| Eutrophication has been one of the main environmental problems all around the world for decades, which attracts more and more attention from engineers and ecologists. In China, restoration of eutrophic lakes has been listed in the front of the schedule in recent years. Among all the eco-technologies, biomanipulation is widely accepted method for ecological restoration of eutrophic lakes. Despite the non-traditional biomanipulation method by using silver carp to control phytoplankton has been extensively studied in China, it showes that the non-traditional biomanipulation can not successfully solve the ecological restoration problem in all the eutrophic lakes of China, due to the high density of silver carp population requirement and difficulty in submerged vegetation restoration. Since many successful traditional biomanipulation exercises have been conducted internationally, this study aims to evaluate the feasibility and effectiveness of application of traditional biomanipulation in China.In this paper, the resources of crustacean zooplankton were investigated in lakes along Yangtze River and in South-west China. Meanwhile, field investigation of reservoirs in Hubei province about effects of common fish on traditional biomanipulation was carried out. In addition, a comparative experiment of the two biomanipulation methods was performed in mesocosm. The results were summaried as follows:1. Resting eggs of cladocerans were collected from the surface sediment of 12 eutrophic shallow lakes and hatched in the laboratory.24 species of Cladocera were found. The most common and dominant genera were Bosmina sp., Ceriodaphnia sp., Alona sp. and Moina sp.. Daphnia was found in most of the studied lakes except Xingyun Lake and Datun Lake. Resting eggs of 5 species of Daphnia all together were found in the sediments, including Daphnia hyalina, Daphnia pulex, Daphnia magna, Daphnia cucullata and Daphnia longispina. The Daphnia egg bundance varied from 39 eggs/m2 to 1605 eggs/m2. The number of large cladocerans(Daphnia) in the hatchlings was less than the small cladocerans(Bosmina, Ceriodaphnia, Alona, and Moina).2. The resting eggs used in the experiments were formed by means of crowding population of Daphnia magna or Moina micruca in lab. Hatching rate of the resting eggs were confirmed under different conditions. These conditions included nitrogen concentration, phosphate concentration, illumination, temperature and the amount of dissolved oxygen. The experiments choosed resting eggs of D. magna and M. micruca as experimental material. According to experimental results, the optimal hatching conditions for resting eggs of M. micruca were 2 mg/L-4 mg/L N,0.1 mg/L-0.3mg/L P, 70μmol·m-2·s-1 and 30℃respectively; Light,0.1 mg/L-0.3mg/L P,25℃and hypoxic condition were more suitable for the hatching of D. magna. In addition, the hatching rate of resting eggs of M. mircrura and D.magna was studied in DongHu Lake and NanHu Lake. The hatching rate of resting eggs of M. mircrura and D.magna were significantly different when at different depths (p<0.05). The results indicated light may be the main limiting factor in the hatching of the two cladocerans. However, it were no significant differences in hatching the resting eggs at same depth (p>0.05).3. Biomanipulation experiments were conducted in experimental enclosures in lake and also in small ponds. The results showed that introduction of Daphnia sp. (100ind./L) into water bodies significantly improved the water quality and reduced the algal biomass. However, water quality in the enclosures with Daphnia sp. and with silver carp (Hyphthalmichthys molitrix,50 g/m3) showed no significant difference (p>0.05). Fish species significant determine the zooplankton size structure and water chemistry. In enclosures with bighead carp (Aristichthys nobilis,50 g/m3) there were significantly greater densities of small zooplankters than in enclosures stocked with silver carp. The outcome indicated that silver carp could efficiently decrease the proportion of cyclopoid copepodids, whereas the bighead carp fed selectively on the cladocerans. The water quality (SD, TP and chlorophyll a) of enclosures with silver carp stocked was better than that with bighead carp (p<0.05).4. We tested the effect of Daphnia on change of the phytoplankton community structure change under the different nutrient levels. The results showed that Daphnia sp. (100 ind./L) caused significant reduction on chlorophyll awhile increase of transparency when TP and TN were below 60μg/L and 1.25 mg/L, respectively. Most phytoplankton taxa were markedly affected by grazing of Daphnia populations. Under the high nutrient level (TP 150μg/L TN 3.1 mg/L); zooplankton community was copepod-dominated.and the phytoplankton community was characterized by the large-sized algae like Gymnodinium eucyaneum and Chroomonas.5. A total of 46 Reservoirs investigation in Hubei province was aimed at presenting a snapshot of crustacean zooplankton communities and their relations to fish species and environmental factors. One-way ANOVA analysis showed total phosphorus in the reservoirs stocking crucian was higher in the reservoirs stocking silver carp, bighead and grass carp (p<0.05). Moreover, the results indicated the species compositon of zooplankton was significantly affected by fish. Zooplankton in the reservoirs with the silver carp and bighead carp stocking were dominant by large copepod and cladoceran. In the grass carp stocking reservoirs, the large copepod was also up to 90%. However, the reservoirs with the crucian stocking were dominant by rotifer, which may result in miniaturization of zooplankton population.High egg abundance of Daphnia was found in sediments in the studied eutrophic lakes. Dissolved oxygen and light may be the main limiting factor in the hatching of D. magna. It can be concluded that biomanipulation by introduction of herbivorous Daphnia can be helpful for restoration of the eutrophic lakes in China under the reatively low nutrient level (TP<60 [μg/ L, TN≤1.25 mg/L). In addition, the stocking of crucian should be highly restricted. |
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