The Feeding Of Limnodrilus Hoffmeisteri And Its Impact On Nutrient Cycling Between Water-Sediment

Because of the rapid development of the economy, human activities impose a tremendous impact on the environment of rivers, lakes and seas, and the ecosystem have been damaged, the most common and serious problems are eutrophication. Lake Taihu located in Yangtze River Delta area is the third largest freshwater lake and an important drinking water source for Wuxi and Suzhou City. In recent years, the continually increasing of surrounding area's population density and the rapid development of economy make rapid eutrophication in Lake Taihu. Because of eutrophication, frequent occurrence of cyanobacteria and bloom in Lake Taihu, also the biodiversity in the lake decline significantly, for example, Oligochaete Tubificidae is the dominant macrobenthos in Lake Taihu now, when cyanobacteria bloom occurs in Lake Taihu, Scenedesmus obliquus is also very common, portion of cyanobacteria and Scenedesmus obliquus sink to the surface of sediments, and will the tubificidae living in sediments feed these alage? Will the tubificidae promote cyanobacteria decomposition? And it is worth attention, in order to understand food web and material recycling of Lake Taihu, and provide the theory basis for protecting and managing Lake Taihu. At the same time, one of the reasons that cause eutrophication in Lake Taihu is that nitrogen and phosphorus nutrient excess in water. The sediments are the important reservoirs of mineral nutrients, it contains organic matter, nitrogen, phosphorus, carbon, sulfur, silicon, iron and metal ion, while tubificidae's entire life cycle is in the sediments, which affects inorganic nitrogen and phosphorus exchange at the sediment-water interface by its bioturbation. All these questions are worth deep studying. This research will focus on tubificidae in Meiliang Bay, Lake Taihu, by means of microcosm experiments, on the one hand we used carbon and nitrogen stable istope to trace the selected feeding of tubificidae on cyanobacteria and Scenedesmus obliquus and the influence on decomposition of settled cyanobacteria; on the other we investigated the effects of phosphorus and dissolved inorganic nitrogen (DIN) exchange between sediment-water interface by tubificidae's bioturbation.We used NaH13CO3 to label Microcvstis and used 15N-NH4+ to label Scenedesmus. then grated these dry labeled Microcvstis and Scenedesmus. We added same amounts of Microcvstis and Scenedesmus mixed with some sedments to test system, then microcosm experiments would be conducted. The concentrations of TN&TP in overlying water of experimental system would be analysed at different time. We also measuredδ15N andδ13C of tubificidae, particulate organic matter in overlying water at every sampling time. Uptake of 13C (and similarly 15N) by the tubificidae at different time was calculated as excess above background and is expressed as specific uptakeΔδ13C (Δδ13C=δ13Csamplc-δ13Cbackground).A selectivity index (Q) was defined as the quotientΔδ13C/Δδ15N, it would help us understand which food tubificidae prefer more at different time. The results showed that:the concentrations of TN&TP in overlying water in control were higher than in experimental group in whole study, these may be relationship with tubificidae's feeding of alage. Theδ15N of tubificidae didn't change in whole study, this told us that tubificidae maybe don't feed Scenedesmus; but theδ13C of tubificidae was higher than before feeding Microcystis, especially in the preliminary study, so the conclusion was that Microcvstis was the food of tubificidae. TheΔδ15N of tubificidae was nearly 0 on Day 7, but theΔδ13C was maximum at the same day, so the selectivity Q was also maximum, this told us that tubificidae maybe feed Microcvstis only before Day 7. This experiment told us that:because Microcvstis is the absolute dominant species in Lake Taihu, tubificidae maybe prefer to Microcvstis in our study for adapting to environment.In the research of tubificidae's bioturbation, based on the natural distribution of macrobenthos of Meiliang Bay, Lake Taihu, we used sediment-water microcosms to study the effects of Limnodrilus hoffmeisteri on the release of inorganic nitrogen and phosphorus from sediments to overlying water. Based on contrastive analysis of phosphorus and dissolved inorganic nitrogen (DIN) exchange between sediment-water interface, recursive rule and the difference of DIN profile at the sediments with and without tubificidae worms activities, the impact and mechanism of Limnodrilus hoffmeisteri (Oligochaeta) activities on the phosphorus and DIN exchange at the sediment-water interface had been studied. It was found that the bioturbation of Limnodrilus hoffmeisteri caused an enhancement of inorganic nitrogen (mainly ammonium nitrogen) and phosphate release from lake sediments to overlying water. Tubificidae's densities markedly influenced the concentration of dissolved inorganic nitrogen(DIN) in sediments and overlying water, and the physiological mechanisms-excretion of tubificidae influenced the release of phosphate. Tubificidae's excretion, bioturbation and the microorganism introduced by tubificidae accelerated the denitrification of nitrogen, accelerated the dissimilation-deoxidization process of nitrate nitrogen to ammonia nitrogen.