Nitrogen And Phosphorus Budget And Energy Characteristics In Monoculture And Polyculture Grass Carp Pond

Grass carp（Ctenopharyngodon idella）is the most productive freshwater fish in China,pond aquaculture is the main mode of grass carp production,accounting for 74%of total grass carp culture production.In the main production areas of China,pond culture of grass carp mainly has two modes:monoculture and polyculture.Due to the high breeding density of monoculture,multiple stocking and multiple fishing can obtain higher returns and develop rapidly,but there are some problems,such as complex water quality management and easy disease outbreak.And polyculture can make full use of the space and resources in the system,improve the recycling rate of nutrients in the system,and thus improve water quality and prevent disease.At present,there are still few studies on the nitrogen and phosphorus budget and energy characteristics of different grass carp pond culture modes,and there are still some problems in these studies.The construction of efficient grass carp culture ponds still lack theoretical basis,and it is of great significance to carry out relevant research to clarify the ecological efficiency of different grass carp ponds and further optimize the pond ecosystem of grass carp.In this paper,the nitrogen and phosphorus budget and energy characteristics of grass carp culture in two ponds were systematically studied,and the main conclusions are as follows:（1）There was a significant difference in water quality between the two grass carp ponds during the aquaculture period.The concentration of TN in monoculture（G）was1.633±0.018 mg/L～6.635±0.267 mg/L,and that in polyculture（GS）was 1.368±0.083mg/L～3.222±0.096 mg/L.In May,November,January and March,the concentration of TN in monoculture was significantly higher than that in polyculture（P<0.05）.The concentration of TP in the monoculture pond was 0.283±0.049 mg/L～1.245±0.361mg/L,and that in the polyculture pond was 0.189±0.053 mg/L～1.255±0.108 mg/L.In May,September,November and January,there were significant differences in the concentration of TP between the two ponds（P<0.05）.The overall water quality of polyculture during aquaculture was better than that of monoculture during aquaculture.（2）During the aquaculture period,the phytoplankton dominant species and biomass of the two grass carp ponds were different.From May to March,8 phylum 138 species were detected in monoculture,while 8 phylum 123 species were detected in polyculture,and there was no obvious difference in phytoplankton species between the two ponds.Cyanobacteria,Chlorophyta and Cryptophyta were the dominant phytoplankton in monoculture,while Cryptophyta were the dominant in polyculture.The biomass of phytoplankton in monoculture was the lowest（6.99 mg/L）in July,and decreased after reaching the peak in January,with the maximum biomass reaching 31.68 mg/L.The biomass of phytoplankton in polyculture reached the highest（30.58 mg/L）in May,and has not significant changed since then.At the end of the aquaculture period,it decreased significantly（P<0.05）to 8.27 mg/L.The average biomass of the polyculture pond is higher than that of the monoculture pond.（3）The phytoplankton community composition and biomass of the two grass carp ponds from May to March were different.44 species zooplankton were detected in monoculture during aquaculture,while 37 species zooplankton were detected in polyculture.The species of zooplankton in monoculture were more abundant than that in polyculture.Except that the biomass of Protozoa in the polyculture were slightly higher in July and November,the biomass of all kinds of zooplankton in monoculture were higher than that in polyculture at other time.The average biomass of Protozoa in the two ponds during the aquaculture period was 0.2853 mg/L and 0.3081 mg/L respectively,and the biomass of Protozoa in polyculture was slightly higher than that in monoculture.The average biomass of Rotifera was 1.2545 mg/L and 0.4920 mg/L respectively.The average biomass of Cladocera was 0.7680 mg/L and 0.0986 mg/L respectively.The average biomass of Copepoda was 0.9284 mg/L and 0.0560 mg/L,respectively.During the aquaculture period,the average biomass of Rotifera,Cladocera and Copepoda in the monoculture pond were higher than that in the polyculture pond,and the zooplankton in polyculture tends to be miniaturized.