Phosphorus(P) species in surface sediments from a shallow,hypertrophic lake,Lake Dianchi,China,were investigated by P fractionation and ^(31)P nuclear magnetic resonance(NMR) spectroscopy during a regional algal bloom.In addition,their potential contributions to the overlying water were also evaluated.Labile fractions of P extracted by NH_4C1,bicarbonate dithionite and NaOH ranged from 340.6 to 1,725.8 mg kg^(-1),accounting for20.5%-67.2%of the total P.A two-step extraction method refinement of P recovery was performed before the^(31)P NMR analysis.Recovery rates of sedimentary TP and organic P,by combining EDTA pretreatment and NaOH or mixed reagents,ranged from 31.8%to 69.3%and from19.8%to 51.7%.^(31)P NMR results showed that ortho-P and monoester-P were the most abundant P components in the sediment extractable P of sediments,followed by diester-P and pyro-P.Spatial distribution of the sum of ortho-P,diester-P and pyro-P detected by P NMR corresponded well with the labile P concentration determined by fractionation.Both exhibited a significant positive correlation with the total P in the water column,suggesting that internal loading may be an important source of P for the lake ecosystem.Biogenic P other than ortho-P may contribute to phytoplankton growth,with the relative proportion being 4.4%-18.7%.The release of labile P fractions fueled algal bloom,and the decay of organic matter,following the bloom events,consumed oxygen and elevated the pH value.This co-dependence might lead to a vicious cycle.Transformation mechanisms of various P species remain ambiguous and are worthy of further investigation.
The distribution characteristics of phosphorus(P) forms in the sediment profile of Lake Erhai,in southwestern China has been investigated by sequential extraction and 31P nuclear magnetic resonance spectroscopy(NMR) of NaOH extracts to understand P dynamics and its potential contribution to lake eutrophication.Contents of P fractions varied in the order of NH4Cl extracted P(NH4ClP) < bicarbonate-dithionite extracted P(BD-P) < HCl-P,Residual-P < NaOH extracted P(NaOH-P).The highly available NH4Cl-P represented less than 1% of total P(TP).BD-P and NaOH extracted reactive P(NaOH-rP) averaged 39%,while the ratio of Fe/P was higher than 15,indicating low P release from the sediments under permanent oxic condition.The less bio-available HCl-P,NaOH extracted nonreactive P(NaOH-nrP) and residual-P contributed 61% of TP.Regression analysis revealed that BD-P,NaOH-rP and HCl-P were positively correlated with the contents of Fe and Mn,Al and Fe,and Ca,respectively.The investigation of P compound groups in NaOH extracts by 31 P NMR showed that ortho-P and monoester-P were the largest two constituents of the P pool,followed by diester-P,phosphonate and pyro-P.A comparison of vertical variations of P groups in the sediment profile suggested that these compounds were involved in the P recycling to different extents in Lake Erhai.In particular,the lake exhibits high potential for labile P release from the surface sediments,which should be taken into consideration even after the outsourced P runoff ceased.
