The Effects Of Filtering Fish Manipulation And Floating Macrophyte Rafts For Improving Water Quality On The Community Structure Of Zooplankton In Tropical Reservoirs: Large Scale Experiments

Reservoirs are man-made water bodies that play a critical role in mitigating the conflict between water demand and supply in industrial,agricultural and daily life.Since there are many reservoirs in Guangdong Province and the majority is becoming more and more eutrophic,we conducted manipulation experiments in Gangcun Reservoir,Xiangang Reservoir and Dajingshan Reservoir with an objective of regulating ecosystem structure against eutrophication.This thesis aims to understand response of zooplankton community to experimental manipulations.Silver carp and bighead carp are the most common fish in tropical reservoirs;their mixed culture is typical of the fishery in tropical reservoirs.The responses of zooplankton to this culture mode were assessed in large enclosures from March 2007 to May 2007.The ratio of silver carp and bighead carp biomass levels was set as follows:0:0,3:7 and 7:3.The density of fish in the fish group was 16.7g.m^-3.The meta-zooplankton was composed of 33 species of rotifers,6 species of cladocerans and 5 species of copepods.Rotifers were dominated by Brachionus angularis,Brachionus forficula,Trichocerca stylata,Anuraeopsis fissa,Polyarthra spp., Conochilus dossuarius and Keratella cochlearis.Moina micrura was the dominant species of cladocera.In Copepoda,Thermocyclops taihokuensis was the most abundant species.The species and abundance was contributed by small-sized species with a body size less than 200μm, such as rotifers and copepod nauplii.The dominant species did not show an evident temporal succession.Relative abundance of rotifers in fish enclosures was higher than that in fishless enclosure due to predation of fishes on big zooplankton,and the abundance of rotifers,cladocera and copepods decreased significantly in fish enclosures.But the abundance and biomass of zooplankton at different ratios of silver carpand bighead carp were similar as the food niche of silver carp and bighead carp overlaps.Xiangang Reservoir is a high fishery yield reservoir,in which silver carp and bighead are mixedly cultured in a biomass ratio of 3:7.In order to understand the community structure of the zooplankton in tropical shallow reservoirs with high fishery yield,zooplankton was sampled in the pelagic zone of this reservoir from June 2007 to February 2009.The meta-zooplankton was composed of 30 species rotifers,7 species cladocerans and 4 species of copepods.Rotifers was dominated by Brachionus angularis,Brachionus forficula,T.stylata,Polyarhtra sp.and K. cochlearis.Bosmina longirostris was the dominant cladoceran.In copepoda,T.taihokuensis was the most abundant species.In the above three groups,the rotifers dominate in species number and total abundance.The total abundance of zooplankton was higher in dry season than flood season,but the total biomass changed inversely.The analysis with CCA showed the precipitation was the most important factor influencing the dynamics of meta-zooplankton.We examined the impact of different stocking levels of fish and different ratios of silver carp and bighead carp in a large scale(667 ha.) and an small enclosured bay(28 ha.) from september 2007 to February 2009 in Xiangang Reservoir.The inside fish biomass of the experimental bay was about 15gm²,the outside one about 30gm².The biomass ratio of silver carp:bighead carp of small enclosured bay was 0:10(september,2007-July,2008) and 7:3(September,2008-February,2009),the large scale one was 3:7.Samples of zooplankton were collected monthly to analyze species composition,abundance,biomass,body size and dominant species.The dominant species of rotifer did not show an evident temporal succession with different stocking levels of fish and different ratios of silver carp and bighead carp.The abundance and biomass of crustacean small enclosured bay decreased significantly than ones of large scale because of low predation induced fishing from september 2007 to July 2008 in large scale.The abundance and biomass of zooplankton at small enclosured bay between from September 2007 to February 2008 and from September 2008 to February 2009,and ones of between small enclosured bay and large scale from September 2008 to February 2009 were not significant difference,which resulted from the low predation of Tilapia nilotica and other rough fishes.To understand the effect of floating macrophyte rafts on zooplankton in Dajingshan Reservoir, pelagic zooplankton was investigated monthly or biweekly inside in experimental zones with macrophyte and without macrophte from July 2006 to December 2008.The meta-zooplankton was composed of 36 species of rotifers,5 species of cladocerans and 9 species of copepods. Rotifers were dominated by Brachionus,such as Brachionus angularis and Brachionus forficula. Bosmina longirostris was the dominant species of cladocera.In copepoda,Limnoithona sinensis was the most abundant species,Limnoithona sisnensis is a small-sized species.It can adapt to high salinity,and invade Dajingshan Reservoir with water pumped from the Xijiang River. Precipitation and pumped water were the most important factors influencing the dynamics of the meta-zooplankton,and they significantly modified the effect of temperature.The zooplankton abundance of three zones was composed of small-sized species with a body size less than 200^tm, such as rotifers and copepod nauplii.The abundance and biomass ofrotifers and cructacean were not significant difference between experimental zones with macrophyte and without macrophyte because three experimental zones were not isolated absolutely and the composition of fishes were adjusted.Towards the third years after the bio-engineering,some oligotrophy indicators, such as Ploesoma hudsoni,Proales sordida,and Scaridium longicaudum occurred,and the abundance of eutrophic indicators such as B.angularis,B.forficula,T.stylata,and B. longirostris,decreased.