Microcosm Study On Phosphorus Exchange On Water-sediment Interface In Taihu Lake

This paper present one of the global environmental problems, Lake eutrophication. Taihu Lake, which is the typical shallow lake and located on the middle and lower reaches of Yangtse Rive, was chosen to study on the micro-entironment of phosphorus exchange on water-sediment interface. According to the equilibrium characteristics of phosphorus adsorption/desorption in three different trophic level sediments in Taihu Lake, temperature and biological factors were the most important factors influenced on phosphorus exchange on water-sediment interface. Then sediments from Meiliang Bay were chosen to investigate the phosphorus release from sediment under high temperature, various light intensities and different disturbance force level conditions in the laboratory experiment. Furthermore biological factors (algae and submerged vegetation) were added in the simulative experimental system to research the influence of phosphorus exchange on water-sediment interface. The results as following:(1)The concentrations of total phosphorus and their fractions were coincided with their trophic states of the overlying water, and changed with seasons. Therefore the characteristics of phosphorus adsorption/desorption process in the water-sediment system were affected by seasons and trophic states. The change trend of adsorption efficiency and the equilibrium adsorption quantity were the same. For the same Lakeland, the changes of equilibrium phosphorus concentration of adsorption/desorption on water-sediment interface were significantly influenced by season factor. For the same season, equilibrium concentration of phosphorus is significantly positive correlation with the trophic level of the sediment. Equilibrium concentration of phosphorus increased with temperature rise and phosphorus concentration in sediment decreased respectively. Temperature variance has significant effects on the adsorption/ desorption process of phosphorus. Desorption process plays big role in all the studied regions in summer,and adsorption process is much important in spring. NaOH extractable phosphorus is the most active phosphorus fractions for the adsorption/desorption process.(2)The competition between two common algae species (Microcystis aeruginosa and Scenedesmus quadricauda) and release process of phosphorus at the water-sediment interface in Taihu Lake at various light intensities in summer were investigated by laboratory experiment. Under high temperature, phosphorus could release from sediment, which accounted for 15.7~18.4% of the total phosphorus in the sediment. HCl extractable phosphorus and Organic phosphorus in the sediments were important contributors during the phosphorus release process. However NaOH extractable phosphorus was active with strong light irradiation, and was not active with low light irradiation. Visible algae bloom could be caused by phosphorus released from sediment at strong light irradiation during summer even no external pollutant input. However, in the no or low light irradiation (<500Lx), the algae could not live in the overlying water. The maximum biomass and growth rate of Microcystis aeruginosa are much higher than those of Scenedesmus quadricauda, which reinforced with the increase of light intensity. Therefore, strong light irradiation and high temperature maybe the important reasons for the frequent algae bloom in summer. In the summer conditions, abnormal growth of alga keeps the low concentration of soluble phosphorus in the overlying water, which promoted increase of alkaline phosphatase activity and further promotes the transformation of OP and release of phosphorus from the sediment, this maybe lead to a vicious circle of frequent algae blooms.(3)The influence of phosphorus exchange on water-sediment interface under disturbance and whether or not having submerged vegetation conditions were investigated in the laboratory experiment. Under experimental condition (light intensities: 4500Lx, temperature: 30℃), Hydrilla verticillata not only adsorbed phosphorus in overlying water for growth but also decreased phosphorus release from sediment, which restrain algae propagation and control the phosphorus in sediment. We also find that organic matter in sediment was sharply utilized by Hydrilla verticillata. However the growth of Hydrilla verticillata was restrained obviously under intensive disturbance.