| Soil is the foundation of the earthen ponds.After overlying water and raising fish,the nutrients and algae stored in the soil will be released into the water through the material exchange process at the mud-water interface,thereby affecting the physicochemical properties of the water and the characteristics of the phytoplankton community structure,and further impacting the primary productivity of water bodies and the result of pond aquaculture.Because of the great differences in properties,nutrient conditions and algae composition in soils developed from different parent materials,the primary productivity of the water body based on different soils will be inevitably different.Based on this,the purple soils of acidic(S1),neutral(S2)and alkaline(S3)developed from different parent materials were used as test soils in the present study.The change of water quality and phytoplankton community characteristics in different purple soils over a period of 150 days were studied in a cylindrical medium aquarium(r=40cm,h=80cm)to stimulate the pond culture conditions.This study will provide a scientific basis for the development of local fishery in purple soil areas.The main results of the study are as follows:1.The physicochemical properties of different purple soil ponds(1)During the experimental period,the overlying water physicochemical properties of the three purple soils all changed except that the water temperature mainly changed with the temperature.The variation range of pH,conductivity,redox potentia and transparency of three purple soils were 7.26-8.23 0.337-0.631 mS/cm,161-369 mv and 9.3~42.7 cm,repectively.The content of dissolved oxygen in water was higher in90-150 days than the first 60 days.Affected by the dissolved oxygen level in the water,the ammonia nitrogen was higher in the early stage than the later stage,while the nitrate nitrogen showed the opposite trend.The active phosphorus content increased first and then decreased.At the same time,the contents of total nitrogen and total phosphorus in the overlying water based on three purple soils were higher than those in the initial period.(2)During the aquaculture period,the pH and conductivity in the overlying water of acid purple soil was significantly lower than that of neutral and alkaline purple soil(P<0.05).The content of active phosphorus and total phosphorus of acid soil was also significantly lower than that of neutral and alkaline soil in the first 120 days(P<0.05).The results of clustering analysis showed that the physicochemical properties of the overlying water of acid purple soil at 30,60,90 and 120 days can be clustered,which indicated that there were significant differences in the physicochemical properties among acidic,neutral and alkaline purple soils.(3)The pH,oxidation-reduction potential and nutrient content of pond mud were significantly correlated with the physical and chemical properties of the overlying water(P<0.05).The activities of catalase,sucrase,urease,alkaline protease and alkaline phosphatase in pond mud were also significantly correlated with nitrogen and phosphorus content in overlying water(P<0.05).2.Characteristics of phytoplankton community structure in different purple soil ponds(1)107 species of phytoplankton had been detected in three purple soils,belonging to 52 genuses and 6 phyla.Among them,there are 52 species of Chlorophyta,23 species of Bacillariophyta,17 species of Euglenophyta,and less other categories.The dominant species of phytoplankton in the three purple soil water bodies were dominated by the species of Chlorophyta in the first 60 d.However,in the middle and late raising period(from 90 to 150 d),the water column of acid purple soil was mainly dominated by species of Chlorophyta and Cyanophyta,especially the Merismopedia punctata,Merismopedia minima,Oscillatoria tenuis and Anabaena had a higher degree of dominance.The dominant species of phytoplankton in the neutral and alkaline purple soil ponds were dominated by species of Chlorophyta,Euglenophyta and Pyrophyta.(2)The variation of phytoplankton biomass in acidic,neutral and alkaline purple soil ponds ranged from 7.5 to 119.2 mg/L,10.1 to 81.8 mg/L and 8.8 to 123.9 mg/L,respectively.The biomass of phytoplankton in acid purple soil ponds were significantly lower than that in neutral and alkaline purple soil ponds at 60 d and 90 d,and was significantly higher than the pond of neutral purple soil at 150 d(P<0.05).The range of phytoplankton density in acidic,neutral and alkaline purple soil ponds were 43~736010~6cells./L,83~565 10~6cells./L and 56~437 10~6cells./L.The density of phytoplankton in ponds of acid purple soil ponds was relatively high at a later stage and may be related to the outbreak of cyanobacteria blooms.(3)The phytoplankton community clustering analysis based on the Bray-Curtis similarity coefficient shows that 15 phytoplankton communities can be divided into five categories.The first category is S1,S2,S3 at 30 days and S2 at 90 days;the second category is S1,S2,and S3 at 60 days;the third category is S3 at 90 days and S2 and S3at 120 and 150 days;The fourth category is S1 at 120 and 150 days;The fifth category is S1 at 90 days.It can be seen that the community structure of phytoplankton in overlying water of acid purple soil in the mid-late period(90-150 days)was significantly different from that in neutral and alkaline purple soil.3.The relationship between phytoplankton community and physicochemical properties of water in different purple soil pondsRedundancy analysis showed that the physicochemical properties of water could explain 73.1%of the phytoplankton community changes,and the first axis and the second axis explained 21.3%and 15.5%of the phytoplankton community changes,respectively.Monte Carlo test results showed that ammonia nitrogen(F=2.16,P=0.008)and nitrate nitrogen(F=1.78,P=0.045)explained 19.2%and 16.4%of the phytoplankton community changes,respectively.Water temperature,electrical conductivity,pH,transparency,active phosphorus,nitrite nitrogen and total nitrogen had no significant on phytoplankton community(P>0.05). |
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