Effects Of Cyanobacteria Bloom On Methane Concentration And Emission In The Northwest Bay Of Lake Chaohu

Lakes are major natural sources of methane（CH₄）.In recent years,the frequent cyanobacteria blooms have been caused by the increasing eutrophication in global lakes.Cyanobacteria blooms are prone to change the physical and chemical properties of water,provide easily degradable organic matter,and accelerate methane production.Although the influence of cyanobacteria blooms on CH₄ is quite important under the background of global eutrophication,the exact effect is still unclear.Therefore,Lake Chaohu,a typical eutrophic shallow lake in China,was selected,and the Northwest Bay of the lake was choose to be the center area.Firstly,the temporal and spatial distribution characteristics and influencing factors of CH₄ concentration in the region were observed and analyzed.Then,the fluxes and pathways of CH₄ emission at the water-air interface in different eutrophication areas of Lake Chaohu were examined using floating chamber and diffusion model to explore the effect of cyanobacteria bloom on the intensity and pathway of CH₄ emission.Finally,the dynamic response of CH₄ concentration and flux to the accumulation of cyanobacterial bloom was studied by simulating the accumulation in the water and decay on the sediment surface of cyanobacteria algae.In the northwestern bay of Lake Chaohu,CH₄ concentrations ranged from 0.171±0.104μmol/L to 0.817±0.872μmol/L in the whole year observation,which is higher in the spring and summer,while lower in the autumn and winter.Spatially,there were higher CH₄ in the area near the estuary and the shore than the center of the west lake.CH₄ concentrations were positively correlated with chlorophyll-a,temperature,total nitrogen and total phosphorus in summer,spring and autumn,while there was no significant relation in winter.So,both cyanobacterial blooms and river input influence the dissolved concentration of CH₄ in this area.In the study of CH₄ emission fluxes and pathways in different eutrophication areas of Lake Chaohu,the highest concentration and emissions flux appeared in Northwest Bay,specifically,dissolved CH₄ in water was 0.178±0.002～1.123±0.026μmol/L,CH₄in surface sediments was70.5±30.7～189±97.0μmol/L,and the total emission flux of CH₄ was 50.1±2.93～1232±28.6μmol/m²/h.Diffusive CH₄ fluxes accounted for 7.3%～42.9%of the total emission,the ebullition accounted for 57.1%～92.7%of the total emission,and ebullition accounted for the highest proportion in the Northwest Bay.In addition,contributions of CH₄ emission pathways changed seasonally,with the highest ebullition contribution accounted for 98.1%in summer.It was found that the accumulation of cyanobacteria promoted the production of CH₄,and the higher the concentration of cyanobacteria,the higher the concentration of CH₄.The concentration of CH₄dissolved in water accumulated by cyanobacteria exhibited obvious diurnal variation,which was higher at night than in daytime.The decay of cyanobacteria on the surface of sediments promoted the release of CH₄ up to 99 times as much as that of the control,and the abundance of methanogens and the proportion of Methanothrix increased in sediments.With the continuous degradation of algal residues in surface sediments,the promotion gradually disappeared.In conclusion,the accumulation and decay of cyanobacteria increase the abundance of methanogens in sediments,enhanced the concentration of CH₄ in water.Therefore,the cyanobacteria accumulation area had higher CH₄ concentration level and emission intensity,and ebullition emission had a higher contribution to CH₄ emission flux in the cyanobacteria accumulation area.These results are meaningful to understand and predict methane production and emissions in the cyanobacterial accumulation area under the background of global prevailing cyanobacterial blooms.