Effects Of Potamogeton Crispus On Internal Phosphorus Release And The Underlying Mechanisms

The phosphorus content in sediments is much higher than that in overlying water in natural lakes.After reducing the external phosphorus loading,the phosphorus release from sediments would maintain the nutrient concentrations in overlying water at a relatively high level for a long time（10-15 years）,which seriously hinders the recovery of eutrophic lakes.Therefore,inhibiting the phosphorus release from sediments is an important goal for eutrophication control in lakes.Submerged macrophytes are the main primary producers in lake ecosystems,and they can inhibit phosphorus release by increasing the dissolved oxygen through photosynthesis and the oxygen secretion in roots,and by stabilizing the sediment.Submerged macrophytes are widely used in ecological restoration projects as they have many advantages.It has been reported that submerged macrophytes can inhibit or promote phosphorus release from sediment.The difference in the conclusions was mainly due to the difference in life-history period and existent density of submerged macrophytes.Therefore,it is significant to study the effect of submerged macrophytes on phosphorus release from sediments for ecological restoration.In this study,the Potamogeton crispus,a common submerged macrophyte in the middle and lower reaches of the Yangtze River was selected as the main representative.Firstly,the influence of Potamogeton crispus with different densities in the rapid growth period on phosphorus release from sediments was analyzed through a 50-day mesocosm experiment.Then,the influence of Potamogeton crispus with different densities in the decline period on phosphorus release from sediments was analyzed through a 50-day mesocosm experiment.Finally,the effect of the death and decomposition of Hydrilla verticillata on phosphorus release from sediments was analyzed through a 30-day mesocosm experiment.The main results are as follows:1.During the rapid growth period,the net fluxes of phosphorus release from sediments(F_Net)and total phosphorus concentration（TP）in overlying water showed similar gradients,both were lowest in the High density group（HM）,followed by the Low density group（LM）,and were highest in Control group（Control）,which indicating that Potamogeton crispus inhibited the phosphorus release from sediments during the rapid growth period,and the inhibition effect increased with the increasing density of Potamogeton crispus.Phytoplankton chlorophyll a（Chl a）was lowest in HM,followed by LM,and was highest in Control,and Chl a was significantly negatively correlated with submerged macrophytes biomass(B_Mac)and significantly positively correlated with TP.Therefore,Potamogeton crispus may reduce the"Pumping effect"of phytoplankton by inhibiting phytoplankton growth,thus inhibiting phosphorus release from sediments.2.During the decline period,F_Net and TP in overlying water in HM were significantly higher than those in LM and Control,which indicated that Potamogeton crispus might increase TP in overlying water by releasing phosphorus from the dead plants during the decline period.F_Net was highest in HM.Hypoxia was observed at the bottom layer in HM（DO<2 mg/L）,and DO was negatively correlated with F_Net and TP.Therefore,Potamogeton crispus may promote phosphorus release from sediments by reducing DO at the bottom layer during the decline period.3.During the death and decomposition experiment of Hydrilla verticillata,F_Gross was lowest in HM,followed by LM,and was highest in Control,which indicated that the death and decomposition of Hydrilla verticillata inhibited phosphorus release from sediments,and the inhibition effect increased with the increasing density.Although the death and decomposition of Hydrilla verticillata caused hypoxia at the bottom layer,it did not promote phosphorus release from sediments.The decomposition of Hydrilla verticillate would make a large amount of phosphorus released into the overlying water,which will increase the pressure that phosphorus transferred from overlying water to sediment,and it is an important reason to inhibit phosphorus release from sediments.This study showed that:（1）Potamogeton crispus inhibited phosphorus release from sediments during the rapid growth period,and the inhibition effect increased with the increasing density.Potamogeton crispus may reduce the"Pumping effect"of phytoplankton by inhibiting phytoplankton growth,thus inhibiting phosphorus release from sediments.（2）Potamogeton crispus might increase TP in overlying water during the decline period.Potamogeton crispus may promote phosphorus release from sediments by reducing DO at the bottom layer during the decline period.（3）Decomposition of Hydrilla verticillata inhibited phosphorus release from sediments,and the inhibition effect increased with the increasing density.The decomposition of Hydrilla verticillate would make a large amount of phosphorus released into the overlying water,which might promote the transfer of phosphorus from overlying water to sediment and decrease the phosphorus release from sediments.The main innovations of this thesis are as follows:the effects of submerged macrophytes on phosphorus release from sediments during different life periods and the underlying mechanisms are systematically clarified.The relevant results can provide a scientific basis and technical support for optimizing submerged macrophytes communities in the ecological restoration project of eutrophic lakes.