Dissolved Methane And Carbon Dioxide Concentration And Flux In Coastal Aquaculture Shrimp Ponds Of Min River Estuary

Aquaculture is an effective way for human to exploit and utilize fishery resources,the construction of various aquaculture ponds has formed a worldwide artificial aquatic ecosystem.As an important aquatic ecosystem type,the increase of carbon and nitrogen load and dense biomass lead to large carbon storage in aquaculture waters,and at the same time becomes an important source or sink of CH₄and CO₂.Nevertheless,aquaculture ecosystem has received less attention because of its narrow area and scattered distribution.Under such circumstances,this study fouced on the typical shrimp culture system in Min River Estuary and determined the CH₄ and CO₂ dissolved concentration profile and flux across water/sediment-atmosphere interface and simultaneously measured physicochemical parameters in situ to reveal the characteristics of carbon flux change in coastal reclamation ponds,improve the carbon emission inventory of aquaculture system on a large scale,and provide data support for the formulation of coastal"carbon sink fishery"emission reduction program.The specific main results are as follows:?1?Temporal variation of environmental factors in shrimp ponds.The environmental factors of shrimp ponds were determined from June 2017 to May next year by in situ observation and indoor analysis.The results showed that the environmental factors of reclaimed shrimp ponds in Minjiang Estuary have significant temporal variations.Meteorological parameters changed with the seasons.In order to regulate the water quality in the suitable range for shrimp growth,the indexes of electrical conductivity,dissolved oxygen,oxidation-reduction potential,pH,chlorophyll a and salinity in aquaculture water fluctuated frequently,but with much smaller variation range.Nutrients in water?dissolved organic carbon,dissolved total nitrogen,ammonia nitrogen and dissolved total phosphorus?and total nitrogen,total carbon in sediments accumulated gradually during the culture period,and the concentration of nutrient substance in the early stage of aquaculture was significantly lower than that in the middle and late stage affected by culture activities?water quality control,feed delivery?and biological metabolism.In the non-aquaculture period,the changes of submergence and drainage status of shrimp ponds and the dry and wet degree of sediments resulted in obvious phase characters of physical and chemical parameters between culture and non-culture period.?2?Variation characteristics of water dissolved CH₄ and CO₂ concentration profiles in shrimp ponds during aquaculture period.CH₄ and CO₂ concentration profile dissolved in water were monitored using headspace balance-gas chromatography technique from June to November 2017.The results showed that CH₄ and CO₂ in aquaculture water are basically in supersaturated state for a long time.The mean values of CH₄ and CO₂ concentration were?13.85±3.57??mol·L^-1 and?44.43±1.63??mol·L^-1,and time-varying ranged from1.28⁵5.97?mol·L^-1 and 18.11⁷9.60?mol·L^-1,respectively.The correlation analysis showd that pH,dissolved total nitrogen and dissolved total phosphorus are important factors affecting the temporal dynamics of CH₄ concentration,meteorological parameters,water temperature,pH and dissolved oxygen are important factors affecting the temporal variation of CO₂ concentration.The deep-varying of CH₄ and CO₂ concentration ranged from 4.31³2.69?mol·L^-1 and 36.40⁵5.68?mol·L^-1,and showed the change characteristics of bottom>middle>surface layer,but the change rule of CH₄ concentration was not obvious.?3?The changes of CH₄ and CO₂ fluxes at water/sediment-atmosphere interface of shrimp ponds during the observation period.CH₄ and CO₂ fluxes at water/sediment-atmosphere interface were monitored using suspension/static chamber-gas chromatography technique from June 2017 to May 2018.The results showed that CH₄ and CO₂ fluxes in shrimp ponds were basically in the state of net emission and absorption,respectively.Mean fluxes of CH₄and CO₂ were?35.74±10.88?and?1.16±4.30?mg·m^-2·h^-1 during culture period,?5.56±1.73?and?-39.33±13.89?mg·m^-2·h^-11 during non-culture period,respectively.CH₄ and CO₂ fluxes ranged from 1.17¹27.09 and-24.99²0.69 mg·m^-2·h^-1 during culture period,0.23²2.16 and-133.96²8.42 mg·m^-2·h^-1 during non-culture period,respectively.The correlation analysis showed that water temperature,dissolved oxygen concentration and biological activity were important factors affecting CH₄flux in aquaculture period.Temperature and sediment moisture content in drainage environment significantly affected CH₄ flux in non-aquaculture period.Photosynthesis of phytoplankton and terrestrial plants played an important role in regulating CO₂ flux in observation period.?4?Contribution of fifferent transport pathways to CH₄ emission fluxes from reclaimed shrimp ponds in Min River Estuary.CH₄ emission flux from shrimp ponds through bubble and diffusion pathways were measured by traditional suspension/diffusion chamber-gas chromatography method from June to November 2017.The results showed that bubbles and diffusion flux of CH₄ ranged from 0.49¹25.69 and 0.08³.00 mg.m^-2.h^-1,respectively.The CH₄ emission from the bubble pathway contributes more than 70%of the total flux,indicating that the main way for CH₄ gas to enter the atmosphere was through bubble transport in shallow shrimp ponds.In terms of time,CH₄ fluxes of both transport routes experienced a change from rising to falling,and bubble and diffusion fluxes peaked in mid-August and late September.Water temperature and dissolved oxygen concentration are important factors affecting CH₄ bubble emission,and dissolved oxygen concentration also affects CH₄ diffusion transport too.