Studies On The Water Environmental Characteristics Of The Grass-Type Zone And The Algae-Type Zone Of Lake Taihu

There are two typical zones in Lake Taihu------East Lake Taihu and MeiliangBay. Though in the same lake, having the similar physical, outer-source pollution, meteorological and climatic conditions , They are in the two completely differentstates------"vegetation dominated clear state" and "algae dominated turbid state",respectively. It is the famous "alternative stable states", which usually appears in the eutrophicated shallow lakes.Water environment in the grass-type zone and the algae-type zone is respectively studied from four aspects: trends analysis, trophic level evaluation, temporal and spatial correlation analysis and master factors analysis. Of which, trends analysis is based on the observed data in 2002;according to the Evaluation standards of trophic states in Taihu lake, fuzzy mathematics and analytic hierarchy process are applied for trophic level evaluation;analysis of temporal and spatial correlations is based on the correlation coefficient;and the analysis of master factors is based on the principal component analysis. The results suggest that: The water environment of the grass-type zone is mainly affected by "the turbidity of the water" , "the nitrogen and phosphorus content of the water" and "the depth of the water", less affected by the algae biomass;the water quality in the growth period is obviously better than that in the non-growth period, and the spatial difference of the former is also obviously larger than that of the latter;the highest trophic level of the water appears in spring, while there is the lowest algae biomass in the water that time. The water environment of the algae-type zone is mainly affected by "the carbon, nitrogen and phosphorus content of the water', "the algae growth and the organic contamination of the water', "the turbidity of the water" and "the water temperature", greatly affected by the algae biomass;the water quality in the growth period is obviously worse than that in the non-growth period, and the spatial difference of the former is also less than that of the latter;the algae growth in the water accords with the trophic level of the water.The water environment difference between the grass-type zone and the algae-type is significant either judging from the average of the whole lake or from the typical spot. The paper analyzes the causes of the difference by comparing algae biomass and nitrogen ^ phosphorus content of the zones. The analyses show that: the nitrogen content of the grass-type zone water is greatly lower than that of the algae-type zone water, while the difference of the phosphorus content between the two typical zones is not very large;the algae growth in the grass-typed zone is observably affected by nitrogen content of the water, less affected by phosphorus;while situation in the algae-typed zone is just opposite;The low value of the algae biomass in the vernal grass-typed zone is perhaps related by the low value of the available nitrogen in the water(ammonium-nitrogen, nitrate-nitrogen).The development analysis of each zone's states indicated that, the "vegetation dominated clear state" of the East Lake Taihu is fluctuating towards unstable state, while the "algae dominated turbid state" of Meiliang Bay is increasingly stabilized. This cautions us the urgency of taking actions to maintain the "clear state" of the grass-type zone, and shows the hardship of shifting the algae-type zone out of its "turbid state".