Phosphorus Speciation And Its Environmental Significance In Sediments Of Major Rivers In Chaohu Lake Watershed,China

Phosphorus(P)is an essential nutrient to support aquatic plant growth,which can also lead to outbreak of harmful algaes and lake Eutrophication.Therefore more attention is paid on P cycling in ecosystem,especially the P which is sent to rivers or lakes due to agricultural runoff、domestic effluents and industrial wastewaters.River sediment is not only the source but also the sink of many pollutants.The role of P in river sediment depends on its form.The form、concentration、release and bioavailability of P can monitor the regional environment and reveal P transformation and exchange between sediment and overlying water as well.With the rapid development of the urban and economy,water quality and safety are playing important role in ecological environment.Eutrophication is a common problem,especially serious in Chaohu Lake.The main inflow rivers that are heavily polluted all located in the upper lake where water bodies receive excess nutrients,causing an accelerated growth of algae.The anthropogenic P inputs increased quickly due to urban and industry development and the increasing non-point pollution caused by untreated domestic discharge and runoff losses.For main inflow rivers in Chaohu Lake watershed,if external phosphorus sources have been reduced,the release of phosphorus from sediments is sufficient to maintain anthropogenic Eutrophication for long periods in many rivers.Therefore,research on P distribution and its environmental significance in surface sediments from rivers in Chaohu Lake Watershed is necessary and of great significance.In the paper,Total Phosphorus(TP)concentration in surface soils、surface sediments of Chaohu Lake and surface sediments from main inflow river of Chaohu Lake were investigated in order to analyze P transformation and exchange.Fractionation、spatial distribution and bioavailability of P in 33 surface sediments from 9 inflow rivers in Chaohu Lake watershed were analyzed,used by various extraction(sequential)methods.According to the scheme,P can be extracted in following forms:Soluble and loosely bound phosphorus(L-P),A1 bound phosphorus(Al-P),Fe bound phosphorus(Fe-P),reductant soluble phosphorus(RS-P),Ca bound phosphorus(Ca-P)and Residual phosphorus(Res-P).Phosphate adsorption isotherm experiment wascarried out to assess the risk of P release.Results are as following:1.In term of TP,content was totally different in surface soil around Chaohu、surface sediments from Chaohu、surface sediments from main inflow rivers.TP content in surface sediments from Chaohu inflowing rivers was the largest while TP content in surface sediments from Chaohu was the lowest.Runoff、domestic effluents、industrial wastewaters and selective erosion due to rainfall may be factors affecting this type of TP distribution.Surface sediments from rivers close to the lake had low TP content because it would release to overlyingwater.2.Fractionation of P shows that Fe-P、Ca-P、Al-P and Res-P were the main P fractions in the sediments from main inflow rivers.The average TP content of sediments from urban rivers was 1119 mgkg-1 while the average TP content of sediments from agricultural rivers was 808 mgkg-1.However,the average contents of Res-P and RS-P in sediments from urban rivers were lower than those in sediments from agricultural rivers.The content of L-P、Al-P、Ca-P and Fe-P in sediments from urban rivers was 1.86、3.53、1.11 and 1.68 times as much as L-P、Al-P、Ca-P and Fe-P in sediments from agricultural rivers,respectively.3.For urban rivers the average contents of P in the various chemical forms in sediments were in the order:Fe-P>Al-P>Ca-P>RS-P>L-P.For agricultural rivers the average contents of P in the various chemical forms in sediments were in the order:Fe-P>Ca-P>Al-P>RS-P>L-P.4.For typical urban river Nanfei,the spatial distributions of L-P、Al-P、Fe-P and RS-P were basically similar.L-P、Al-P、Fe-P and RS-P contents in sediments totally increased obviously from the upper reach to lower one.And the tendency is more obvious at he sample sites near the Chaohu.The spatial distributions of Ca-P and Res-P were basically similar.Ca-P and Res-P contents in sediments totally decreased obviously from the upper reach to lower one.L-P、Al-P、Fe-P can be regarded as important parameters which can reflect anthropogenic original P.5.For agricultural rivers,TP content in sediments decreased obviously from the upper reach to lower one.And the tendency is less obvious at the sample sites near the Chaohu.The spatial distribution of L-P、Al-P、Fe-P、RS-P、Ca-P and Res-P were basically similar to that of TP.6.Bioavailable phosphorus(BAP)content in sediments varied from 111.6 to1512.7 mgkg-1.BAP content was higher in sediments from urban rivers than in those from agricultural rivers.The value of BAP to TP ranged from 19.2%to86.3%with average 51.2%.7.L-P and Ca-P were both positively correlated to CaO content in the sediments.TP、L-P、Al-P、Fe-P、Ca-P and Res-P were both positively correlated to OM content in the sediments.BAP was positively correlated to TP、OM content in sediments and pH in overlying water.8.In the adsorption isotherm experiment,Pr can be described as P that can be released from sediment.Different Pr was in different sediment,Pr mainly consisted of L-P、Al-P and Ca-P.The change of pH caused by emission of sewage and wastewater can influence P release.9.Equilibrium phosphorus concentration(EPC0)content in sediments varied from 0.005 to 0.324mgL-1.EPC0 content was higher in sediments from urban rivers than in those from agricultural rivers.For the same river,EPC0 content decreased from the upper to lower reach.EPCo was positively correlated to CaO、OM content in sediments and Cond in overlying water.EPC0 was negatively correlated to DO in overlying water.10.EPC0 value was high in the sediments from Nanfei River.However,compared with soluble reactive phosphorus(SRP)of the overlying water,it was still low.According to the P release risk based on EPC0,the estuarine sediment of the Nanfei River where EPCo was lower than SRP tended to act as sink and would absorb P from overlying water.When it comes to the dredging of Nanfei River,different measures should be taken carefully according to the different parts of Nanfei River.It is wise and necessary to analyze the P release risk based on EPC0 before taking measures.When facing rivers whose sediment may act as sink of P,large dredging project should be avoided or carried seriously.