（4）The species of microorganism community in two grass carp ponds were similar while the composition has some differences during aquaculture.At the initial stage of aquaculture,Proteobacteria and Firmicutes were dominant in the water and sediment of the two ponds,and the proportions of Firmicutes and Proteobacteria were 81.86%and6.40%in the water of monoculture,while 23.42%and 44.61%in the sediment,respectively.And the proportions of Firmicutes and Proteobacteria were 35.08%and44.15%in the water of the polyculture while 16.11%and 54.56%in the sediment.Proteobacteria and Actinobacteria were dominant in the water at the middle and late stage of aquaculture,Chloroflexi and Proteobacteria were dominant in the sediment.The potential pathogenic bacteria of Mycobacterium（15.97%）were dominant in the water of monoculture at the middle period of aquaculture.Mycobacterium（2.72%）and Flavobacterium（1.93%）appeared in polyculture at the end of aquaculture.The monoculture pond was more prone to pathogenic bacteria and had a higher proportion.（5）During the aquaculture period,the utilization rate of nitrogen and phosphorus in polyculture was higher than that in monoculture,and the utilization rate of nitrogen in the two grass carp ponds was higher than that of phosphorus.Feed was the main input source of nitrogen and phosphorus in ponds,which accounted for 67.82%and 68.57%in monoculture,that in polyculture was 56.51%and 57.43%respectively,and the proportion of nitrogen and phosphorus input from feed in polyculture was higher.Nitrogen and phosphorus input of aquaculture organisms accounted for 12.04%and 8.66%in monoculture respectively,and that accounted for 12.11%and 9.97%in polyculture,which was higher.Sedimentation and aquaculture harvest were the main source of nitrogen output from ponds,with 37.66%and 28.54%in monoculture,40.73%and 27.11%in polyculture.The nitrogen output from harvest in polyculture was higher.Phosphorus in ponds was mainly exported by sedimentation,which accounted for 55.77%and 50.06%respectively in the two ponds.And the proportion of polyculture was slightly lower.（6）The nitrogen conversion of grass carp pond was mainly concentrated at the sediment-water interface.The nitrification rate,denitrification rate and nitrate reduction rate of sediment-water interface were higher than those of sediment,the denitrification rate and nitrate reduction rate were significantly different（P<0.05）.The nitrogen conversation was the fastest both in sediment and sediment-water interface,followed by nitrate reduction and denitrification was the slowest.Analyzed the key enzyme activity of nitrogen cycle in the sediment of grass carp pond,the activity of AMO was 0.02656±0.00309 U,significantly higher than the other four enzymes（P<0.05）,and Nar has the lowest activity of 0.00119±0.00116 U.The denitrification rate in the pond was limited by the step of NO₃^-to NO₂^-.（7）The energy transfer efficiency（TE）of polyculture was higher than that of monoculture.Ecopath models were constructed for the two grass carp ponds to analysis.The ecological nutrient structure of the two ponds was simple,and the energy flow was concentrated in trophic level I and II.The range of effective trophic level of the two ponds was low,and the food web was simple.The EE value in polyculture was 0.140～0.996,and that in monoculture was 0.042～0.960.The EE value of fish in polyculture was higher than that in monoculture,and the conversion efficiency of energy contribution to the ecosystem was higher.Both ponds relied on the detrital food chain,and the feed plays an important role in the food chain,with more than 65%of energy.The proportion of total flow originating from detritus can reach more than 65%.Some of the energy in both ponds had not been utilized and flows into detritus,among which the energy of bacteria and phytoplankton had not been utilized is more,and the unused energy of monoculture was more than that of polyculture.In this study,the biocharacteristics,nitrogen and phosphorus budget and energy characteristics of monoculture and 80:20 polyculture grass carp pond were mastered,and the advantages of polyculture were analyzed from the aspects of biology,material and energy.It is found that polyculture can make full use of feed,adjust the microorganism community in the culture system,change the richness of phytoplankton and zooplankton,reduce the waste of energy flow in the system,increase the transfer efficiency,and improve the stability of the ecosystem,which provides a theoretical reference for optimizing the grass carp pond aquaculture system.