Spatial Pattern Of CH₄ Flux And Its Impact Factors Analysis In Lake Taihu

Lake is an important methane(CH4)source for the atmosphere.The diffusion,ebullition,and plant-medium are the main pathways for CH4 emission across the lake-air interface.And,temperature,nutrient load,water depth,biomass and son on impact the CH4 emission from lake.Considering the climate change and water eutrophication,such as algal bloom,the temporal and spatial variation of CH4 emissions and its environmental biological control mechanism at large shallow eutrophic lake has become the hot topic for lake greenhouses cycle.This objective of study was to investigate the temporal and spatial variation of CH4 emission and its environmental biological control mechanism at Lake Taihu,a large(area 2400 km2)shallow(mean water depth:1.9 m)eutrophic lake in Eastern China.Sampling at five lake flux sites and whole-lake 29 spatial sites were conducted continuously with different time scales from 2011 to 2016.To explore the influences of algal blooms and submerged macrophytes biomass on the lake CH4 emission,the flux-gradient and eddy covarance method with in-situ and continuous measurement were deployed in the eutrophic zone and in a habitat of submerged macrophytes,respectively.The main results in the present study are as follows:(1)CH4 diffusion flux across the lake-air interface showed strong temporal and spatial variabilities.The CH4 diffusion flux increased with water temperature,the average flux was six times higher in summer than that in winter.The spatial pattern of the diffusion CH4 emission was correlated with water depth and pollution level,and was also associated with the spatial distributions of algal and submerged vegetation.Multiple stepwise regression analysis indicated NDVI,water clarity,dissolved oxygen concentration,and water depth together explain 78%of observed spatial variability in the diffusion CH4 flux(R2 = 0.78,p<0.01,n = 29).(2)Algal blooms caused by water eutrophication significant changed the CH4 emission flux between lake surface and the air.The emission pattern and control factors for lake CH4 are different at different time scales in eutrophic zone.The average total CH4 flux(0.460 mmol m-2 d-1)and diffusion CH4 flux(0.136 mmol m-2 d-1)in eutrophic zone were significantly higher than that in central zone(total flux:0.102 mmol m-2 d-1;diffusion flux:0.030 mmol m-2 d-1).The in-situ and continuous measurement using flux-gradient method indicated the CH4 flux in eutrophic zone did not show diurnal variation.Half-hourly CH4 flux was sensitive to the atmospheric pressure and water buoyancy flux at night.CH4 flux also showed significant daily,monthly,and seasonal variation.The CH4 flux variations were correlated with water temperature.In the hyper-eutrophic Northwest Zone with higher allothogenic nutrient load,the poor correlation between water temperature and CH4 flux was found.But NDVI explains 67%observed temporal variability in the diffusion CH4 flux(R2 =0.67,p<0.01).(3)The submerged vegetation significantly enhanced the CH4 emission flux.The CH4 total and diffusion flux are the highest in the submerged vegetation zone.The in-situ and continuous measurement using eddy covariance method showed a diurnal cycle of CH4 flux.Driven by the higher waterside convection,the CH4 flux during nighttime was 40%higher than that during daytime.The half-hour and daily mean eddy flux did not show temporal variation.Half-hour CH4 flux was influenced by the atmospheric pressure,water temperature,and friction velocity.Daily mean CH4 flux is controlled water buoyancy flux during nighttime.Monthly and seasonal mean CH4 flux positively correlated with water temperature and NDVI with high values during warm season and low values during cold season.(4)Based on multiple-years' measurements by three different methods,the mean CH4 flux of the whole lake was 0.265 ± 0.095 mmol m-2 d-1 or 1.55 ± 0.55 g m-2 year-1,which consisted of diffusion emission,0.54 ± 0.29 g m-2 yr-1 and ebullition emission,1.0 ± 0.27 g m-2 year-1.The ebullition contributed 65%of total CH4 emission at Lake Taihu.However,ebullition contributed to the total CH4 flux was 71%and 49%in an open-water eutrophic zone and in a habitat of submerged macrophytes,respectively.The in-situ and continuous measurement revealed that CH4 flux is significant higher in clear water vegetation-dominated status(BFG site:5.519 g m-2 year-1)than that in turbid water alga-dominated status(MLW site:2.208 g m-2 year-1).Despite the lake Taihu was the eutrophic,the whole-lake mean CH4 emission was smaller than that reported in a synthesis study for lakes in Eastern China with an order of magnitude.The results implied that temporally and spatially distributed measurements were necessary to estimate accurately whole-lake CH4 emissions.