Research On The Characteristics Of Greenhouse Gas Emissions From Mariculture Ponds In Tianjin And Their Influencing Factors

Aquaculture ponds as a special aquatic ecosystem that is significantly disturbed by human activities,the study on the key processes of carbon and nitrogen cycle in the aquaculture ponds will help to better understand the biogeochemical laws in different types of aquatic ecosystems,and that is of great significance to further explore the level and impact mechanism of greenhouse gas emissions in aquaculture waters.In this paper,shrimp ponds in the Dagang Area,located in Binhai New District,Tianjin,were selected as the research area.From May 2019 to October 2019,we conducted 6 monthly samplings and two consecutive 24h daily samplings throughout the breeding period.Dissolved concentrations,saturation status,and emission fluxes of greenhouse gases（GHGs）in two mariculture ponds,were measured using the static headspace gas chromatography method and computed using a two-layer model of diffusive gas exchange.And the main influencing factors of GHGs emissions from the ponds under the disturbance of artificial management were identified using Spearman correlation analyses and multiple stepwise regression analyses.It is expected to provides data support for the in-depth study of greenhouse gas emission laws of aquaculture ponds in the entire coastal area,at the same time,it also could provides a certain reference for the development of low-carbon fisheries and policy formulation.The main conclusion of this paper includes the following several aspects:（1）Diurnal variation of GHGs concentrations and saturation:In the middle stage,the range and average value of the CO₂ concentration of the lower-salinity shrimp pond（A）were（12.64¹4.63）μmol/L and（13.88±0.73）μmol/L,and it’s changing trend is"M-type";the range and average value of the CH₄ concentration were（0.009⁰.030）μmol/L and（0.017±0.007）μmol/L;the range and average value of the N₂O concentration were（7.04⁷.39）nmol/L and（7.24±0.13）nmol/L,the minimum value occurs at 20:00.The range and average value of the CO₂concentration of the higher-salinity shrimp pond（B）were（12.06¹5.74）μmol/L and（13.31±1.41）μmol/L,and there were several peaks during the observation;the range and average value of the CH₄ concentration were（0.008⁰.087）μmol/L and（0.034±0.026）μmol/L;the range and average value of the N₂O concentration were（5.12⁵.26）nmol/L and（5.19±0.06）nmol/L.The saturation of the three greenhouse gases in the two aquaculture ponds during the mid-day aquaculture scales are relatively high.The fluctuation trend is basically consistent with the dissolved concentration,and the whole is close to saturated or even supersaturated.Except for CH₄,the saturation levels of CO₂ and N₂O are slightly lower than those in the middle stage,and both are in an unsaturated state.The analysis of statistical results shows that although the dissolved concentrations and saturations of the three greenhouse gases show certain daily dynamic characteristics.However,the diurnal difference was not significant,there only the concentration of CH₄ at the end stage of B showed a significant diurnal difference.On the whole,the concentrations and saturation levels of the three greenhouse gases have certain differences in the two aquaculture ponds A and B.A is higher,and only the dissolved concentration and saturation of CH₄ in the middle stage is lower than B.The reason is there are differences in various environmental factors of the two breeding ponds,such as the temperature,water temperature,DO,salinity,and NH₄⁺-N of A are smaller than B.Lower temperature results in lower gas diffusion rate.Lower DO levels not only provide a better anaerobic environment for methanogens and denitrifying bacteria,but also promote biological respiration.Lower salinity also inhibits microbial activity.The lower NH₄⁺-N level significantly affected the production of CH₄ because it provided less nitrogen source for methanogens.In addition,it can be inferred that the denitrification may be the main process of N₂O production on this time scale according to the change of N₂O concentration.