| Based on the theory and technique of urban river water body ecological restoration,this paper aims to develop the submerged plant cultivation technique and to select suitable submerged plants for the current situation and characteristics of river water in Suzhou.It is hoped that the planting technique will restore and rebuild the submerged plants community,reduce the eutrophication of the rivers,and create a good habitat for the urban rivers,which laying a foundation for the construction and improvement of the river water ecosystem in Suzhou.Through a large number of field investigations and laboratory experiments,this paper draws the following conclusions:(1)Through a one-year survey of the distribution of submerged plants in nine sections of Suzhou urban river from January in 2018 to November in 2018,it was found that the main submerged plants in Suzhou urban river were Cabomba caroliniana,Hydrilla verticillata,Ceratophyllum demersum,Potamogeton crispus,and Vallisneria spiralis.And Cabomba caroliniana had successfully became the invasive species.According to the results of RDA analysis,water SD,TN and Chla have significant effects on the growth and distribution of submerged plants.(2)The response surface design experiment was carried out to study the most suitable illumination,water temperature and planting density for the absorption of nutrients of Elodea canadensis,Vallisneria spiralis,Ceratophyllum demersum and Hydrilla verticillate,as well as the growth.It was found that Elodea canadensis was resistant to low temperature(The optimal growth conditions is 5433 lx,18 ? and 2.69 g/L),but it was poorly grown under high temperature conditions and had low efficiency in removing nutrients.Contrary to Elodea canadensis,Ceratophyllum demersum and Hydrilla verticillata are resistant to high temperature.However,they cannot grow normally and remove nutrients under low temperature conditions(The optimal growth conditions are 6454 lx,33 ?,4.46 g/L and 6474 lx,29 ?,3.37 g/L,respectively).Vallisneria spiralis is resistant to both low temperature and high temperature(The optimal growth conditions is 6061 lx,22 ?,2.60 g/L).It is more suitable for different water environment than other three submerged plants,and is suitable for the new planting technique.(3)The ability of four submerged plants to remove suspended solids and the turbidity resistance of submerged plants under different turbidity conditions were studied under the optimal growth conditions of each submerged plant which was studied in chapter 2.The indoor study on 168 h showed that,when the turbidity of water was 20 NTU,the turbidity change in the control group was higher than that in the blank control group.The turbidity removal rate in the groups of Elodea canadensis,Vallisneria spiralis,Ceratophyllum demersum and Hydrilla verticillate reached the maximum at 168 h,and the removal rate of the suspended matter was 86.31%,64.62%,75.98% and 76.05%,respectively.Compared with the blank control group,the turbidity removal rates of Elodea canadensis,Vallisneria spiralis,Ceratophyllum demersum and Hydrilla verticillate reached the maximum at 96 h,168 h,96 h and 168 h,respectively on the condition of 40 NTU,and the removal rates were 53.66%,64.58%,62.91% and 65.02%,respectively.When the turbidity of water was 60 NTU,the turbidity removal rate reached the maximum in the groups of Elodea canadensis,Vallisneria spiralis,Ceratophyllum demersum and Hydrilla verticillate at 168 h,and the removal rates were 68.29%,44.49%,50.70% and 73.04%,respectively.When the turbidity of water was 80 NTU,the turbidity removal rate of Elodea canadensis,Vallisneria spiralis,Ceratophyllum demersum and Hydrilla verticillate reached the maximum at 96 h,and the removal rates were 67.45%,29.34%,50.11% and 67.03%,respectively.It can be found that,under the condition of 0 ~ 40 NTU,the removal rate of suspended matter of the four submerged plants is basically the same(P > 0.05).However,the removal rate of the suspended matter of Vallisneria spiralis,Ceratophyllum demersum was much lower than that of Elodea canadensis and Hydrilla verticillate(P < 0.05)when the turbidity of water reaches above 40 NTU.It was also found that the growth state of four submerged plants was all relatively better under the condition of 0~40 NTU.When the turbidity of the water became 40 ~ 80 NTU,the growth state of Ceratophyllum demersum and Elodea canadensis was far worse than the initial stage of the experiment(P < 0.05).However,Vallisneria spiralis and Hydrilla verticillata can adapt to higher turbidity water environment.Therefore,Vallisneria spiralis and Hydrilla verticillata are the best submerged plants for the ecological restoration of rivers in Suzhou.(4)In order to solve the problems of poor growth condition of aquatic plant,low self-purification ability and large flow rate changes in the hard and straight river channel.In this experiment,the ecological restoration of river was simulated by hanging wall planting technique and net bed planting technique,completing the restoration and reconstruction of submerged plant communities in rivers and improving water habitats.Using Vallisneria spiralis as the hanging wall planting and net bed planting technique pilot test materials,it was found that Vallisneria spiralis can grow normally by the hanging wall planting technique under the condition of 0~0.22 m/s flow rate.And,the removal rates of TN,TP and NH3-N by this technique were 43.54%,49.00%,and 82.58%,respectively.However,Vallisneria spiralis could not adapt to the flow rate of 0.22 m/s,and the growth state was poor by the net bed technique.The removal rates of TN,TP and NH3-N for water were 46.45%,57.85% and 97.78%,respectively.Therefore,the following conclusions are drawn: the hanging wall planting technique is more suitable for urban rivers where the water velocity is faster but the water body has less impact on the river bank,while the net bed planting technique is applicable to urban rivers with severe eutrophication and low water transparency. |
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