Characteristics of Zooplankton and Phytoplankton Functional Groups and Evaluation of Water Quality in Recirculating Aquaculture Ponds
In order to probe into the medium-and long-term effects of circulating aquaculture mode on aquatic ecological environment and the succession and interaction of plankton and plant communities,the changes in physical and chemical factors of water quality and the structure of plankton and plant communities were investigated and analyzed in 9 sampling points at water pushing facility area,aquaculture tank and in the downstream area of 24 breeding gutters with 22 m long,5 m wide and 2.7 m deep with pushing water at both ends and sewage collection facilities at the bottom in a 12.7 hm2 tank stocked with 2 or 3 years old silver carp(Hypophthalmichthys molitrix)at stocking density of 1 200 kg·hm-2,bighead carp(Aristichthys nobilis)at stocking density of 600 kg·hm-2,grass carp(Ctenopharyngodon idella)at stocking density of 120 kg·hm-2 and common carp(Cyprinus carpio)at stocking density of 480 kg·hm-2 in late March of 2018 and 2019 to investigate aquatic ecological environment and the succession and interaction of plankton at the initial stage of the circulating aquaculture mode and one year after the culture.The results showed that there was little change in number of phytoplankton species and functional groups,with sharp change in relative importance index(IRI)in each functional group.The dominant functional groups of phytoplankton were shown to be changed from functional groups E and D to functional groups TD and J,and the succession was derived from the spatial difference of water layer mixing caused by frequent turbulence of water flow.The dominant functional groups of zooplankton were changed from functional groups PSF and PMF to functional groups PMF,PLF and PSP,showing a trend of zooplankton enlargement with more ecological stability,and the low stocking density of filter-feeding fish may be an important reason for this phenomenon.Small filter-feeding zooplankton was mainly promoted by nutrient-tolerant,flagellated and small sized phytoplankton.On this basis,large filter-feeding zooplankton were also promoted by attached green algae and diatoms adapted to medium trophic habitats.The promotion effect of phytoplankton functional groups G,W1 and X on predatory zooplankton includes both direct conduction of upward effect and indirect conduction with small filter-feeding zooplankton as the link of the food chain.After one year of recirculating aquaculture,the spatial difference of water quality at each point was further increased,but the pond was still in the state of light-medium pollution,which met the aquaculture water quality standard and was stable as a whole.
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