Study On The Pollution Characteristics Of Sediment In Crab Ponds And Simulation Of Sediment Improvemen

Chinese mitten crab（Eriocheir sinensis）,commonly known as the river crab,is a traditional,high-quality breed in China and the freshwater fishery single species with the largest output value.In recent years,due to the increase in culture scale and culture years,the culture management measures have been extensive,leading to a decrease in crab quality and serious water pollution in some local areas,which has constrained the healthy and sustainable development of the river crab culture industry.Among them,the impact of pond sediment on the crab culture environment has not been systematically studied so far.This paper conducted a survey and analysis of the pond at three different culture years（0-1 years,NP;5-7 years,MP;and over 30 years,AP）from March to October 2021,and systematically analyzed the dynamic changes of environmental factors during the crab pond cycle culture process and explored the main controlled factors affecting the crab culture benefits.Based on this,the improvement effect of different proportions of sediment improvers（potassium sulfate compound salt versus compound microbial agent）on sediment was compared,in order to provide scientific decision-making solutions for river crab pond culture management.The main research results are as follows:（1）Various indicators of the pond water showed significant fluctuations in the sampling period,with COD_Mn,TN,TP,and Chl a content achieving the highest values in October.The water quality of the crab pond differed significantly among the various culture years,and overall showed a pattern of eutrophication degree of AP>MP>NP.During the sampling period,TN,TP,and COD_Mn of the water exceeded the standard,and the average excess rates were 100%,41.6%,and 75%,respectively.In the three crab ponds,a total of 95 kinds of phytoplankton were identified,belonging to Chlorophyta,Bacillariophyta,Cyanobacteria,Euglenophyta,Cryptophyta and Chrysophyta.The annual average densities of phytoplankton and their biomass densities in NP,MP,and AP ponds were 367.92×10⁴,691.32×10⁴,and 2116.34×10⁴ cells/L,and 3.39,3.83,and 18.88mg/L,respectively.Microcystis sp,Cryptomonas ovata,Euglena sp and Trachelomonas sp were the dominant species in all three crab ponds,with AP having more concentrated and single species of dominant phytoplankton compared to NP and MP.（2）The thickness,loss-on-ignition（LOI）,TN,and TP contents of the sediment increased with the increase in culture years,while the average particle size and oxidation-reduction potential（ORP）decreased.In addition,the sediment of all three crab ponds was neutral,with a p H between 6.97～7.13.It is worth noting that the TN of the sediment in all three crab ponds was mildly polluted,and the TP pollution level was higher than TN,and in a state of moderate-to-heavy pollution.The comprehensive pollution index showed that the sediment of AP and MP was in a state of moderate organic pollution during the survey period,while NP was in a state of mild organic pollution.Proteobacteria,Bacteroidetes and Desulfobacterota were the main microorganisms in the crab pond sediment at the phylum level,accounting for 34.00～61.06%of the total microbial communities.The enrichment of nutrients and anaerobic environment drove the composition of the microbial community structure in the sediment.In addition,the abundance and diversity of the microbial community in the sediment of the pond in NP were higher than in AP during most of the culture period.During the survey period,the differences between the three pond sediment samples gradually decreased,and the proportions of common OTUs in each sampling period were 33.43%,41.78%,51.28%,and 55.26%,respectively.（3）The culture benefits of river crab ponds were significantly negatively correlated with the content of TP,TN,NH₄⁺,and LOI in the sediment（p<0.05）,and significantly positively correlated with the ORP and average particle size of the sediment（p<0.05）.Partial least squares path modeling（PLS-PM）results showed that culture years had a direct impact on the crab pond culture benefits.It is worth noting that the culture years changed the comprehensive pollution status of the sediment and made a difference in the crab pond culture benefits by affecting the composition of the microbial community.（4）Applying sediment improvers can effectively reduce the nitrogen and phosphorus load in the crab pond sediment while improving the oxidation-reduction status and particle size distribution of the sediment.However,it did not significantly affect the diversity and species abundance of the microbial community.The C2 experimental group,which applied a lower concentration of potassium persulfate compound salt combined with a higher dose of compound microbial agent,showed the highest reduction rates in LOI,TN,and TP of the sediment,which were 34.90%,13.52%,and 24.35%,respectively.In addition,the relative abundance of Firmicutes was higher in the C2 group,reaching 9.00%,indicating that the C2 group was more conducive to the settlement of beneficial bacteria in the culture environment.