Study On The Succession Trend Of Submerged Macrophyte Community In Restored Shallow Lake

Submerged macrophytes can reduce resuspension and sediment phosphorus release,improve sediment activity,absorb nutrients from water,and help maintain clear-water conditions in shallow lakes.Reestablishment of the macrophyte community is therefore of key importance in lake restoration.But the submerged macrophyte community reestablished often collapsed in 3-5 years after lake restoration.As the vegetation biomass decreased and even disappeared,water clarity declined and water quality deteriorated,the turbid-water conditions dominated by phytoplankton would return.In view of this phenomenon,we propose the following hypothesis: the submerged macrophyte community reestablished will be affected by interspecific competition,fish herbivory and water nutrient increase.With irreversible succession,species richness and biomass of submerged macrophytes decreased,gradually being replaced by planktonic algae,and thus resulting water quality decline.To test the hypothesis,we conducted several field experiments.The competition between different growing forms of submerged plants and also different submerged plants and their consequences were studied in the first and second experiment.The regularity of nutrient release in aquatic plant degradation process and its effect on water quality were studied in the secondthird experiment.The effects of fish selective herbivory on the competition among different species of submerged plants and the consequences were studied in the last experiment.Through the above studies,we discussed the succession trend of the submerged macrophyte communities rebuilt impacted by interspecific competition,water nutrient increase(released from the dead submerged plants),and fish selected herbivory and its effect on water quality after lake restoration in this paper,and therefore provide theoretical support for the ecological restoration of eutrophication of lakes and long-term management.The results showed that: 1)the results of competition among different life forming(canopy-forming and meadow-forming)submerged macrophytes were that the canopy formers gradually become the dominant species,and meadow type submerged plant will gradually become the companion species and even die out in lake ecosystem.In this process,water quality of the treatments with meadow formers was better than that of the treatments with canopy formers.Particularly,TN and TP in the treatments with meadow formers were less than that in the treatments with canopy formers.Hence,meadow-forming macrophytes have a better ability to improve water quality than canopy-forming ones.2)competition trend of different canopy-forming submerged macropyte species is that stronger competitive canopy former gradually become the dominant species,and the weak competitiveness one will gradually become companion species or even disappear in lake ecosystem.In this process,the two canopy-forming species both decreased TN in the water body,and the comprehensive water quality index of the Hydrilla verticillata group was better than that of the Myriophyllum spicatum group and the mixed group.Accordingly,canopy-forming submerged macrophytes are effective to reduce nutrient concentration in the water body,and H.verticillata is better than M.spicatum.3)the rules of nitrogen and phosphorus release in the process of degradation of different types of aquatic plants are different,and the plant debris released lots of nutrient into water,so it is necessary to remove part of the plant residues after a large number of vegetation death in the lake ecological restoration.4)Herbivorous fish(Megalobrama amblycephala)preferred feeding canopy-forming submerged plant.Selective herbivory can inhibit the competitive advantage of canopy formers to assist the meadow-forming submerged plant building group,and can also reduce plant residual,and thus reducing the endogenous pollution of lake water body.The results show that submerged plant communities will gradually turn to single canopy-forming species and the vegetation will decline due to interspecific competition after the ecological restoration in eutrophic lakes.The succession is enhanced by the release of nutrient salts of dead plant.Stocking of proper densities of Wuchang bream may help control the canopy-forming species and maintain meadow-forming species and thus the clear-water state.Hence it is recommended to be a management method after lake restoration.