| Pond culture is still the main way of freshwater aquaculture in China,but the problems in the waste of water resources,fish diseases,and the pollution of surrounding water by the wastewater discharge are exposed gradually in the long-term development process.In recent years,with the management of the aquaculture wastewater discharge are increasingly strict and standardized,novel aquaculture modes,such as recirculating aquaculture system with containers,are becoming increasingly popular.The system relies on the sewage treatment facilities like ecological ponds for harmless treatment of aquaculture wastewater,showing advantages in water conservation,emission reduction and wastewater recycling,which can meet the requirements of the Construction of Eco-civilization and fish safety production.However,there is still a lack of systematic research on the capacity,efficiency and mechanism of the ecological ponds,and no consensus on the factors affecting the stable operation of the system are evoked.Herein,based on the new-built recirculating aquaculture system with containers,a series of experiments were conducted to explore the factors affecting the system stability:(1)The water physicochemical parameters,which in different functional areas in the direction of water circulation,were continuously monitored for seven weeks.The experiments focused on the concentrations of nitrogen and phosphorus compounds and the wastewater treatment capacity of the ecological pond;(2)The microbial community structures of the aquaculture water and sediments in different functional areas in the direction of water circulation were carried out using 16 S rRNA sequencing to investigate the abundance,diversity and composition of microbial communities,then the relationship between environmental factors and microbial community structures were analyzed;(3)The ecological ecosystem model considering plankton and benthos was built using AQUATOX model,in order to predict the development of the pond ecosystem and simulate the several kinds of scenario conditions according to the factors affecting the stability of the system.Combined with the in-field experiments and numerical model,the following conclusions were obtained:(1)The dissolved oxygen(DO)concentration in aquaculture water satisfied the needs of fish growth,but it was disadvantageous to create anaerobic and facultative anaerobic situations in ecological ponds #1 and #2;(2)p H value increased to more than 8.5 in the fourth and seventh weeks affecting by human and environmental factors,which is not conducive to the fish welfare and system evolution;(3)The results showed that the system capacity in the nitrogen and phosphorus compounds degradation and transformation were insufficient in the new-built stage,and the concentrations of nitrite,nitrate,total nitrogen and total phosphorus reached 6.16 ± 0.49 mg/L,15.61 ± 2.32 mg/ L,14.12 ± 0.42 mg/L and 11.18 ± 1.00 mg/L at the end of the experiment,respectively,indicating the water was eutrophic;(4)The dominant phyla of the microbial communities in aquaculture water and the sediments were Proteobacteria and Bacteroides,but the abundance and diversity of the microbial communities in the sediments were far higher than that in the water body,indicating that the sediments were the main places for microbes to carry out life activities;(5)The microbial community structures in the sediments were relatively stable at new-built stage,but which in the aquaculture water were affected greatly by environmental factors over feeding time.What’s worse,Cyanobacteria and Flavobacterium dominated in the water at the end of the experiments,which may result to the algal bloom and severe fish diseases;(6)The relative abundance of the ammonia-oxidizing organisms(AOA and AOB,0.03% ～ 1.28%)and nitrifying bacteria(NOB,2.57% ～ 8.26%)in the sediment were higher than that in the water,while the relative abundance of phosphateaccumulating organisms(PAOs)was close to 0.Additionally,the shortage of carbon source and the high concentration of nutrients were the critical factors affecting the microbial community structure;(7)Numerical simulation showed that the diatom biomass was rapidly decreased due to predator loss,and green algae and blue-green algae finally became dominantly.Especially,the blue-green algae biomass increased by more than 300% during the simulation,and aquatic animals were more easily adapted to the changes in the system environment;(8)The system could effectively reduce the nutrient concentrations in the long-term cultivation process relying on self-purification capacity,but lowing the input of nitrogen and phosphorus was still the key way to avoid eutrophication.Simultaneously,a proper amount on the filterfeeding fish adding to ecological pond and submerged plant in the ecological pond # 3 were able to restrict the aquatic animals biomass,which was benefit the recovery of pond systems.In conclusion,the lack of carbon source,high DO concentration and increased p H value in ecological pond inhibited the life activities of microorganisms in the early stage of the newbuilt recirculating aquaculture system,which affected the capacity of nitrogen and phosphorus degradation and removal,resulting to a large amount of nitrogen and phosphorus compounds accumulated in the aquaculture water.Therefore,the algae are under the stress of aquatic animals’ growth,and the system function was unbalanced.We suggested that the system functions can be effectively restored by reducing nitrogen and phosphorus input,such as optimizing feeding scheme and feed formula,cultivating submerged plants and filter-feeding fish.The results of this paper provide critical reference for improving the design and daily management of recirculating water aquaculture system with containers and other new aquaculture modes,and provide technical support for reducing the uncertain risks of new aquaculture in the actual aquaculture operation and further promotion. |
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