Long - Term Characteristics Of Eco - Environmental Changes In The Three Large Lakes

Lakes in Yunnan have been impacted by multiple environmental stressors(i.e. eutrophication, fish introduction and macrophyte degradation) over the past few decades, experiencing the deterioration of water quality and degradation of ecosystem funcitoning. The large lakes of Dianchi, Erhai and Fuxian are among the nine largest lakes in Yunnan, but differ markedly in terms of lake depth and trophic state. This thesis focuses on these three lakes aiming to examine the long-term pattern of cladoceran communities during the last century at each lake, and test for the heterogeneity in the spatiotemporal patterns among the three lakes. Particularly, we applied paleolimnological technologies and multi-proxy analyses to reveal how cladoceran communities have responded to long-term limnological changes.Sediment pigment records suggest that primary productivity increased significantly in the past few decades at each lake, but the magnitude and trajectory of these changes were different among lakes. The sediment pigment concentrations(estimated via Chlorophylla) in Dianchi and Fuxian Lake were consistently low before the middle of 20 th century. The Chl-a level began to increase dramatically and rapidly since 1960 s in Dianchi Lake. The Chl-a concentrations began to increase slowly since 1940 s in both Erhai and Fuxian, but the degree was lesser at Fuxian.Sediment records revealed that cladoceran community, body sizes and abundance had changed significantly during the last century at each lake. When the lake productivity was low as shown by Chl-a, the abundance of Bosmina(a pelagic cladoceran herbivore) remained low in Dianchi and Fuxian Lake and Bosmina longispina, an oligotrophic species, dominated the community. With increased trophic status, B. longispina decreased and was generally replaced by B. longirostris which prefers more eutrophic. The total abundance of Bosmina had increased in these two lakes concurrently, but experienced a decreasing trend in the past ~40 years at Fuxian. The body sizes of Bosmina decreased clearly after ice fish(Neosalanx taihuensis) was introduced when compared to the pre-introduction period. For example, years after the fish introduction in both lakes in early 1980 s, the carapace and antennule lengths of B. longispina generally decreased, indicating the intensified predation pressures on zooplankton with negative impacts on body size structure of bosminids. In Lake Erhai, the cladocerans abundance increased obviously before the 1980 s, then decreased, became to recover since 1996 and decreased again in recent years, corresponding a fluctuation of ice fish population associated with fishery activities(i.e. fish ban and fish stocking).In order to evaluate the role of top-down and bottom-up effects in driving aooplankton communities, we use sediment pigments and bosminid carapace length to approximate lake productivity and predation pressure, respectively. With the application of multiple regression and variance partitioning methods, our analyses suggested that top-down control(i.e. predation pressure) exerted stronger impact on bosminid production at Dianchi than that at Fuxian Lake(15.46% and 10.39%, respectively), while there was a much stronger control of bottom-up(i.e. nutrient enhancement) in Fuxian than that in Dianchi(69.74% and 19.67%, respectively). In addition, we found that the interaction between bottom-up and top-down is much more significant in Dianchi than that in Fuxian(42.12 % and 3.46 %, respectively), despite that both forcings exerted a similar degree of control on bosminid production in Dianchi(77.25%) and in Fuxian(83.59%). We conduct principle component analysis(PCA) of cladoceran assemblages to identify the potential driving factors in Erhai Lake,. The results showed that only the first axis was statistically significant accounting for 61.64% of the total variance, suggesting that only one environment gradient was significant in driving cladoceran compositional changes.Our results clearly show that there existed strong spatiotemporal heterogeneity in cladoceran community changes due to different limnological features among our study lakes. For example, the strength of top-down and bottom-up effects in affecting cladoceran communities differed in our large lakes depending on trophic states. In shallower systems,macrophyte degradation could significantly change the cladoceran community. In conclusion, our researches indicated eutrophication, macrophyte degradation and fish introduction lead to the dramatic changes of plateau lake ecosystems. So differences in limnological features should be considered for sustainable ecological restoration and lake management in Yunnan.