Large-scale Enclosure Experiment And Its Insight On Lake Ecological Management

The Taihu Basin covers an area of 3.69 × 104 km2,and its territory includes parts of Jiangsu,Zhejiang,Anhui and Shanghai.It is crisscrossed by a network of rivers and dotted with numerous lakes and ponds.This region boasts abundant water resources and a developed economy.The Taihu Lake,covering a water area of 2,425 km2,is a geographic center and water conservancy hub of the Taihu Basin and the third largest freshwater lake in China after the Poyang Lake and Dongting Lake.In 2012,the total water resources of the Taihu Basin reached 33.3 × 108 m3,and the population of the basin 59.2 million,making up 4.4%of the total population in China;its GDP reached 5.4188 trillion Yuan,making up 10.4%of the national total;GDP per capita reached 92,000 Yuan,2.4 times the Chinese average.In terms of GDP per capita,it has already achieved the level of the developed countries.But over the past decades,the practice of exclusive and excessive pursuit for rapid growth of GDP has led to the increasingly serious pollution problems,casting a cloud over the brilliance of the developed economy.According to 2012 Bulletin on River Water Sources in the Taihu Basin and the South East,only 18.7%section of the Taihu Basin of which the annual water quality achieved or excelled Type ? water quality;73.9%of the waters of the Taihu Lake were inferior to Type V water quality;7%of the waters were classified as Type V water and 19.1%Type IV water.The Taihu Lake ecological improvement works have been started successively.The generic ecological improvement works include:dredging soil,topography alteration,wave dissipation,enclosure,waterfront restoration,wetland vegetation restoration,food chain(network)restoration and water purification system.In each of the above lake ecological improvement measures,a variety of specific ecological improvement initiatives are made.The designs of ecological enclosure works vary with the different types of lakes in different regions,and the effects differ as well.Two different types of ecological enclosure experiments were designed in this paper to verify the effect of in situ eco-restoration and ex situ eco-restoration.Based on the comparative analysis of the areas within and without ecological enclosure,the effects of the enclosure works of different types were studied and the inspiration on lake ecological improvement from the large enclosure experiments was disused in this paper with a view to providing useful reference for the future lake ecological improvement works.Results of the study show that:(1)In the distinct enclosure experiment,based on the evaluation of the eutrophication of China's lake(reservoir),the annual average eutrophication index(EI)of the 4 sections in this experiment was 62.26,and all of the 4 sections were of moderate eutrophication;according to the phytoplankton cell abundance evaluation,the annual eutrophication level of the natural lake area was high eutrophication,the water intakes of Gonghu Waterworks moderate eutrophication,the enclosure-leading area high eutrophication and the in situ experiment area moderate eutrophication.The water of the intakes of Gonghu Waterworks was superior to that of other regions in many aspects,including water quality,phytoplankton,zooplankton and eutrophication evaluation.Therefore,it can be assumed that the enclosure and wetland restoration works in Gonghu Bay have exerted positive effects on Gonghu Waterworks.(2)No nutritional limiting factor for phytoplankton growth was found in the distinct enclosure experiment,but significant correlations existed among water quality,phytoplankton and zooplankton,and there were obvious bottom-up effects among them.As for water quality status,significant linear correlations existed among water temperature,pH value,turbidity,dissolved oxygen,total nitrogen and ammonia nitrogen,and between Chl-a and phyoplankton density.Phytoplankton density was shown to be highly linearly correlated to zooplankton density and biomass.Linear regression equation:zooplankton density=247.24+0.91*phytoplankton density;zooplankton biomass=1.09+0.003*phytoplankton density.The enclosure experiment is conducive to the restoration of the health food chain relationships.(3)In the phytoplankton biological control experiment,the top-down effects of filter-feeding fishes increased with individual growth over time and presented obvious inhibitory effects to phytoplankton density,and the phytoplankton density of the in situ area A was lower than that of the in situ area B by up to 61.39%;the grazing effect of filter-feeding fishes can promote the phytoplankton biodiversity and species richness.The phytoplankton diversity index of the in situ area A was higher by 14.53%on average than that of the in situ area B in and the twelve months throughout the year.There were significant differences between them(p<0.05).(4)In the phytoplankton biological control experiment,the phytoplankton diversity of the in situ area A was lower than that of the in situ area B by 74.62%in spring and winter when zooplankton density is lower,and the top-down effects of filter-feeding fishes presented obvious inhibitory effects to the low-density zooplankton;the phytoplankton diversity of the in situ area A was higher than that of the in situ area B by 20.65%in summer and autumn when zooplankton density is higher,indicating that the top-down effects of filter-feeding fishes cannot inhibit the zooplankton of a high density or ultra-high density level;the number of the zooplankton species in the in situ area A was higher than that of the in situ area B by 19.44%,and the biodiversity index of zooplankton higher by 7.29%;the dominant species in the in situ area A appeared seven times while those in the in situ area B appeared nine times.The grazing effect of filter-feeding fishes was of positive to inhibit the expression of the dominant species,increase zooplankton biodiversity and maintain species richness.