（2）Monthly changes of GHGs concentrations and saturation:The range and average value of CO₂ concentration of A were（12.46⁶9.71）μmol/L and（24.74±22.33）μmol/L,and it followed the order:initial stage>middle stage>final stage.The range and average value of CH₄ concentration are（0.02⁰.07）μmol/L and（0.05±0.02）μmol/L,and it followed the order:final stage>initial stage>middle stage;The range and average value of N₂O concentration are（6.48⁹.95）nmol/L and（7.86±1.37）nmol/L,and it followed the order:initial stage>final stage>middle stage.The range and average value of CO₂ concentration of B were（9.60¹5.62）μmol/L and（12.50±2.46）μmol/L,and it followed the order:middle stage>initial stage>final stage.The range and average value of CH₄ concentration are（0.01⁰.16）μmol/L and（0.05±0.02）μmol/L,and it followed the order:initial stage>final stage>middle stage;The range and average value of N₂O concentration are（5.25⁷.31）nmol/L and（6.28±0.83）nmol/L,and it followed the order:initial stage>final stage>middle stage.The analysis of statistical results shows that the dissolved concentrations and saturation levels of the three greenhouse gases are significantly different at different stages.They all show a high level in initial stage.This is because,on the one hand,the temperature and water temperature in the initial stage are already at a relatively high level,the microbial activity is large,and the reaction rate is high.Nutrient elements in aquaculture ponds are enriched,and microorganisms have a sufficient reaction matrix.In addition,the dissolved concentration and saturation levels of the three greenhouse gases in this study are within the range of existing related research results,and the dissolved CO₂ concentration level is relatively high,which indicates that Tianjin marine aquaculture ponds have higher greenhouse gas emissions potential.（3）Diurnal variation of GHGs emission fluexs:The three greenhouse gases show different source and sink roles in different stages,and show different diurnal variation.Among them,during the middle stage,the range and average value of CO₂emission flux of A were（-137.97⁵2.61）μmol·m^-2·h^-11 and（-2.16±60.49）μmol·m^-2·h^-1,The overall performance is the sink of atmospheric CO₂,and the absorption capacity of atmospheric CO₂ is expressed as higher during the day and lower at night;the range and average value of CO₂ emission flux of B are（-35.67¹54.49）μmol·m^-2·h^-1and（43.78±68.75）μmol·m^-2·h^-1,and the overall performance is shown as atmospheric CO₂.Source,showing the characteristics of high day and low night,and it’s emission pattern is high during the day and low at night.The range and average value of CH₄ emission flux of A are（-2.89^-1.13）μmol·m^-2·h^-11 and（-1.72±0.64）μmol·m^-2·h^-1,and B are（-1.25¹.11）μmol·m^-2·h^-11 and（-0.25±0.91）μmol·m^-2·h^-1.The overall performance is the sink of atmospheric CH₄ in two ponds,and their changing rules are high during the day and low at night.The range and average value of N₂O emission flux of A are（27.10⁶5.80）nmol·m^-2·h^-11 and（42.52±13.01）nmol·m^-2·h^-1,and showing the characteristics of low day and high night.The range and average value of N₂O emission flux of B are（-36.98^-16.51）nmol·m^-2·h^-11 and（-27.24±7.45）nmol·m^-2·h^-1.The overall performance is the sink of atmospheric N₂O,and the absorption capacity of atmospheric N₂O is expressed as day low and night high.In final stage,the range and average value of CO₂ emission flux of A were（13.56¹5.24）μmol·m^-2·h^-11 and（14.45±0.56）μmol·m^-2·h^-1,the range and average value of CO₂ emission flux of B were（12.04¹2.57）μmol·m^-2·h^-11 and（12.37±0.17）μmol·m^-2·h^-1.They all appear to be the source of CH₄,and the changing rules are higher during the day and lower at night.The range and average value of CH₄emission flux of A are（0.05⁰.08）μmol·m^-2·h^-11 and（0.06±0.01）μmol·m^-2·h^-1,and of B are（0.05⁰.07）μmol·m^-2·h^-11 and（0.06±0.01）μmol·m^-2·h^-1.They all appear to be the source of CH₄,and the changing rules are higher during the day and lower at night and higher during the night and lower at day respectively.The range and average value of N₂O emission flux of A are（-27.04^-14.86）nmol·m^-2·h^-11 and（-23.69±5.02）nmol·m^-2·h^-1,and showing the characteristics of low day and high night.The range and average value of N₂O emission flux of B are（-104.59¹3.37）nmol·m^-2·h^-11 and（-28.70±38.22）nmol·m^-2·h^-1.They all appear to be the sink of CH₄,and the changing rules are higher during the day and lower at night.The analysis of statistical results shows that only the diurnal variation of CH₄emission flux of B in final stage was significant.There are some differences in the emission characteristics of GHGs between two stages:（1）In final stage at A,and in two stages at B were source of atmospheric CO₂,but the diurnal variation were different,which was affected by the intensity difference of algae photosynthesis and biological respiration in the pond.（2）CH₄ has changed from sink to source in different aquaculture stages of A and B.This is because on the one hand,residual bait,shrimp and other biological excreta have a higher accumulation level at the end of aquaculture.Provides a richer reaction substrate.On the other hand,when the daily change sampling is carried out at the end of the breeding,the breeding pond has started to drain water,the water depth is shallow,and the degree of CH₄ oxidation during the discharge process is reduced,thereby promoting the release of CH₄.