| Lake Gehu is the secondary big freshwater lake in South Jiangsu province, China.With the area of164km2, Lake Gehu is relative lager in watershed of Lake Taihu andplay multiple roles of social and ecological such as irrigation, aquaculture, water supply,shipping and tourism. Development of agriculture and industry in Lake Gehu watershedwas so quick and Lake Gehu has becoming one of the most vigorous area in China inthe past two decades. Otherwise more and more nutrients loading entered Lake Gehuand fisheries density continue increasing, accompanied economic booming, made LakeGehu suffered serious ecological damage.To find out reasons of deteriation of Lake Gehu ecology and the recovery ways,resolve problems of retreat of fisheries resource and degradation of environment, manyinstitutes investigated the environment, geograph, ecology, fisheries and biology andaccumulated plenty of foundamental data. During “eleven five” plan, many sub-projectsof state water pollutant control and handle programm were related to Lake Gehu andplenty of work has made. But most of these researches was carried out from one aspectof ecosystem and lake of systematic decision and behavior under the frame ofecosystem architecture. Computer model of food web based on mass balance couldanalysis ecosystem holistic and integrate basic research data and documents. Domesticscholars have conducted a preliminary study using computer models on the ecosystemof Taihu Lake and East China Sea. In this thesis we constructed a EwE model on behalfof the Gehu ecosystem history and current situation by synthesizing theory and methodsfrom biology, ecology, ecological stoichiometry and computer science, analysized andquantified the food web composition, distribution of trophic levels, ecosystem maturityand stability, influence of fisheries policy. Results would future guiding fisheries policydevelopment and ecological restoration of Gehu. This contribution first constructedmodel of Gehu and made a comparison with model in1986-1989and found that Gehuwas under degradation compare to1986-1989.Silver and bighead carp are major kindes in china traditional aquaculture and oneof the most introduced fishes in the world. Since the theory of non-classicalbiomanipulation of control cyanobacterial blooms was put forward, researches aboutniches of silver carp and bighead carp in the lakes is gradually increased, a number of researchers in different institutes studied filtration on algae and influence on nitrogenand phosphorus cycling of silver and bighead carp through enclosure, indoor controlledexperinments. The researchers draw different and even opposite conclusions for theyused different experimental methods and in different environments. About140hectarepen culture with structure improved set up in the open area of Gehu, stocking differentdensity of silver carp and bighead carp, to verify influence that the silver carp andbighead carp in natural water bodies on the lake ecology, provide guidance on controlcyanobacterial blooms and eutrophication recovery.1Ecopath model study of Lake Gehu ecosystemAn ecopath model with21functional groups was constructed based on the data ofecological investigation and fisheries statistics of1986-1989. Structure andcharacteristics of Gehu ecosystem was analysized by network analysis moduleembedded in ecopath. The analysis results demonstrated that ecosystem in1986-1989included seven trophic levels and major trophic flow occurred in the first four levels.Range of groups from within1-3.584. Total system throughput was12131.76t·km-2·year-1, among this flow originated from first trophic level accounted for74.91%.Total net system primary production was3722.117t·km-2·year-1, among thismacrophyte production accounted for33.31%. Mean catched trophic level was2.78.Ecotrophic efficience of most target fishes were high, indicated that most of these fisheswere constrained by combination of fisheries and predatory. Value of P/R（Production/Respiratory, P/R） was2.76, value of P/B （Production/Biomass, P/B） was1.76, Finn’s cycling index was14.76%. According criteria from Odum of matureecosystem, we considered that Gehu ecosystem in1986-1989was mature.Another ecopath model of Lake Gehu on2010was constructed using data ofinvestigation and fisheries statistics. The model included seventeen groups. Analysisresults indicate that ecosystem of2010Gehu Lake comprised of seven trophic level.Range of trophic level of these groups was1-3.691and major flow occurred in the firstfour trophic level. Total system production was1974.82t·km-2·year-1, total systemthroughput was8562.544t·km-2·year-1, connectance index, system omnivore index,Finn’s cycling index and Finn’s mean path length were0.219,0.189,7.99and2.841respectively. Values of P/R and P/B were2.189and3.509respectively. Mean catchedtrophic level was2.56. Through comparison of total system throughput, P/R, P/B,Finn’s cycling index, connectance index and dominant food chain between1986-1989and2010, we could conclude that Gehu was degrading from mature status, resouces offisheries declined, K-strategists decreasing and r-strategists increasing, present biomassof silver and bighead carp could not control biomass of algae. 2Experinment study of influence of silver carp and bighead carp onphotoplanktonPen culture was set up in the west of Zhihekou, south of Gehu, with the area ofabout140hectares. Three treatments with different density of silver and bighead carpwas carried out. Density of silver carp were2910,3090and3630/hm2respectively anddensity of bighead carp were1995,2220and2775/hm2respectively. Seventeensampling sites were configured outside and inside the pen culture. Investgatedplankton, water quality, zoobenthos and nitrogen and phosphorus in sediment montyly.The analysis results showed that stocking of silver and bighead carp could decreasingchlorophyll a significantly, but could not decrease total nitrogen, togal phosphorus andammonia nitrogen significantly. Density of phytoplankton ouside of the pen culturewas significant greater than that of inside the pen culture but there is no significantdifference of biomass betweeen inside and outside of the pen culture.3Influence of silver carp and bighead carp on zoobenthosSilver and bighead carp influenced zoobenthos mainly through altering nutrientscycling and modifying habitat of zoobenthos. We found16zoobenthos in the area ofpen culture within studied period and they belonged to Annelida, Arthropoda andMollusca. Among these zoobenthos11belonged to annelida,67%;4belonged toArthropoda,28%; and1belonged to Mollusca. Limnodrilus hoffmeisteri, Aulodriluspigueti and Bellamya aeruginosa are dominant species in the pen culture area.4Influence of silver carp and bighead carp on nitrogen and phosphorus insedimentTotal nitrogen inside the pen culture was lower than that of outside of thepenculture but there was no significant difference. Total phosphorus inside of the penculture was significant greater than that of outside the pen culture. The conclusion wasthat stock of silver carp and bighead carp could decrease phosphorus accumulation inthe sediment and had no vital influence on nitroten.This contribution was a positive explore to understand the mechanism that silvercarp and bighead carp played in the lake nutrients cycling. Limitatios of this study wasthat the area of pen culture was small compare to the whole lake, with the interere of lake currence the silver carp and bighead carp could influenced the whole lake profund.Research on this field still need data come from bigger scale. |
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