|Other Abstract||For shallow lakes, macrophytes-dominated clear water and algae-dominated turbid water are the two typical states in the regime shift process of ecological systems. The two states have different feedback mechanisms to maintain the system stability. As to the lake ecological services concerned, macrophytes-dominated clear water is more valuable to environmental protection and production action than algae-dominated turbid water. However, with more and more effect of human activities, some lakes in China have been suffering eutrophication of different degrees and a big amount of shallow lakes shift from macrophytes-dominated clear state to algae-dominated turbid state following the change of food-net structure. In the present study, Lake Dianchi and the other 14 shallow lakes in Yunnan Province, South-west China were investigated in spatial and temporal scales to elucidate the difference and respective maintenance mechanisms of the two typical states with the characteristics analysess of various physic-chemical factors, as well as parameters of phytoplankton, zooplankton, aquatic plants and sediment. The main results are summarized as follows:
1) The physic-chemical characteristics were studied not only in different areas of Lake Dianchi but also in 14 lakes of Yunnan Province with different steady states. The results on Lake Dianchi showed that there were obvious divisions of trophic state in the Lake, and the trophic level decreased from north to south. Moreover, transparency in the Lake was mainly affected by total phosphorus, chlorophyll a and turbidity. While the total phosphorus increased, the transparency decreased; it decreased sharply when the concentration of total phosphorus reached up to above 50μg/L. In addition, the relationship of transparency with chlorophyll a and turbidity were significant, which indicated that algal bloom was one of the main causes to enhance the turbidity. Results of similar ananlysis on the other 14 lakes showed that in the macrophytes-dominated lakes there were significantly lower loads of biologically available nitrogen and phosphorus, as well as particulate nitrogen and phosphorus, than those in the algae-dominated lakes, which suggested that submerged plants played significant effects on nutrient uptaking and sediment fixation.
2) The phytoplankton characteristics in spatial and temporal scales of macrophytes-dominated lake were compared with those of algae-dominated lake. The results showed that lakes or regions with different steady states had different annual succession rules: the macrophytes-dominated lake had the succession rule of Bacillariophyta- Aphanizomenon- Chlorophyta- Cyanophyta, but the algae-dominated lake was Aphanizomenon- Anabaena- Microcystis which belonged to Cyanophyta. In addition, it was found that the abundances of Cyanophyta, Chlorophyta and Bacillariophyta were good index to evaluate steady state of lakes.
3) Analysis based on aquatic vegetation succession in different areas characterized with dominance of submerged higher plants or planktonic algae in Lake Dianchi, features of different steady states were studied to exame the main factors which determine the plant biomass and species diversity. Results showed that the species diversity of aquatic plants significantly correlated with the biomass, the biomass decreased obviously while the diversity decreased; further more, bio-available phosphorus in water and sediment was closely related to biomass and species diversity, and the biomass and diversity decreased prominently while the bio-available phosphorus increased. It means an important role played by the bio-available phosphorus in the growth and maintenance of submerged vegetation. Results also revealed that, when soluble reactive phosphorus (SRP) in the water column was below 5μg/L and bio-available phosphorus in sediment was below 60μg/L, the aquatic vegetation was stable. However, when the bio-available phosphorus in sediment was over 60μg/L and the soluble reactive phosphorus in water column become changing in a wide range it would be hard for vegetation protection and species diversity recovery.
4) Sediment characteristics were studies in different areas of Lake Dianchi and also in the investigated 14 lakes of different steady states. Results showed that the most contribution to total phosphorus and to chlorophyll a in water column originated in the NaOH-phosphorus; phosphorus fractions in sediment were affected by the growth activities of phytoplankton and aquatic plant; total phosphorus in sediment of different lakes varied widely and was affected by various factors, thus it is not employed to divide exactly the two typically steady states of lake ecosystem, but NaOH-phosphorus can be used to divide availably the two types of lake states with cluster analysis.
5) Results in this study explained that，total phosphrus is the most important nutrient factor in driving the regime shifts of lake ecosystems. Further results from study on shifts between the two typical regions in Lake Dianchi with the piecewise function method, showed that total phosphorus in macrophytes-dominated regions was always below 120μg/L, but in algae-dominated regions it was always over 150μg/L in the same lake. In addition, studies on lakes of different steady states explained the threshold of total phosphorus was 70~110μg/L in the shift process from macrophytes-dominated state to algae-dominated state, and the TP threshold was 25~40μg/L in the shift from algae-dominated state to macrophytes-dominated state.|