Characteristics Of Phytoplankton In The Main Grass Carp Pond And Its Correlation With Environmental Factors

Pond culture is a common mode in freshwater culture at present.However,water pollution caused by aquaculture will not only affect the quality of aquatic products,but also may pollute the environment,thus affecting the sustainable development of aquaculture industry.In order to solve this problem,this study constructed a model combining constructed wetland and culture pond,and compared the in-situ water quality control model with the traditional water culture model by adding beneficial algae species.The main idea of the composite constructed wetland model is to pump the cultured water from the pond into the constructed wetland through the adsorption and settlement of different aquatic plants,so as to remove the excessive nutrients in the water,and then discharge the purified water into the pond.However,the in-situ water quality regulation mode mainly aims at increasing the phytoplankton diversity in the water environment by adding beneficial algae to pond species,making the phytoplankton community structure more stable and enhancing the self-regulation ability of the water ecosystem.The changes of phytoplankton composition and community structure and their correlation with the physicochemical factors of pond water were studied under three different cultivation conditions,which provided a basis for the ecological environment regulation of pond culture.The main research results are as followed:1.Changes of main physical and chemical factors of pond water quality and breeding effectDuring the whole breeding process(June to October),the main physicochemical factors of grass carp ponds under different conditions showed certain differences.The concentration of suspended matter,chemical oxygen demand,chlorophyll a,and nitrogen and phosphorus in the compound culture mode were significantly lower than those of the algae-regulated pond and the traditional culture pond.To some extent,the compound pond culture model had a significant effect on water quality regulation.The in-situ regulation mode had no significant regulation effecton water quality.From the perspective of aquaculture effects,the survival rate and total yield of the compound aquaculture ponds were significantly better than the algae control mode and the traditional aquaculture mode.2.Composition and structural changes of phytoplankton in pond water179 phytoplankton species were identified in the compound pond,belonging to 7 phyla and 64 genera.14 dominant phytoplankton species were identified in the compound pond culture model,including Chlorella vulgaris,Crucigenia tetrapedia,Scenedesmus insignis,Scenedesmus bicaudatus,Scenedesmus granulatus,Scenedesmus dimorphus and Scenedesmus bijuga from Chlorophyta,Merismopedia tenuissima,Merismopedia punctata,Merismopedia minima and Microcystis aeruginosa from Cyanophyta,Euglena oxyuris from Euglenophyta,Melosira granulate and Synedra acus from Bacillariophyta.Among them,the chlorophyta dominated.The cell density ranged from 0.79×108 to 9.48×108 cells/L.117 phytoplankton species were identified in the in-situ algae regulation pond,belonging to 7 phyla and 47 genera.8 dominant phytoplankton species were identified in situ model,including Chlorella vulgaris,Crucigenia tetrapedia and Scenedesmus granulatus from Chlorophyta,Merismopedia tenuissima,Merismopedia punctata,Merismopedia minima and Microcystis flos-aquae from Cyanophyta,Melosira granulate from Bacillariophyta.There were more dominant species under cyanobacteria.The algal cell density ranged from 0.07×109 to 1.06×109 cells/L.107 phytoplankton species were identified in the control pond,belonging to 8phyla and 46 genera.7 dominant phytoplankton species were identified in control model,including Chlorella vulgaris,Crucigenia tetrapedia and Scenedesmus granulatus from Chlorophyta,Merismopedia tenuissima,Merismopedia punctate and Merismopedia minima from Cyanophyta,Melosira granulate from Bacillariophyta.The algal cell density ranged from 0.13×109 to 1.16×109 cells/L.The shannon-wiener indices of phytoplankton in composite culture ponds,in-situ algae regulation ponds and traditional ponds were 2.98~3.76,2.63~3.18 and2.81~3.27,respectively.Margalef indices ranged from 3.20~4.53,2.83~3.96 and2.54~4.15,respectively.The Pielou evenness index ranged from 0.48~0.55,0.43~0.61,and 0.46~0.55,respectively.The comprehensive evaluation shows that the composite mode was light pollution,the in-situ algae regulation mode was medium pollution,and the traditional mode was medium pollution.3.Correlation between water quality change and phytoplankton community structure change in pondThe number of phytoplankton species was significantly positively correlated with transparency,nitrate nitrogen,total nitrogen,and total phosphorus,and significantly negatively correlated with temperature.Algal cell density was significantly positively correlated with total phosphorus,while total nitrogen was extremely positively correlated with nitrite nitrogen and negatively correlated with nitrite nitrogen.The Margalef diversity index had a very significant positive correlation with transparency,a significant positive correlation with nitrite nitrogen,nitrate nitrogen,total nitrogen,and total phosphorus,and a significant negative correlation with temperature.RDA showed that chlorophyta was mainly affected by temperature,dissolved oxygen,nitrous nitrogen and chemical oxygen demand;cyanobacteria was affected by dissolved oxygen,ammonia nitrogen,total nitrogen and nitrous nitrogen;diatoms are affected by p H,suspended matter and nitrous nitrogen;while eutrophia was greatly affected by nitric nitrogen.Researches showed that the operation of the composite mode can effectively promote the growth of green algae and inhibit the outbreak of cyanobacteria,and improve the breeding water.In addition,there were more dominant species of phytoplankton in this model,the diversity level of phytoplankton was high,and the community stability was healthy.In the in-situ algae regulation mode,the phytoplankton diversity index was lower than that of the composite mode,but the addition of beneficial algae could increase the number of dominant species of phytoplankton.Compared with the traditional mode,the phytoplankton community diversity level in this mode was high and the community structure was stable.