Effects Of Aquatic Re-vegetation On Phosphorus Fractions In And Release From The Sediments In Urban Rivers

Rapid urbanization has caused serious eutrophication in urban rivers. Restoration of aquatic plants is the most effective way for treatment and control of water eutrophication. However, few studies have focused on the assessing effectiveness of eco-restoration. As external phosphorus inputs have been controlled effectively, internal phosphorus release from sediments can't be ignored. In this thesis, long-term effects of revegetation have been studied in two urban rivers (Liwa River and Caoyang River) with eco-restoration of different time. Phosphorus fractions and release from sediments have been studied by both in-situ field investigation and lab culture experiments. The results are as followings.(1) The relative abundance of phosphorus fractions in sediments are:Ca-P> OcP>Fe-P,RP>Ex-P>AI-P. Ex-P and Al-P, Fe-P and Ca-P respectively have a significant correlation. Changes of phosphorus fractions are responsible for phosphorus release. Among all the fractions, Ex-P can directly affect phosphorus concentration in overlying water, while it can be released though interstitial water of sediments.(2) Aquatic plant restoration can reduce the release of phosphorus in sediments. But the effectiveness varies with life forms of hydrophytes, restoration time and community composition. The longer the restoration time, the more Ca-P in sediment. Comparing with submerged plants, floating-leaved plants and emerged plants are more capable of oxidizing rhizosphere to inhibit the release of phosphorus by increasing Fe-P. Mixed planting of different kinds of aquatic plants can occupy more space in the sediments due to varied root length, it shows the highest ability to immobilize phosphorus. Heavy rains disturbance can cause phosphorus release from sediments, and aquatic revegetation can effectively restrain the impact of the heavy rains. The longer the restoration time, the more obvious the effect. The wrong way of aquatic plants harvest brings direct disturbance of sediments, which will promote the sediment suspension and phosphorus release in a certain period.(3) Temperature has a significant effect on the phosphorus release in sediments. Phosphorus release increases with temperature rises. Phosphorus release and release rate significantly increase at15℃possiblely due to the decomposition of a substantial number of organic phosphorus. At this time, DIP in water are:Caoyang River> Changfeng Park> Liwa River. DIP changes suggest that the liberation of organic matter may cause eutrophication in early spring. To reduce phosphorus and organic matter in sediments play an important part on limitation of DIP rise in early spring. DTP release rates of Caoyang and Liwa River are low when the temperature just begins to rise. In the cultivation process from10℃to28℃, DTP release rate of Changfeng Park is higher than that of Liwa River. In the cultivation process from15℃to28℃, DTP release rate of Caoyang River maintains stability, which will be little effected by temperature rise. Average DTP release rates suggest that aquatic revegetation can restrain the increase of phosphorus release rate caused by temperature rise when season changes. The longer the restoration time, the less the effect of temperature rise.(4) Redox potential has also showed effects on phosphorus release. The sequence of release rate is highly anaerobic> aerobic> anaerobic treatments. Highly anaerobic and anaerobic conditions respectively influent the formation of Fe-P effected by sulfate or nitrate reduction, which lead to the difference of the amount of P release from sediments. In oxidation condition, the decomposition of organic matter is the main cause of phosphorus release. At the early and middle stages of treatments, DTP release rate increases first then falls down. DTP release rate in the late treatments is highly anaerobic> anaerobic> aerobic conditions. Ex-P is lower in anaerobic condition, while Al-P obviously decreases in highly anaerobic condition. Therefore, aquatic plant growth will eventually reduce phosphorus release rate in spring and summer. When plants died in winter, release rate will increase, finally. Phosphorus release of Changfeng Park is more than that of Liwa or Caoyang River in all the conditions. Aquatic revegetation reduces the contents of Ex-P, Al-P and Fe-P in sediments, which are bioavailable. Thus, phosphorus release is a little and slow. It reduces the influence of redox potential.In this thesis, results show that aquatic revegetation can effectively restrain the, and have good ecological effects. Aquatic plants can inhibit phosphorus release from sediments by rhizosphere oxidization and reducing the disturbance effect caused by heavy rains. At the same time, aquatic plants uptake nutrients uptake and enhance biodiversity and biomass of aquatic ecosystems. Aquatic revegetation causes the changes of phosphorus fractions, which fundamentally reduces phosphorus release from sediments. It also reduces the influence of redox potential and temperature rise during the periodic aquatic plant growth process. Therefore, to reduce sedimentary of phosphorus and organic matter is conducive to alleviate eutrophication and control phosphorus release in the aquatic vegetation restoration process. In addition, water quality results suggest that comprehensive standard in which biological index is dominated should be used to evaluate the ecological effects of the long-term aquatic revegetation.