Study On The Causes Of Disappearance Of Vallisneria Natans In Urban Landscape Water

In recent years,the disappearance of submerged plants is frequent.A park in City introduced Vallisneria natans as the main vegetation to restore the water body when treating the eutrophic outer river,which obtained good treatment effect,but in the later stage,Vallisneria natans began to disappear.This research first carried on the preliminary investigation to this eutrophication water.It was found that the nutrient content of the water was higher（TN 8.59mg/L,TP 0.40mg/L）,the phytoplankton（Chl.a）,turbidity and EC were higher,and the transparency was only 0.3 meters.The results showed that the nutrients,the substrate and the light intensity in the water might be the main factors that caused the withering and dying of Vallisneria natans.In order to find the reason of the disappearance of Vallisneria natans,imdoor experiments were set up to study the effect of the bottom material and water quality on Vallisneria natans,and field experiments were carried out to study the effect of water depth and bottom material on Vallisneria natans.Laboratory experimental results showed that the plant height,number of leaves and biomass increased（sediment>quartz sand,fifty days>zero day）,the Chl.b content of leaves decreased,the SOD,MDA,POD and CAT increased then decreased,the MDA and POD content of roots increased then decreased,which indicated that the water body and sediment of the eutrophication water promoted the growth of Vallisneria natans when the light is sufficient,but also had a stress effect on the roots and leaves of Picea,The decrease of nutrition and the decrease of stress.When the light is insufficient,the stress of Vallisneria natans in the substrate is lower than that of quartz sand,so the substrate is more suitable for the growth of Vallisneria natans.In the field experiment,it was found that the water body was always in the state of high nutrients（the photosynthetic characteristics and light tolerance of Vallisneria natans were inhibited）and low transparency（the quantum flux of underwater light decreased）.When the depth is 0.5 meters,the quantum flux is 100μmol/m^2.s,Chl.a and Chl.b content first decreases and then increases.When the water depth is 1.2 meters,the quantum flux is50μmol/m^2.s,Chl.a and Chl.b decreases and then tends to be stable.When the water depth is 2 meters,the quantum flux is zero,the content of Chl.a and Chl.b tends to be zero.In addition,the content of SOD,MDA and CAT in the roots and leaves of Vallisneria natans was 2 m>1.2 m>0.5m in the early stage and decreased sharply in the later stage at 2m,which indicated that Vallisneria natans could grow at the depth of 0.5 m-1.2 m,grew well at 0.5 m,and died at 2 m.It was also found that a tilapia grazed15±5g Vallisneria natans a day.Analyze two experiments,the main reasons for the disappearance of Vallisneria natans in the eutrophication water may be the joint action of the following factors:First,the EC,Chl.a and turbidity are too high,transparency is low,resulting in too low underwater photon flux,suitable for Vallisneria natans growth in water depth is lower.Second,the water body was in high nutrient concentration for a long time,which inhibited the photosynthetic activity and light tolerance ability of Vallisneria natans leaves.Third,the fish eat the Vallisneria natans.