Dynamics Of Carbon Dioxide And Methane In Coastal Reclaimed Shrimp Ponds During Culture Period Of Two Main Estuaries,Fujian

Aquaculture shrimp ponds is an important part of aquaculture ecosystems in the estuary area.Study on the key processes of carbon fluxes from the aquaculture shrimp ponds in the estuary area plays a vital role in many important aspects,such as developing our knowledge of the biogeochemical cycles of carbon in coastal aquaculture systems,as well as better improving the establishment of related policies for carbon sink fishery and low carbon technology fishery.In this paper,shrimp(Litopenaeus vannamei)ponds in the Min River estuary and the Jiulong River estuary,located in Fujian,southeast of China,were selected as the research area.The variations of key processes of carbon dynamics during different culture periods in the shrimp ponds of the two estuaries were studies via field investigation and laboratory experiments.The main results showed as follows:(1)During the study period,sediment CO2 production rate from shrimp ponds in the Min River estuary(MJ)and the Jiulong River estuary(JLJ)ranged between 29.97 and 116.23,and 29.26 and 107.19 ?g g-1 d-1,respectively.Average CO2 production rates showed significant seasonal variation in order of middle stage>initial stage>final stage.Sediment CH4 production rates from shrimp ponds in the Min River estuary(MJ)and the Jiulong River estuary(JLJ)ranged between 1.26 and 1.92,and 0.96 and 1.44 ?g g-1 d-1,respectively.The CH4 production rates from shrimp ponds sediment in the MJ was significantly lower during the initial stage compared to other stages,while the CH4 production rates in the JLJ showed a increasing trend over the study period.Correlation analysis showed that sediment CO2 production rates from shrimp ponds in the MJ were significantly correlated to sediment temperature,pH value,salinity,porosity and TN contents,while CH4 production rates were significantly correlated to sediment temperature,TC content,TN content and C:N.In the JLJ,sediment CO2 production rates from shrimp ponds were significantly correlated with sediment temperature,pH value,TC and TN contents,while CH4 production rates were significantly correlated with sediment conductivity and salinity.Average CO2 production rate in the ponds overlying water in each estuary followed the order:middle stage>final stage>initial stage,varying from 0.32 to 0.50 mmol CO2 m-2 h-1 in the MJ and 0.09 to 0.36 mmol CO2 m-2 h-1 in the JLJ.The potential CH4 oxidation(MO)rate from the ponds overlying water in the MJ and the JLJ during the study period ranged between 0.80 and 5.66 ?mol CH4 m-2 h-1,and 0.36 and 5.17 ?mol CH4 m-2 h-1,respectively.Significant temporal variations in MO in the overlying water of shrimp ponds were observed over the study period,which showed an increasing trend over time.Temporal variations of CO2 production rate in the MJ were probably related to the water temperature and TOC,while those of MO were controlled by the interactions of pH,dissolved oxygen,DOC,salinity and Chl-a.In the JLJ,the temporal variations of CO2 production rate may be related to the variations and interactions of water temperature,salinity,substrates and Chl-a,while those of MO were primarily affected by pH and dissolved oxygen.The above results indicate that organic matter(OM)degradation in anaerobic sediments and aerobic water column is the main source of shrimp ponds CO2.The primary source of CH4 in shrimp ponds is the degradation of OM in anaerobic sediments,while the water column is an important place for CH4 consumption because of its high dissolved oxygen.(2)The porewater CO2 and CH4 concentrations in shrimp ponds of the the MJ and the JLJ over the study period ranged between 35.47 and 199.42 ?mol CO2 L-1,14.09 and 48.05 ?mol CO2 L-1,13.47 and 94.44 ?mol CH4 L-1,and 2.29 and 29.52 ?mol CH4L-1,respectively,with significantly higher concentrations during the middle stage.The CO2 concentrations in the ponds water column of the MJ and JLJ ranged between 7.95 and 213.57 ?mol CO2 L-1,and 13.03 and 66.40 ?mol CO2 L-1,respectively,showed a decreasing trend over time.CH4 concentrations in the ponds water column of both estuaries generally increased over time,varying from 0.14 to 1.22 ?mol CH4L-1 and 0.14 to 0.68 ?mol CH4 L-1.Temporal variations of ponds sediment porewater CO2 in the MJ were probably related to the porewater TOC concentration and sediment pH,while those of CO2 concentrations in the JLJ were regulated primarily by sediment pH and temperature.The variations of ponds sediment porewater CH4 in both estuaries during the study period,to a large extent,were dependent on porewater TOC concentration.In the water column of shrimp ponds,the temporal variations of CO2 concentrations in the MJ were probably related to the water salinity,dissolved oxygen,NH4+-N