A Study On The Decomposition Of The Aquatic Macrophytes

Aquatic macrophytes is an important part of aquatic biodiversity. Decomposition of aquatic macrophytes can play a considerably role in elements cycling in aquatic ecosystems, and may therefore regulate aquatic ecosystem nutrient balance. The study on the decomposition of aquatic macrophytes in the lake had an important significance for understanding the eutrophication, biological sedimentation, nutrition balance and so on. Most studies of decomposition processes have focused on terrestrial plants, however, few study focused on the aquatic macrophytes. In the thesis, on the one hand, we studied the effect of mixed floating (Nymphoides peltatum) and submerged macrophytes (Potamogeton crispus) on decomposition, on the other hand, we also studied the effect of macro-invertebrates on the decomposition of N. peltatum, P. crispus and the mixture of them. Besides, we explored the digested methods to the aquatic macrophytes’ determination of phosphorus.The results were mainly as follows:1. The decomposition rate of both N. peltatum and P. crispus was significantly and positively correlated with initial N content (p<0.05, r=0.862),24.74%and44.91%dry mass remaining after90days, respectively. The observed remaining mass of the mixture at the early stages of decomposition in ten days was6.63%(p<0.05) higher than the expected, indicating the occurrence of negative, non-addtivie effects of mixed species early on. In contrast, there was no significant mixing effect after ten days in subsequent samplings. After90days, the remaining dry mass of the mixture was30.39%. The N and P contents of both N. peltatum and P. crispus released rapidly at the early stages and then slowed down. The remaining percentage of N and P of N. peltatum were lower than that of P. crispus. During the early stages of decomposition of mixed material in ten days, the observed N and P remaining were14.36%and12.88%(p<0.05) higher than the expected, indicating the occurrence of antagonistic effects on N and P release in the mixture. However, there were no significant antagonistic mixing effects in subsequent times for N. After90days, the observed P remaining was4.26%(p<0.05) lower than expected, indicating a synergistic effect on P release occurred. The remaining percentage of N and P were43.60%and15.88%, respectively. Nutrients and polyphenol concentrations in the mixture decreased rapidly at the early stages and then decreased slowly through the end of the study, in a manner similar to that of the single species. The results showed that there were negative, non-additive effects on decomposition rate, N and P releases when two species were mixed together at the early stages, while there was a synergistic effect on P release in the final stage of the decomposition. This suggested that neither decomposition nor nutrient release patterns can be assessed on basis of single species dynamics. In addition, there was a significant time-independent non-additive effect of species interactions. We further suggested that different aquatic macrophytes of contrasting life forms such as floating-leaved plants and submerged plants may differ in initial chemical quality and may exhibit major determinants for decomposition of mixed aquatic macrophyte.2. The presence of macroinvertebretes increased decomposition rates of N. peltatum, P. crispus and the mixture by51.04%,71.30%and62.86%, respectively. The N relesse rates by41.42%,75.82%, and62.50%, respectively, and the P relesse rates by36.87%,68.61%, and54.31%, respectively. When macro-invertebrates were allowed or excluded, there was no significant difference between the observed and expected decomposition rates in the N. peltatum-P. crispus mixture, respectively, therefore, there were no mixing effects in both of them. The nitrogen release rates of the mixture in the treatment that excluded macro-invertebrates had a negatively interaction, however, the negatively interaction disappeared when the macro-invertebrates were allowed. Additionally, both the treatment had a negatively interaction for phosphorus release rates in the mixture. The results showed that the decomposition rates, nitrogen and phosphorus release rates of the N. peltatum and P. crispus were accelerated by the macro-invertebrates, and which were positively related to the relative density of family and individuals of macro-invertebrates. Compared with the treatment that excluded macro-invertebrates, there were significant changes for the mixing effects of nitrogen release rates of the N. peltatum-P. crispus mixture in the treatment that allowed macro-invertebrates. Our results provided information to further study the influence factors which effect the decomposition of aquatic macrophytes, and also provided references for the protection and management of wetland biodiversity.3. The total phosphorus in the sixteen kinds of aquatic macrophytes were determined by the H2SO4-K2SO4-CuSO4-Se and H2SO4-H2O2digested methods, and the accuracy of the two digested methods were very high. Although there was a significant difference between the two methods for the total phosphorus content, it had a significant positive correlation between them. Besides, both them almost had completed the digestion in the same time. The results showed that the determination of total phosphorus content in aquatic plants by H2SO4-K2SO4-CuSO4-Se digested method was more convenient and safer than by H2SO4-H2O2digested method, thus the H2SO4-K2SO4-CuSO4-Se digested method was a ideal method, and we could make a conversion between the two digested methods by the regression equation.