will mineralize under the effect of microorganisms and release the phosphate after decomposing, then the phosphate may be coupled with the active Fe3+,Al3+and Ca2+in sediment and transform into Fe-P, Al-P and Ca-P, so that there are transformation between Fe-P, Al-P and Ca-P; there is no conversion between O-P and other various phosphorus forms, which can be speculated that O-P mainly comes from the product of weathering of rocks. However, biogeochemical cycle of phosphorus in sediment is quite complex, so the process and mechanism of transformation between various phosphorus forms remains to be further research.The spatial distribution characteristics of TP and other various phosphorus forms in the surface sediment of Gufu River have great similarities, and present obvious regularities. For the trunk stream of Gufu River, the contents of TP, O-P, Ca-P, DP and Al-P remain the same from GF01to GF05basically, but show the tendency of growth from GF06to GF09. The contents of Fe-P and OP have no obvious regularity and present a larger volatility, but there is an increased tendency from GF06to GF09like other phosphorus forms. GF03and GF04are two abnormal points in the distribution of Fe-P and the contents of which are all higher than other points; For the tributary of Gufu River, Zhuyuan River, its distribution rules of all phosphorus forms in surface sediment are not the same with the river reach from GF06to GF09which also located in XingShan County. The contents of all phosphorus forms decrease in turn from ZY01to ZY03, and all the annual average contents of all phosphorus forms are higher than the river reach from GF06to GF09except Fe-P. The study divide Gufu river into three river reaches including Gufu-Shennongjia, Gufu-Xingshan County and Zhuyuan-Xingshan County for analyzing the effect of human activities to the surface sediment of Gufu River, and the results show that the contribution of human activities to phosphorus in sediment is small.This study analyzes the species and distribution characteristics of bioavailable phosphorus in order to research the risk of phosphorus in surface sediment of Gufu River, and come to the conclusions:bioavailable phosphorus in surface sediment contains four kinds of phosphorus forms, including OP, DP, Al-P and Fe-P. For the trunk stream of Gufu River, the distribution of bioavailable phosphorus present a great fluctuation but there is an obvious regularity except an abnormal point, GF04. There is no big change from GF01to GF06but present a growing trend from GF07to GF09; For the tributary of Gufu River, Zhuyuan River, the contents of bioavailable phosphorus in surface sediment decrease gradually from ZY01to ZY03. The annual average content of bioavailable phosphorus in the surface sediment of Gufu River range from54.93to83.28mg/kg, accounting for17.30%of TP only, which show that only a small fraction of phosphorus in surface sediment may be used by organisms and the risk of phosphorus is small. Then we speculate that the surface sediment of Gufu River plays the role of phosphorus "sink" and the endogenous phosphorus is not the main factor of threating the eutrophication of overlying water. So that this study propound some protection measures about water of Gufu River, On the one hand, we must strengthen management and reduce the phosphorus pollutants which produced by human activities into water body to control the input of exogenous phosphorus; On the other hand, we must prevent acidification of water body to control the release of Ca-P in surface sediment of Gufu River.