Effects Of Aquatic Plants During Their Decay And Decomposition On Water Quality

In recent years, with the rapid development of population growth and industries, agriculture, stockbreeding, water pollution has become severer. Nitrogen and phosphorus, sediment and other pollutant emissions discharge into the river system, due to the irrational development and utilization of human, causing deterioration of water quality. Eutrophication of water bodies has become the major environmental problems in the world today. Aquatic plants are significant in water environment quality improvement and ecosystem restoration. Aquatic plants as the primary producer of the ecosystem, and the environmental effect produced by them are the foundation of water environment treatment.Aquatic plants can absorb nitrogen and phosphorus in water body during the process of their growth, and they also can enrich and remove the organic matters and heavy metals. At present, using the method of ecological restoration of aquatic vegetation has become an important approach to control of the eutrophic lakes. But in practice, the decomposition of plant residues is likely to cause the secondary pollution of water when aquatic plants enter the decline phase. However, there were some studies had found that keep a certain amount of aquatic plants in autumn and winter season,which would improve the circulation of nutrition element in water and avoid the water quality deterioration. The decomposing process of aquatic plants and their impacts on water quality are different under different plant species and the amount of plant residues. A64-day decomposition experiment was undertaken to understand the effects of the decaying processes of Phragmites australis and Zizania latifolia, Nymphoides peltatum and Nelumbo nucifera, Potamogeton crispus and Myriophyllum verticillatum on the water quality under three plant biomass densities. The early spring’s temperature was simulated when aquatic plants were decomposed in large-scale, used common6aquatic plants as the experiment material in Taihu Lake. This research investigated the decomposing process of aquatic plants and their influence on water quality. To study the environmental influence of aquatic plants decomposing process. The result shows:(1) The responses of water quality to plant decomposition were varied by plant species. For Phragmites australis, the water bodies presented the lowest concentrations of total nitrogen, chemical oxygen demand and total phosphorus. For Zizania latifolia, the concentrations of total nitrogen and chemical oxygen demand would increase in the late period of its decomposition, and lead to the deterioration of water quality. The concentrations of total nitrogen and chemical oxygen demand in containers with Nymphoides peltatum and Nelumbo nucifera were higher than other four plants. In contrast, Potamogeton crispus and Myriophyllum verticillatum had the highest concentrations of total phosphorus, ammonia nitrogens and nitrates during the decomposition.(2) The effects of the decomposition processes of aquatic plants on the water quality were dependent on the plant biomass density. For a certain plant species, the main water quality indicators had same treads under different biomass densities. Under the condition of the experiment, the biomass density of0.2g-L-1had lower nitrogen content in the water. And the nutrient cycle of nitrogen and phosphorus can be effectively enhanced at relatively moderate plant residues. Overall, our results suggested that nitrates can be availably reduced to some extent by retaining a small amount of aquatic plant residues, thereby nitrogen loading of the system can be decreased in eutrophic waters. (3) The decomposition processes of aquatic plants nutrient had an influence on the recycling process of elements in sediment. At the end of the experiment, the content of organic carbon in the sediment was increased, while the contents of nitrogen and phosphorus were reduced. The results indicated that the existence of the aquatic plant was beneficial to the nitrification and denitrification, and it also had a certain role in promoting the internal phosphorus.This study investigated the decomposing process of aquatic plants and their impacts on the water quality. Result showed that at the early stage of the decomposition of aquatic plants, it had lead to the high concentration of nitrogen and phosphorus and local hypoxia conditions. While the concentrations of the nitrates and nitrites would decrease in the late period of decomposition. Overall, our results suggested the existence of the aquatic plant would increase the dissolved oxygen of the water and promote the nitrification. In addition, nitrates can be reduced to some extent by retaining a small amount of aquatic plant residues, thereby nitrogen loading can be decreased in eutrophicated waters. However, a moderate amount of the harvest and salvaging residues of appropriate aquatic plants in non-growing season, in order to reduce the secondary pollution caused by the decomposing process of aquatic plants.