(5)In the phytoplankton biological control experiment,in the in situ area A,the total nitrogen was higher by 12.68%,ammonia nitrogen by 33.3%and chlorophyll a by 7.2 1%compared with the in situ area B,showing that the feeding and drainage activities of filter-feeding fishes had played a negative role in water quality improvement.In the in situ area B,the total nitrogen decreased by 7.79%,ammonia nitrogen by 16.67%and chlorophyll a by 10.52%compared with natural lake areas,showing that submerged vegetation is of positive significance to water purification.Therefore,it will be a mutually complementary and reinforcing measure to improve water quality while controlling phytoplankton through the joint actions of submerged vegetation and filter-feeding fishes in the future lake ecological improvement works.(6)In the phytoplankton biological control experiment,phytoplankton was positively correlated with zooplankton.Phytoplankton was significantly positively correlated with Cladocera(p<0.01)as well as Copepodas(p<0.01)and was not correlated with Rotifers(p>0.05),indicating that zooplankton has weak top-down effects on phytoplankton and presents more bottom-up effects of phytoplankton on zooplankton by means of resource constraints.Linear regression equation:in the in situ area A,zooplankton density=207.48+1.28*phytoplankton density,and cladocera density=-119.55+1.07*phytoplankton density;in the in situ area B,copepodas density=20.42+0.28*phytoplankton density,and cladocera density=-3.04+0.56*phytoplankton density.(7)In the indistinct enclosure experiment,according to eutrophication status evaluation criteria for China's lakes(reservoirs),the control group outside the core area presented the highest eutrophication status index(59.37),which was followed by the pre-processing group(55.09),riverway group(55.02)and central lake(52.68);the phytoplankton cell abundance evaluation shows that the annual eutrophication level of the control group was high eutrophication and that of the pre-processing group,riverway group and central lake moderate eutrophication.Both evaluation methods show that:the water quantity improved gradually from the external to the internal of enclosure,and phytoplankton were brought under effective control;the design of constructing indistinct enclosures of constructed wetlands,ecological penetration-filtration system and winding river riparian zone played a significant role in water purification and algae inhibition.This indicates that the practice of constructing indistinct enclosures of wetlands,ecological penetration-filtration system and winding river riparian zone has obvious effects over water purification and exerts a positive influence on the water environment within a region.(8)In the indistinct enclosure experiment,the indistinct enclosure had a positive influence on eutrophication and zooplankton density inhibition.In the control group of indistinct enclosure with the worst water quality,phytoplankton density was significantly higher than that of the three groups within enclosure by 30.43%?33.83%and zooplankton density by 72.20%?93.05%(p<0.05).However,the indistinct enclosure showed a negative influence on biodiversity and species richness.In the control group of indistinct enclosure with the worst water quality,phytoplankton species were significantly more than those of the three groups within enclosure by 6.67%?25.84%,zooplankton species by 61.29%?194.12%,phytoplankton diversity significantly higher by 21.93%?39.57%and zooplankton diversity significantly higher by 14.89%?20.54%(p<0.05).(9)In the indistinct enclosure experiment,no nutritional factor restricting phytoplankton growth was found,and there was a low correlation between water nutrients and phytoplankton;water nutrients did not seriously affect the bottom-up effects of phytoplankton,but the correlation coefficients of phytoplankton density and zooplankton density,and of diversity index and zooplankton density were 0.808 and 0.597 respectively,both of which reached the extremely significant level(P<0.01),indicating that phytoplankton has a significant bottom-up effects on zooplankton.Linear regression equation:zooplankton density=74.78+1.57*phytoplankton density,zooplankton density=95.87+174.54*phytoplankton diversity index,and zooplankton diversity index=1.48+0.002*phytoplankton density.In conclusion,both distinct and indistinct enclosure experiment showed positiveinfluences on water quality improvement and plankton control,but there were somenegative effects.Generally speaking,however,the enclosure experiment should beregarded as positive and effective,and the effect of ex situ eco-restoration is muchbetter than in situ eco-restoration.The study revealed that the ecological engineeringmethods,such as strongly separating water bodies with enclosure and applyingvegetation absorption and matrix adsorption,can effectively reduce the nutrient load of water and lower the eutrophication level;the top-down effects of filter-feeding fishes can be effective in inhibiting blue algae density;it is necessary to adjust the food chain structure and build a healthy food-chain-oriented aquatic ecosystem.