Abstract:According to the theory of multiple stable states of shallow lake ecosystems, it is possible to speed the ecological restoration process by recovering aquatic vegetation in eutrophic lakes. In Wuli Lake, both emerged plant and floating-leaved plant could adapt to the eutrophic habitat, but submerged plant was limited by low light intensity at the lake bottom. Permanent e-merged plant community and floating-leaved plant community was built under natural conditions of Wuli Lake, and some submerged plant communities were set up in the enclosures where the water quality was improved and the underwater light intensity increased. The submerged plant community could not survive the summer for the high water temperature and high water level in the season. To make the habitat in Wuli Lake suitable to submerged plants one possible way is to control the water level and build shallow littoral zone by moving the fertile mud from the center area to the littoral zone. In the present studies, some usable techniques have been mastered for water quality control and aquatic vegetation recovery in shallow eutrophic lakes, and more detailed quantitative studies were needed on the relationship of structure and environmental function of aquatic vegetation.

Abstract:Under natural environmental conditions of Taihu Lake, Elodea Nuttallii and six other species of submerged plant depended mainly on vegetative reproduction to survive and spread their population. In the restoration process of shallow lakes, aquatic macrophyte should be planted on a large scale in a relatively short time, and artificial vegetative reproduction and plantation skill is needed for this purpose. Transplanting the cuttings of Elodea Nuttallii, Hydrilla verticillata and Cerotophyllum demersum was proved to be very easy and effective for sufficient shoot supply, long planting season, and large-scale operation. However, Valiisneria spiralis, Potamogeton maackianus and P. malaianus had low vegetative reproduction capacity As a result, seeding transplantation was still effective although it was difficult and slow. It's more difficult and undesirable to transplant subterraneous tuber of Valiisneria spiralis and stock of Potamogeton maackianus unless it was particularly needed.

Abstract:Experimental studies on the interaction of submerged plant and phytoplankton in an eutrophic lake revealed complex mutual inhibition competition. Keen competition for the light in the water was found and phytoplankton took remarkable dominance. Submerged plant was the stronger competitor for mineral nutrients because it could get nutrients from both the lake water and the fertile sediment at the bottom. When light and nutrients were abundant, phytoplankton was suppressed by submerged plant with some unknown biochemical which caused falling of phytoplankton. The competition equilibrium was obviously affected by water depth, water temperature and nutrient concentration in the water. Phytoplankton would prefer high temperature, deep water and high nutrient level, bat submerged plant the opposite. Once the community of submerged plant grew up, it gave stronger suppression to the phytoplankton and kept its dominance more stable. Thus, submerged plant should be planted in the cool and low water level season in eutrophic waters with the external nutrient source strictly controlled.

Abstract:Experimental studies were carried out on the basic light demand for brood-bud germination of submerged plant Vallisneria spiralis L., Hydrilla verticillata Royle, Ceratophyllum demersum and Potamogeton crisptts L. Intense light stimulation could break dormancy of brood-buds of P. crispus and lead to germination in the early summer In natural water body, summer dormancy of P. crispus brood-buds may be caused by low light intensity in the bottom. Light was not indispensable for brood-bud germination in the other three plant, but low relative radiation intensity (less than 5%) would cause albino young shoot which could not carry out normal photosynthesis. Thus, light condition at lake bottom in brood-bud germinating season was the key environmental factor for submerged plant to survive.

Abstract:In the eutrophication process of Wuli Lake, large amount of nitrogen, phosphorous and organic matter accumulated in the sediment of the lake bottom. In summer, oxygen deficiency often occurred in the sediment and bottom water, because high temperature enhanced biochemical oxygen consumption in the sediment and water, but the high water level greatly limited oxygen supply of the bottom water and sediment. The sediment condition was suitable for emerged plant and floating-leaved plant all the time, and even favorite to submerged plant when the temperature was below 30℃ or the water depth was less than 1m. In summer season when the temperature was over 30℃ and the water depth was beyond 1m, the anaerobic condition in the sediment and bottom water could destroy the root of submerged plant and cause it floating-up. It became an obstacle for restore aquatic vegetation in Wuli Lake that submerged plant could not survive the summer. To create suitable habitat for aquatic plant in Wuli Lake, the water level should be controlled, and the fertile sediment in the center of the lake be moved to the littoral zone to make it more shallow.

