Study On Aquatic Vegetation Change And Its Mechanism In Liangzi Lake During The Last200Years

Aquatic vegetation is one of the most important ecological characteristics of shallowlake ecosystems. Understanding its evolution feature is crucial for lake ecologicalrestoration. In this thesis, Liangzi Lake, a typical macrophyte-dominated lake from theYangtze floodplain, was chosen for investigation. To reconstruct the historical evolution ofaquatic vegetation, a sediment core from its deepest area was analyzed with radiometricdating (210Pb/137Cs) and multi-proxies, including grain size, loss on ignition, geochemicalindicators, diatoms, cladocerans. Based on the data from reconstructed historical aquaticvegetation succession and the historical documentary data from its catchment, the drivingfactors of aquatic vegetation succession since1950s were analyzed. The main conclusionswere listed as follows:1. To explore the potential of using diatoms to infer historical evolution of aquaticvegetation, radiometric dating (210Pb/137Cs), sedimentary diatom analysis in the sedimentcore and the monitoring aquatic vegetation records in the Liangzi Lake basin since1950swere compared. This study justified that diatom-based reconstruction on aquatic vegetationwell reflected the succession of aquatic vegetation in historical period, which also wascorroborated by cladoceran data. Therefore, diatom can be used as one effective biologicalindicator for historical aquatic vegetation reconstruction.2. Based on the validation results above and combined with radioactive dating, wereconstructed historical changes of aquatic vegetation over the past200years for LiangziLake. There were several distinct periods: the episode I (ca.1850-1942AD;65-37cm) wascharacterized by natural aquatic vegetation development. During that period, the growth ofaquatic vegetation in the Liangzi Lake was relatively poor but exhibited a relatively highdiversity, mainly dominated with the submerged vegetation, followed by small amounts offloating-leaved vegetation and emergent vegetation, less with floating vegetation. Theepisode II (ca.1942-1963AD;37-23cm) was a recovery period of aquatic vegetation, withincreasing aquatic vegetation and also dominated by submerged vegetation. Meanwhileemergent vegetation, floating-leaved vegetation and floating vegetation still exist with different degrees of increase and diverse types of aquatic vegetation. In the episode III(ca.1963-1994AD;23-11cm), the amount of aquatic vegetation was increased significantly.At this stage, aquatic vegetation developed significantly, and was mainly dominated bysubmerged vegetation. Emergent vegetation and floating-leaved vegetation still existed,but the floating vegetation was decreased. The episode IV (ca.1994-2011AD;11-0cm) wasa recession period of aquatic vegetation in1990s, and a recovering of aquatic vegetation in2000s. At this stage, submerged vegetation was dominant and showed an increasing trend,with some floating aquatic plants and floating-leaved vegetation and emergent vegetationdecreased or even disappeared.3. Redundancy analyses on the period post-1950were carried out to extract thesignificant driving factors for changes in diatom populations based on the responsevariable (epiphytic diatoms-on behalf of aquatic vegetation coverage) and explanatoryvariables (i.e. human activity indicators, hydrological and meteorological data, physicaland chemical indicators). Partial redundancy analysis was used for calculated thecontribution of each of factors in explaining diatom variances. The results show thathuman activities play a leading role in the aquatic vegetation evolution in Liangzi Lakesince1950s, and floods are also an important factor on aquatic vegetation recession.4. In the macrophyte-dominated lakes, most of diatoms are epiphytic, whoseabundance is mainly controlled by aquatic vegetations. Meanwhile, the dataset of surfacesedimentary diatoms also exhibited an insensitive response of epiphytic diatoms to nutrient.Therefore, in the macrophyte-dominated lakes, diatom-based quantitative reconstruction ofhistorical water total phosphorus concentrations may be of significant errors.