Effects On Methane Fluxes Of The Brackish C.malaccensis Marsh Conversion To Aquaculture Pond In Min River Estuary

The reclamation of wetlands for aquaculture ponds had become main environmental issues to estuary,and affected the biogeochemistry cycle of methane（CH₄）in estuaries.In this study,we determined the influence of coastal marsh conversion to aquaculture ponds on CH₄ emissions fluxes and it’s temporal dynamic characteristics in the Min River estuary of southeast China from June 2019 to January 2020.By using lab incubation experiment and molecular biology techniques,we also determined the sediment CH₄ production rate、methanogenesis community structure and the CH₄emission pathway.The results will improve our understanding of the effects of coastal marsh conversion to aquaculture ponds on the methane biogeochemical cycle,which will provide essential scientific basis for effective regulation of CH₄ emissions and achieve sustainable development of coastal aquaculture industry.The main results showed as follows:（1）The main physical and chemical properties of aquaculture pond showed time dynamic characteristics and are greatly affected by the stage of aquaculture.Salinity,EC,DOC,Cl^-and SO₄^2-concentrations in middle stage and late stage were significantly higher than early stage（P<0.05）,but the difference in depth was not significant（P>0.05）.（2）The physicochemical properties of sedments showed time dynamic variations in the brackish C.malaccensis marsh（P<0.05）.SOC of the brackish C.malaccensis marsh was 16.01±2.07 g·kg^-1,which was significantly higher than that in the aquaculture ponds of 5.56±1.18 g·kg^-1（P<0.05）,and TN of the brackish C.malaccensis marsh was2.27±0.76 g·kg^-1,which was significantly higher than that in the aquaculture ponds of1.17±0.63 g·kg^-1（P<0.05）.Organic matter in sediments decreased with the Brackish C.malaccensis marsh conversion to aquaculture pond.（3）The relative abundance of Methanobacterium in the sediments（0-15 cm）from the brackish C.malaccensis marsh and aquaculture pond were 59.72%and 41.09%,respectively,which were dominant group of methanogens.Based on the analysis of the main groups of methanogens from sediments,we found that the main pathway of sediment methane production in the brackish C.malaccensis marsh and aquaculture pond were hydrogenotrophic.（4）Sediment CH₄ production rates showed large monthly variations in the brackish C.malaccensis marsh and aquaculture pond,with the highest CH₄ production rates generally in June and August,and August and October,respectively.The sediment CH₄production rates ranged from 3.34±0.66 to 134.16±28.35 ng·g^-1·d^-1 and 6.76±1.73 to52.74±11.40 ng·g^-1·d^-1,respectively,in the brackish C.malaccensis marsh and aquaculture pond,with with average values of 51.86±14.87 and 24.03±6.27 ng·g^-1·d^-1.Overall,the mean sediment CH₄ production rates in the brackish C.malaccensis marsh was higher than those in the aquaculture pond（P<0.05）.This results indicated that the conversion of brackish marsh to aquaculture ponds could decrease sediment CH₄production rates.（5）The CH₄ emission fluxes from the brackish C.malaccensis marsh ranged from0.25±0.07 mg·m^-2·h^-1 to 3.26±0.56 mg·m^-2·h^-1,with the highest CH₄ flux in June,and the lowest flux in January.In contrast,CH₄ emission fluxes from the aquaculture ponds ranged from 0.01±0.01 mg·m^-2·h^-1 to 27.25±2.84 mg·m^-2·h^-1,with the highest CH₄ flux in August,and the lowest flux in January.There were large difference in the temporal dynamic characteristics in CH₄ emission both among ecosystems.This results indicated that the coastal marsh ecosystem converted to aquaculture ponds considerably alter the the temporal dynamic characteristics of CH₄ emissions.（6）The mean CH₄ emission fluxes from the brackish C.malaccensis marsh was1.17±0.20 mg·m^-2·h^-1,which was significantly lower than that in the aquaculture ponds of 4.80±2.05 mg·m^-2·h^-1（P<0.01）.Our results highlighted that the conversion of brackish marsh to aquaculture ponds could increase CH₄ emissions,whihc could contribute to global warming substantially.We considered that CH₄emissions incresed following conversion of brackish marsh to aquaculture ponds,to a great extent,were dependent on the interactions of the change of water flooded condition and gas emission pathway,and organic matter supply.