（3）A is a N₂O emission source in the middle stage,while both the final stage at A and two stages at B are sink of atmospheric N₂O,which is different from existing related research results.This is because on the one hand,higher levels of salinity inhibit the activity of nitrifying and denitrifying bacteria,on the other hand,algae has high photosynthesis intensity,the water body is greatly disturbed by wind,and the anaerobic environment at the bottom of the pond is easily damaged,which inhibits denitrification The effect of the process,resulting in low N₂O production.（4）Monthly changes of GHGs emission fluxes:The range and average value of CO₂ water-air interface emission flux of A are（-60.09²307.16）μmol·m^-2·h^-11 and（626.03±969.10）μmol·m^-2·h^-1.The range and average of the water-air interface emission flux of B are（-74.20¹548.62）μmol·m^-2·h^-11 and（339.42±611.63）μmol·m^-2·h^-1.The range and average value of CH₄ water-air interface emission flux of A are（-1.33¹.77）μmol·m^-2·h^-11 and（-0.03±1.15）μmol·m^-2·h^-1.The range and average of the water-air interface emission flux of B are（0.07⁵.71）μmol·m^-2·h^-1and（2.41±2.29）μmol·m^-2·h^-1.The range and average value of N₂O water-air interface emission flux of A are（-23.32³50.26）nmol·m^-2·h^-11 and（96.58±143.57）nmol·m^-2·h^-1.The range and average of the water-air interface emission flux of B are（-16.51²07.85）nmol·m^-2·h^-11 and（32.02±89.30）nmol·m^-2·h^-1.The GHGs emission fluxes of the two ponds all showed the pattern of the initial stage>middle stage>final stage,and the overall performance was a source.The analysis of statistical results shows that the emission levels of the three greenhouse gases are quite different at different aquaculture stages.This is because it is affected by the alternate control of photosynthesis,biological respiration,and mineralization of organic matter by microorganisms in the water.This phenomenon also shows that the source-sink conversion mechanism of greenhouse gases is more complex and needs further exploration.（5）Influencing factors:Spearman correlation analysis results show that,the CO₂emission flux of A is significantly or very significantly related to environmental factors such as air temperature,water temperature,DO,pH,humidity,wind speed,redox potential,NO₂^--N and SO₄^2-while The CO₂ emission flux of B is related to air temperature and water temperature.Environmental factors including DO,pH,pH,humidity,NH₄⁺-N and SO₄^2-are significantly or very significantly related.The CH₄emission flux of A is significantly or very significantly related to air temperature,water temperature,humidity,salinity,redox potential and NO₃^--N,while the CH₄emission flux of B is only significantly correlated with redox potential.The N₂O emission flux of A is significantly or very significantly related to environmental factors such as air temperature,water temperature,pH,humidity,wind speed,redox potential,NO₂^--N and SO₄^2-,while the N₂O emission flux of B is related to air temperature,water temperature,DO Environmental factors including salinity,salinity,wind speed,redox potential,NH₄⁺-N and NO₂^--N were significantly or significantly related.The results of multiple stepwise regression analysis showed that there are many differences in the main influencing factors of three greenhouse gases in the two aquaculture ponds.（6）Comparative analysis with existing research:The maximum CO₂ emission flux is 101.52μmol·m^-2·h^-1,which is close to the largest in Fujian Province(164.3μmol·m^-2·h^-1)and the southern of Jiangsu Province(104.88μmol·m^-2·h^-1).This shows that the Tianjin marine aquaculture pond has a large CO₂ emission potential during the entire aquaculture period and is an important source of emissions.The maximum CH4 emission flux is 0.091μmol·m^-2·h^-1,which is close to the Hubei Province(0.151μmol·m^-2·h^-1),Anhui Province(0.12μmol·m^-2·h^-1),and the southern of Jiangsu Province(0.069μmol·m^-2·h^-1).The maximum N₂O emission flux 15.41nmol·m^-2·h^-11 is significantly lower than Fujian Province(143.00 nmol·m^-2·h^-1)and Jiangsu Province(3.48×10³nmol·m^-2·h^-1).This shows that the emission potential of CH₄ and N₂O in Tianjin marine aquaculture ponds cannot be ignored.At the same time,it was found that in this study,the three greenhouse gases had higher emissions in the early stage of breeding,which was different from other research results.This is mainly due to the high initial temperature and high protein content of the bait,resulting in large microbial activity and a rich reaction matrix.The results also show that how to improve the utilization rate of bait and reduce the impact of human activities on greenhouse gas emissions is also one of the directions that need to be further studied in the future.