Study On Phosphorus Release From The Sediments And Its Influencing Factors In Rongcheng Swan Lake

Rongcheng Swan Lake is a natural coastal lagoon. In recent years, the ecologicalenvironment of Swan Lake has deteriorated as a result of frequent human activities, and alarge scal of filamentous green algae have broken out frequently. In present paper, trophicstatus and binding forms of phosphorus （P） in sediments from Swan Lake wereinvestigated, and the effects of environment factors and the breakout of Cladophora sp.（the growth and decomposition stages） on P release from the sediments were studied. Theinfluence mechanisms of pH and dissolved oxygen （DO） on P release and the releasepotential from the sediments in different lake regions were clarified. The main results weregiven as follows:（1） Total phosphorus （TP） concentrations in surface sediments from Swan Lakeranged from79.15mg·kg^-1to616.31mg·kg^-1. TP and various P forms in sedimentsshowed great spatial difference. High TP concentrations were found in northwestern andcentral regions, and low TP concentration in the southeast （a prawn pond） and tidal deltaof the lake. The concentrations of potentially bioavailable P （sum of organic P andiron/aluminum binding P） were much higher in the northwest and west end of Swan Lake.（2） The pH value in overlying water had a great influence on P release from the dryand fresh sediments in Swan Lake. P release amount was the lowest under the neutralconditions, while it increased under acidic and alkaline conditions. For the sediments inthe northwest and west regions, P release amounts were higher under alkaline condition,while for the other regions, the sediments had strong release capacity only under acidiccondition. The sediments in northwestern area showed great release capacity under bothacidic and alkaline conditions. Because water pH was weak alkaline in Swan Lake, thesediments in the north and west regions had high potential of P release.（3） Dissolved oxygen （DO） level had obvious affect on P release from the sedimentsin Swan Lake. P release amounts from the sediments were significantly greater underanaerobic condition than that under aerobic condition. The anaerobic condition can provesediments to release P, while aerobic condition was disadvantage for P release. There was great difference in P release capacity among different regions. P release amounts from thenorthwestern sediments were larger, with a maxminm of12.56mg·kg^-1under anaerobiccondition, which was only3.77mg·kg^-1for the western sediments.（4） Indoors incubation experiments showed that Cladophora sp. growed well inenriched sea water, whose biomass were much higher than that original sea water.Cladophora had great accumulation ability for P, and it can reduce P concentration inoverlying water obviously. In enriched treatments, soluble reactive P （SRP） concentrationdecreased by91.22%⁹3.84%. P concentration in the sediments also declined during thegrowth period of Cladophora, and the greatest impact was observed on redox-sensitivephosphorus （BD-P）.（5） Due to the decomposition of Cladophora, DO concentration in overlying waterdeclined significantly and reached the anoxic condition (DO<0.20mg L^-1). pH value ofdifferent treatments also decreased. During the algal decomposition, SRP concentrations inoverlying water increased significantly, ranging at0.01¹.51mg L^-1. The decompositionof Cladophora had an obvious effect on the P level of lake water. In dominated zone ofCladophora, the released P from sediments constituted the most part of P in overlyingwater, while in non–dominated zone, the released P from algal decomposition were higherthan that from the sediments.（6） Because of Cladophora decomposition, concentrations of TP and different Pforms all increased obviously, and the increasing extent was larger with the adding algaeamount. TP concentrations of different treatments varied from0.04mg·L^-1to1.34mg·L^-1.In early period of experiment, the rate of decomposition was fast and P release fromCladophora decomposion was the main source in overlying water, while P release fromsediments was the source of P in later period. The P forms released from decomposingCladophora were mainly dissolved organic phosphorus （DOP） and particulate phosphorus（PP）.