Abstract:Wuli Bay, a small and shallow bay of Taihu Lake in the northwestern comer, is a drinking water source and famous scenic spot. In the 1950's, Wuli Bay still remained its original beauty. It was covered by aquatic macrophyte and the lake water was clear enough to see the bottom. The water was in mesotrophic level and nearly saturate with oxygen. The system had very strong buffering capacity to nitrogen and phosphorous pollution. The sediment was well-oxidized and had low content of TP(0.023%) and TOC(0.75%). Phytoplankton was intensely suppressed by aquatic macrophyte to an annual mean density of 26.7×10⁴ ind./L with diatom dominated. More than 190 species of zooplankton was found with a mean density of 5 660 ind./L. Abundant large benthic animals was dominated by Macrobrachium nipponens, Stenothyra glaloroa and Corbicula fluminea in the bay. It was rich in fishery resource and piscivorous fish took the dominance. Since 1950, about half of the lake surface has been diked and the littoral zone environment, the best living habitat for aquatic macrophyte, has been destroyed. In the late 1960's, grass carp was introduced to Wuli Bay which led to the disappearance of macrophyte, and eutrophication occurred with increasing pollution. The dike-dam and sluice broke the connection between Wuli Bay and Taihu Lake. So, the waste water from Wuxi City became the main inflow of Wuli Bay and eutrophication was enhanced. The lake water in Wuli Bay is now in hy-pereutrophic level with low secchi depth(<0.5m) and serious oxygen shortage. TP and TOC has accumulated in the sediment to 5.17 and 2.87 times as high as in 1951, and heavy "bot-tom-up" effect was found in the late spring of 1993 and 1994. Algal-bloom occurred over the year with a mean density of 4 174*10⁴ ind./L, which was 156 times as high as in 1951. Zooplankton decreased to 902 ind./L(16% of that in 1951) and very few large benthic animals found alive. The whole bay was used as a fish pond for silver carp and big head, pisciverous fish disappeared from the bay. Wuli Bay is still the important drinking water source and scenic spot of Wuxi City although the water is eutrophic and the environment is terrible. We could not return Wuli Bay to its original condition, but it is possible to control the pollution and restore the present lake area to clear water and mesotrophic state. It is an efficient way to restore the Wuli Bay by recovering aquatic vegetation, but the deep water and low transparency made it difficult for the macrophyte to live. It is very important to improve water transparency before recovering vegetation. Wuli Bay is a typical sample of shallow eutrophic lakes, and the achievements in research on eutrophication and restoration of Wuli Bay could be used to other similar lakes.

Abstract:During the passed decade, in shallow lakes of the middle and down stream basin of the Yangtze River, especially in urban lakes, there has been an explosive development of pen fish farming, and a rapid increase in fish yield and economic benefit. However, due to the input of artificial food, over-exploitation of aquatic higher plants and increase in population, submerged vegetation have decreased and even disappeared, the process of eutrophication has been quickened and the total function of lakes has been weakened. Elodea Nuttallii was introduced into the East Taihu Lake from abroad in 1986, and soon became established. It has been used as food of cultivated herbiverous fishes by the local fishermen. Based on the biological and ecological property of Elodea, in this project we studied:(1) the planting method, cutting time and mode of Elodea in shallow lakes in order to increase the yield per unit area; (2) the digestibility and food conversion efficiency of Elodea by grass carp so as to determine the ratio of fish yield to Elodea planting area; (3) the role of Elodea in the ecosystem. Finally, we designed and established an artificial compound ecosystem of Eiodea-grass carp pen farming, i. e., on the basis of planting Elodea which is used for grass carp culture and functions as nutrient cycling medium, the grass-fish balance system is established within large lake ecosystems, which should lead to a better development of lakes.