and PO43--P,while those of CH4 concentrations were controlled by the interactions of water pH,salinity and Chl-a.In the JLJ,the variations of CO2 concentrations in water column of the shrimp ponds were probably related to the NH4+-N,NO3--N,dissolved oxygen and Chl-a,while those of CH4 concentrations were controlled by the interactions of water salinity,DIC,NH4+-N and temperature.Significant temporal variations in CO2 and CH4 fluxes across the sediment-water interface(SWI)of shrimp ponds were observed over the study period,with minimum and maximum fluxes occurred at the initial and final stages,respectively.The CO2 fluxes in the MJ and JLJ over the study period ranged between 0.28 and 3.46 mmol CO2 m-2 h-1,and 0.18 and 3.01 mmol CO2 M-2-1,respectively.The CO2 fluxes in both estuaries over the study period ranged between 5.26 and 168.81 mmol CO2 m-2 h-1,and 2.93 and 56.74 mmol CO2 m-2 h-1,respectively.In the MJ,the temporal variations of CO2 fluxes from the SWI were probably related to the sediment temperature,TC content and porewater DIC,while those of CH4 fluxes were controlled by the interactions of sediment TC content,salinity and porewater DIC.In the JLJ,the variations of CO2 fluxes across the SWI were probably related to the sediment temperature,while those of CH4 fluxes were controlled by the interactions of sediment C:N and porewater NH4+-N concentration.Over the study period,we observed that the mean concentrations of CO2 and CH4 were higher in the sediment porewater than those in the ponds water column,and consistently positive CO2 and CH4 fluxes across the SWI in the shrimp ponds,indicating that the shrimp pond sediment is generally a net release source of C02 and CH4.(3)The CO2 fluxes across the water-air interface(WAI)were quite variable over the study period,ranging from positive values(3.37 and 1.58 mmol CH4 m-2 h-1 in the MJ and the JLJ,respectively)during the initial stage to negative values(-2.09 and-0.49 mmol CH4 m-2 h-1 in the MJ and in the JLJ,respectively)during the final stage.Overall,the shrimp ponds in each estuary acted as a net source of CO2 at the initial stage and middle stage with a positive mean flux and a net uptake of CO2 at the final stage with a positive mean flux.From the view of sampling time,there were large variations in CO2 fluxes across the WAI both among estuaries and observation periods.In the MJ,the shrimp ponds acted as a net source of CO2 at the initial stage and middle stage and a net uptake of CO2 at the final stage for all sampling time.In the JLJ,the CO2 fluxes across WAI were always positive at the initial stage,while the fluxes were quite variable during the middle stage and final stage ranging from a net release on 08:00 to a net uptake on 14:00 or 11:00.These results suggest that C02 fluxes from the estuarine shrimp ponds greatly varied among the three stages.The variations of CO2 fluxes were dependent on the interactions of algal photosynthesis,biorespiration and the mineralization of organic matter.The CH4 fluxes across the SWI were always positive over the observation period,which corresponded to net nutrient releases from the shrimp ponds to the atmospheric environment.The total CH4 fluxes from the shrimp ponds in the MJ and JLJ over the study period ranged between 0.28 and 16.28 mmol CH4 m-2 h-1,and 0.13 and 1.25 mmol CH4 m-2 h-1,respectively,with the highest and lowest values during the final and initial stages.The diffusive CH4 fluxes in both estuaries during the study period ranged between 0.28 and 79.85 ?mol CH4 m-2 h-1,and 0.58 and 32.52 ?mol CH4 m-2 h-1,respectively,which showed an increasing trend over time.The chamber fluxes are consistent with ebullition being between 5 and 1500 times greater than the calculated diffusive fluxes.It appears that ebullition is prevalent in these observation sites,which is the dominant pathway of CH4 release from the shrimp ponds to the atmosphere.(4)CO2 and CH4 fluxes across the WAI from the shrimp ponds were positively correlated to gases production rates and gases fluxes across the SWI.Although there is not reach the significant level in some cases,but this positive correlation suggested that high gases production rates and high gases fluxes across the SWI is an essential prerequisite for high gases fluxes across the WAI both among estuaries and observation periods.(5)Statistical analysis results showed that the key processes of carbon fluxes of coastal land aquaculture shrimp ponds in the MJ were generally higher than that in the JLJ.Salinity is an important factor causing the differences of key processes of carbon fluxes from the coastal land aquaculture shrimp ponds across the two estuaries.The results indicate that it is much important to study the carbon biogeochemical cycles in the aquaculture ponds in different estuaries with different salinity in future.