Changing Characteristics And Health Assessment Of Sedimentary Environment In Large Water Surface Shrimp Pond

Litopenaeus vannamei is one of the important economical shrimps,with the advantages of fast growth rate,short breeding cycle and wide range of salt tolerance.But shrimp farming activities can lead to water quality deterioration.Large-surface pond culture is a growing mode of Litopenaeus vannamei in recent years,and has great advantages in terms of shrimp quality and resource utilization,but there are problems such as low aquaculture yield and volatile aquaculture environment.According to the related research progress and existing problems at home and abroad,the research is carried out in the coastal saline-alkali pond in Binzhou City,Shandong Province.This paper discusses the changing characteristics of sediments and their potential impact on water quality from four aspects:nitrogen and phosphorus exchange fluxes at the sediment-water interface,sediment microbial community structure,sediment organic matter sources,and sediment environmental assessment.The purpose is to provide basic data support for the shrimp ecological aquaculture model in large water,and to provide scientific advice for aquaculture activities.Below are key research findings:1.Calculation of nitrogen and phosphorus exchange fluxes at the sediment-water interface in ponds with different salinities using Fick’s first law.The results showed that the average salinity of the low,medium and high salinity groups in 2019 were 27.89,44.74,and 56.03,respectively;in2021,they were 32.55,40.29,and 54.48,respectively.The average exchange fluxes of DIN in the low,medium and high salinity groups in 2019 were 4.13,7.71,and 6.72 mg/（m²·d）^-1,respectively,which were lower than those in 2021(15.6,8.52,6.72 mg/（m²·d）^-1).The average exchange fluxes of PO43+-P in the low,medium and high salinity groups in 2019 were 0.01,0.01,0.07 mg/（m²·d）^-1,respectively,and 0.01,0.01,0.02 mg/（m²·d）^-1in 2021.NO₂^--N,NH₄⁺-N,DIN,DON,TN in shrimp ponds with different salinities in two years diffused from sediment to overlying water,and the sediment was a pollution source,NO₃^--N diffuses from the overlying water to the sediment,and the sediment behaves as a pollution sink;In general,the nitrogen exchange flux in 2021 is greater than that in 2019,and the nitrogen exchange flux in the low-salt group is the largest,followed by the high-salt group,and the medium-salt group is smaller;the two-year difference in phosphorus exchange flux between the low--salt groups and medium-salt groups is small.2.Using the Illumina sequencing platform,based on the 16Sr RNA gene sequencing of the microbial community in the pond,the results showed:The seasonal changes of the medium and high salinity groups are different.In May,shannon,ACE and Chao indices of water and sediment in Medium salt group were lower than those in August,while in High salt group,they were opposite,indicating that microbial diversity in Medium salt group increased with culture time,while that in high-salt group decreased with culture time.The dominant bacteria in the two salinity groups were firmicutes,actinobacteria,hipbacteria,verrucomicrophyla,Bacteroidetes and desulphuria.Firmicutes and actinobacteria were the dominant bacteria in the medium salt group,while firmicutes and hipbacteria were the dominant bacteria in the high salt group.In May,the relative abundance of Firmicutes in the two salinity groups was lower than that in August,and the abundance of actinobacteria,hipbone bacteria,verrucomicrophyla,Bacteroidetes and desulphurization bacteria was higher than that in August,with obvious seasonal change.The abundance of Firmicutes,hipbone bacteria and verrucomicrophyla in medium salt group was lower than that in high salt group,while the abundance of actinobacteria and Bacteroidetes in medium salt group was higher than that in high salt group.The dominant bacteria in the sediments were Firmicutes,actinobacteria,desulphurization bacteria,verrucomicrophyla,Bacteroidetes,hipbone bacteria and Proteobacteria.Firmicutes and actinobacteria were dominant in the salt and salt groups.Nitrogen,phosphorus,salinity,p H and DO are the main environmental factors that have significant correlation with microbial community in the pond water of Litopenaeus vannamei.Eh,porosity and water content are the environmental factors that have significant correlation with microbial community in the sediment.The results showed that the microbial community structure was affected by many environmental factors,and the change of microbial community structure might be the result of many factors.3.Elemental analyzer was used to measure the carbon and nitrogen content and stable isotope values of pond sediment,residual bait and feces during aquaculture,and to explore the source of sediment organic matter.The results showed that in 2019,the average values of TN content in the sediments of the low,medium and high salinity groups were 0.06,0.05,and 0.07 mg/g,respectively;The average values of TOC content were 1.60,0.97,and 1.02 mg/g;the average values ofδ¹⁵N were 3.74‰,3.65‰,and 4.59‰,respectively;The average values ofδ¹³C are-8.00‰,-4.86‰,-9.10‰,respectively.In 2021,the average values of TN content in the low,medium and high salinity groups were 0.12,0.14,and 0.06 mg/g,respectively;the average values of TOC content were 2.49,3.71,and 1.52 mg/g,respectively;The average values ofδ¹⁵N were 4.14‰,2.22‰,2.05‰;the average values ofδ¹³C were-9.26‰,-8.32‰,-8.66‰.The contents of TN and TOC in the sediments during the breeding period in 2019 were lower than those in 2021,and the contents during the year were the largest in the low-salt group,followed by the medium-salt group,and the smallest in the high-salt group.The C/N ratio andδ¹³C of pond sediments in 2019 and 2021 were combined as abscissa and ordinate to analyze the source of organic matter.The results showed that the main source of pond sediment was C4 plants,with a contribution rate of more than 80%;the contribution rate of residual bait and manure was the highest in the middle salt group.4.Using organic pollution index,nitrogen and phosphorus comprehensive pollution index,heavy metal,sulfide,Nemerow comprehensive pollution index to evaluate the sediment environment of aquaculture ponds.The results showed that the organic pollution index and nitrogen and phosphorus comprehensive pollution index pollution index of the low-salt group were higher than those of the medium-salt group and the high-salt group,and the pollution index of the low-salt group in 2019 was lower than that in 2021;The heavy metal content increased with the breeding time.Except for the high-salt group in July（930.75 mg/kg）and the mid-salt group in June（301.93mg/kg）,the sulfide content of sediments in other salinity groups was less than 300 mg/kg.The Nemerow comprehensive pollution index showed that the low-salt group had the most serious pollution,followed by the middle-salt group,and the high-salt group had the best sediment environment.