Abstract:Chemical composition and energy content of Elodea Nuttallii were determined by common methods of biochemical analysis. The digestibility, maximum consumption, protein efficiency ratio and food conversion coefficient of N. elodea by grass carp and Chinese bream were studied. In N. Elodea, water content amounted to 89.8%, protein, fat, cellulose and non-nitro-gen contents in dry matter were in proper orders:23.52%, 2.88%, 26.71%, 29.27%. Its protein contained 17 common amino acids, including 9 essential amino acid for fish. The content of amino alids was 198.04 mg/g(dry). Under mean water temperature 20.4℃, digestibilities of dry matter, protoin, fat, carbo-hydrates and amino acids in, grass carp fed N. Elodea were in proper order:56.02%, 73.9%, 58.46%, 77.00%, these in Chinese bream fed N. Eoldea were in proper order:54.97%, 72.96%, 64.63%, 51.36%, 77.00%. The maximum consumption (Y, g/24h) of grass carp fed N. Elodea was related to the body weight of fish (X, g) by equation Y=3.9449X^0.8357 (r=0.9154). In cage, protein efficiency and food conversion coefficient of N. Elodea by grass carp were in proper order:17.1%, 48.2. In enclosure, these of N. Elodea by grass carp were in proper orders:13.1% and 63.4%.

Abstract:The biomass, production and competitive growth of Elodea Nuttallii and other submerged plants was studied in a pond by Taihu Lake side during 1993-1995. The study shows that means of harvest affect the production of the submerged plants, cutting its half biomass per time make peak annual output, and Elodea Nuttallii is dominant species under the environmental condition of experiment.

Abstract:Elodea-grass carp pen farming is a relatively independent subsystem artificially established in the lake, in which the grass carp feeds on Elodea Nattallii, the excretory matters of fish transform into nutrient salts, and nutrient salts transform into the circulatory system of Elodea. The transformed organic matters are moved out of the system through fish and there are no extra organic matters remained in the system, so fish-farming environment is optimized. Based on the above-mentioned, we selected East Taihu lake as the experiment base and established a semi-closed experiment region, in which there are two fish-farming districts, Elodea-growing district and no-weed district. The changes of bacteria, phytoplankton, zooplankton, zoobenthos and organic detritus have been analysed. The results show:(1) The density of heterotrophic bacteria decreases gradually from fish-farming district to outside, decrease 88.9% in Elodea-growing district and 73.9% in no-weed district. The amount of coliform group is similar to heterotrophic bacteria, decreases 75.3% and 66.7% respectively in both districts. (2) The average density and biomass of phytoplankton are 4 612.187×10⁴ ind./L and 7.1776mg/L in Elodea-growing district and 6070.853×10⁴ ind./L and 8.3108mg/L in no-weed district respentively.(3) 92 and 60 species of zooplankton (including Rotifera, Cladocera and Copepoda) have been found in Eiodea-gvoWmg and no-weed district, respectively. The average density and biomass are 4 752 ind./L and 4.868mg/L and 6 749 ind./Land 3.9407mg/L respectively. (4) 19 and 14 species of zoobenthos (including Annelida, Mollusca and Arthropoda) have been found respectively in the two districts. Although the densities are the same in two districts, the compositions of community and biomass are different. The average density and biomass of Annelida (mainly oligochaete) are 59 ind./m² and 0.575mg/m², 73 ind./m² and L 453mg/m²; Mollusca are 80 ind./m² and 34.822mg/m², 29 ind./m² and 12.065mg/m²; Arthropoda(mainly chironomid larvae) are 78 ind./m² and 0.673mg/m², 115 ind./m² and 0.688mg/m² in two districts respectively. (5) According to the average depositional rate of organic matters, it is as 5 times in no-weed district as that in Elodea-growing district. Due to the macrophyte, the buffer capacity effected by environment is strengthened in the Eoldea-growing district. Not only is the water body purified and the water quality recovered. but also the community structure of hydrobios is more stable and species diversity is increased. Eutrophication is also controlled effectively. So, Elodea-grass carp pen farming is a optimum way of eco-fishery development.

Abstract:Impacts of Elodea Nattalii on water quality of grass carp pen culture area were studied in man-made elodea-grass-carp compound ecosystem. The result shows that water in plantation was better than that in area of non-macrophyte. Content of TN and TP in water were not increased because of purification of Elodea in planted area. By estimated balance of nitrogen and phosphorus in plantation, the appropriate of pen culture area to planted elodea area is 1:3.58. In addition, it was only partitive that breeded fish effected water quality in weeds-filled water body. So far as whole area of planted, there was little influence on water quality.