Effects Of Aquatic Macrophytes On Nutrient Cycling In Shallow Freshwater Lake Ecosystems

Eutrophication of water bodies is a common problem in the world. Studying the effects of aquatic macrophytes on nutrient cycling in the aquatic ecosystem will have great significance in understanding the mechanisms of macrophytes in purifying the water bodies, revealing the principle of eutrophic lakes control and restoration, and utilizing the lakes resource sustainablely.To research the effects of aquatic macrophytes on the nutrient cycling in the shallow and eutrophic lakes in middle to lower reaches of the Yangtze River, several experiments have been performed, such as the effects of submerged macrophytes on the sediment resuspension, breakdown and nutrient dynamics of macrophytes, macrophytes in nutrients assimilating and turnover in two submerged communities. The main results are as follow:Submersed macrophytes can reduce sediment resuspension and decrease internal P loading. During the study period, total phosphorus of 334, 253 and 589 mg m^-2, were brought back to the water column due to the sediment resuspension in V. denseserulata, P. maackianus communities and in aquiculture stand, respectively. The P. maackianus community was more effective in reducing sediment resuspension than the V. denseserulata. Herbivorous fish could destroy the aquatic vegetation, induce sediment resuspension and promote the internal nutrient loading. The results show that exploiting the resources of aquatic macrophytes must be reasonably, maintaining the biomass at 300 g m^-2 is necessary to control the sediment resuspension effectively.AFDW/TN ratio determined the litters breakdown rate, litters with lower AFDW/TN ratio decayed fast. According to the decay rate the sequence of the five litters is: leaves of N. nucifera > leaves of Z. latifolia > culms of Z. latifolia > P. maackianus > petioles of N. nucifera. 20% of their biomass remained as detritus after decay in a year, which promoted the lake silt and swampiness; 75% and 80% of their initial TN, TP lost in a year, respectively. V. natans grew rapidly (0.055 d-1) and absorbed the N, P nutrients efficiently (136.45 and 33.5 mg m^-2 d^-1, respectively). Therefore, V. natans should be used as a preferred aquatic plant in restoring aquatic vegetation. While the P. maackianus with high carbon accumulation rate (2.63 g m^-2 d^-1) and high biomass was of great significance in transferring nutrition from the sediment.In V. denseserulata and P. maackianus communities, the cycle coefficient of TC, TN and TP were all above 0.7, suggesting that most of the nutrients assimilated in plants would be released back to the environment as the plants senescence and decay. Litters of the macrophytes served as a nutrient "source" rather than a "sink".The study stated sediment was the richest nutrient bank, followed by the macrophytes communities. Through detritus accumulation and sedimentation, the sediment would become the nutrient "sink" in the macrophytes communities.