The Study Of Nitrogen And Phosphorus Budget And Estimation Of The Carrying Capacity Of Crab Ponds

In recent years,the development of the freshwater aquaculture industry has led to a shift from low-intensity traditional aquaculture methods to intensive aquaculture production systems.With the use of commercial pellet feeds and the continuous increase in aquaculture intensity,these changes have caused many researchers to pay attention to the actual effectiveness of nutrient resource utilization and its related environmental impact issues on the current production system.Commercial pellet feed is one of the main sources of nitrogen and phosphorus input in semi-intensive and intensive aquaculture systems.With the increase in breeding density and the large use of commercial feeds,the intensification of freshwater aquaculture will cause a large amount of nutrients to be released into the water.This in turn leads to deterioration of water quality and eutrophication.Therefore,the nutrient absorption efficiency of cultured organisms has an important impact on the enrichment of nitrogen and phosphorus in the water and sediments.In most cases,the amount of nitrogen and phosphorus introduced into ponds through fertilizers and feeds will be recovered below 20%to 50%at the time of harvest,and the rest will be diluted into the water column,resulting in relatively high levels of pond culture tail water,such as nutrients,suspended solids and oxygen demand,and pond sediments have a strong ability to capture nutrients such as nitrogen and phosphorus,so that a large number of nutrients accumulate in the sediments.Therefore,pond sewage is a potential source of pollution for receiving water bodies.Deteriorating water quality will adversely affect the long-term sustainability of the aquaculture industry.Understanding the process of nutrient regeneration and absorption is very important to improve water quality and the overall ecosystem health of the aquaculture system.This paper analyzes the environmental monitoring data of the in 2019 during a breeding cycle to study the temporal and spatial change characteristics of various water environmental factors and sedimentary environmental factors,and uses a comprehensive nutritional status index to evaluate the eutrophication status of the water body in various periods.The organic pollution index method and the comprehensive pollution index method were used to evaluate the ecological risk of sediments,and the reasonable breeding capacity of the breeding area was estimated based on the nitrogen and phosphorus load of the water body,and the following conclusions were obtained:（1）The total nitrogen concentration in the pond group was between 0.50 and8.35 mg/L,with an average value of 2.29 mg/L,and the highest value appeared in May.The total phosphorus concentration is between 0.11 and 0.45 mg/L,the average value is 0.24 mg/L,and the highest value appears in August.The total nitrogen concentration of the water source group is between 1.49～5.70 mg/L,the average is2.79 mg/L,the total phosphorus concentration is between 0.13～0.26 mg/L,the average is 0.18 mg/L,and the highest value also appears at August.Both nitrogen and phosphorus show a downward trend in terms of time.Taking the Grade III water standard in the"Surface Water Environmental Quality Standards"as a reference,the TN content in the water source group was higher than that in the pond group and exceeded the standard in the whole process,with the maximum exceeding range of470.44%.In the pond group,TN and COD_Mnexceeded the standard and the amplitudes were 412.00%and 74.48%,both appeared in the initial stage of aquaculture.After that,TN,TP,COD_Mnshowed a downward trend,and the rate of exceeding the standard was greatly reduced.Compared with the water source group,only COD_Mnwas much higher than the water source group.The total phosphorus was slightly higher than that of the water source group in the first three months,but was equal to the water source group at the end of aquaculture.（2）The water quality was evaluated for eutrophication and found that the pond group was in a moderate eutrophication state most of the time,and only August was mild eutrophication;the water source group was mild eutrophication most of the time and only May was moderate eutrophication.Total phosphorus,total nitrogen,and transparency are the main reasons for the eutrophication of the water body in the breeding area.The main factor of eutrophication in the pond group is total phosphorus,and the water source group is transparency.（3）By measuring the total nitrogen and total phosphorus in the bottom sludge,it is found that the total nitrogen and total phosphorus in all months exceed the reference standard value（total nitrogen 1111 mg/kg,total phosphorus 600 mg/kg）,but this standard is actually low for the qualitative environment.The content of total nitrogen and total phosphorus reached their highest values in September and August respectively;the comprehensive sediment pollution index EF is between 1.39～3.11,and it is mild pollution in May and November,moderate pollution in August,severe pollution in September.Nitrogen as the main pollution factor in September,and phosphorus in the other three months;the overall level of organic pollution in the breeding area is in good condition,with light pollution in most months,only moderate pollution level in September.（4）The nitrogen budget equation of the four river crab ponds is:juvenile crabs28.40kg（1.23%）+diversion 522.53kg（22.76%）+feeds 1747.40kg（76.01%）=adult crabs 269.05kg（1.23%）+drainage 140.51kg（6.14%）+waterweed 178.57 kg（7.78%）+sediment 1664.79 kg（72.34%）.The equation of phosphorus budget is:juvenile crab 0.81kg（0.20%）+diversion 29.73kg（7.23%）+feeds 385.13kg（92.58%）=adult crab 3.58kg（0.86%）+drainage 11.65kg（2.81%）+waterweed14.70 kg（3.54%）+sediment 323.77kg（78.52%）.Consistent with the results of other studies,feeds is the largest source of nitrogen and phosphorus,and sediment is the main destination of nitrogen and phosphorus.（5）The nitrogen load calculated by the chemical analysis method,the Takeuchi method and the material balance method were-54.27 kg/hm²,17.36kg/hm²and 47.12 kg/hm²,respectively,and the phosphorus load was-2.61 kg/hm²,3.74 kg/hm²and 13.04 kg/hm²respectively,the chemical analysis method results are the lowest,which is related to the higher nitrogen and phosphorus content in the diversion water,and the difference between the results of Takeuchi method and the material balance method may be affected by the parameter values;the nitrogen deposition rate of river crabs is 13.74%,similar to the results of previous studies,the phosphorus deposition rate is 0.72%,which is slightly lower.（6）In terms of aquaculture capacity,the estimated results of the two methods are greater than the actual aquaculture.The estimation results based on the nitrogen emission standard show that the 4 ponds can also increase the culture by 19.75%to54.83%,and the phosphorus emission standard can also increase the culture by60.06%.～99.71%.Based on the evaluation results of the two methods,the recommended breeding capacity of river crabs is 2004～2681 kg/hm²（10229～13296tails/hm²）,that is,the average value estimated by the nitrogen emission standard to the average value estimated by the phosphorus emission standard.In summary,the breeding area urgently needs to improve the quality of the water diversion,such as treating the tail water before discharging it,especially in the upstream area,while controlling agricultural non-point source pollution and the messy discharge of domestic sewage,and adopting river channel in-situ ecological restoration technology reduce nitrogen and phosphorus content and improve water transparency.In addition,artificial feed can be used to replace trash fish or develop new feed formulas in the breeding process.By adopting high-efficiency breeding technology,trash fish can be processed into pellets to improve food conversion rate,reduce pollutant discharge,and realize the healthy development of aquaculture.