Spatial And Temporal Variation Characteristics Of PCO₂ And CO₂ Efflux From Small Reservoir On The Loess Plateau

Reservoir ecosystems contribute significantly to CO₂ emissions.However,estimates of global reservoir CO₂ emissions remain greatly uncertain owing to the absence of a comprehensive and temporal resolved CO₂ emission measurement.As an important part of the carbon cycle of inland waters,the efflux of CO₂ from reservoirs plays an important role in the global carbon cycle.By using the floating chamber and headspace equilibrium methods along with a Licor-850 gas analyzer,this study investigates the partial pressure of carbon dioxide?pCO₂?and CO₂ emission?FCO₂?across water-atmosphere interface in the small reservoir,located in the upper reach of the Beiluo River on the Loess Plateau.A monitoring of CO₂ evasion at 3h intervals for five days?in total 200 measurements?were conducted in March,May,July,August and October in 2019.Moreover,the accompanying physicochemical parameters were also measured to explore the seasonal variation of surface water pCO₂ and CO₂ emission and its influencing factors.The temporal and spatial variations of pCO₂ and FCO₂ were compared to reveal the relationship between the reservoir and the atmosphere is a carbon“source”or a“sink”.Our study provides the data basis for including small reservoirs on the Loess Plateau into the current carbon emission estimates.The main conclusions of the paper are as follows:1. The water environmental factors?p H,Chl a,DO,DOC,DIC and water temperature.?in nangou reservoir have obvious seasonal variation characteristics.The reservoir water exhibited weak alkalinity.The variation of Chl a,DO,DIC and T_w were greatly during the seasons,and the DIC concentration of the water is extremely high.The diurnal p H,DO pattern showed the same variation with the Tw declining during the nighttime and increasing during the daytime.There is little difference in the horizontal distribution of water environmental factors in spatial variation.The p H and DO decreased with the increase of water depth.The DOC decreased slightly with the increase of water depth,and other factors did not change significantly with the depth.2. The pCO₂ was characterized by pronounced seasonal and diurnal variations.The pCO₂averaged 3248±594?atm,1249±141?atm,2284±312?atm,2431±369?atm and 1698±265?atm in March,May,July,August and October,respectively.The diurnal pCO₂ pattern showed the same variation as the whole data set,with pCO₂ dropping during the daytime?09:00-18:00?and increasing during the nighttime?21:00-06:00?.The pCO₂ during the nighttime were not significantly?p>0.05?different from the daytime,except in August.From a spatial perspective,pCO₂ levels are significantly different?p<0.05?at the D4 and D1.In July and August,the bottom water pCO₂ was higher than the surface water,and the surface water pCO₂ was higher than the bottom water in October.3. The surface water pCO₂ presented an extremely significant negative correlation with water temperature,p H and Chl a while an extremely positive correlation with dissolved inorganic carbon?DIC?and dissolved organic carbon?DOC?.The carbonate system?DIC and p H?and water temperature plays a major role in affecting the seasonal variation in reservoir CO₂ partial pressure?pCO₂?.The pCO₂is negatively correlated with p H and positively correlated with DIC,and the correlation coefficient is-0.67 and 0.76,respectively.It indicates that the soil strong chemical weathering,soil erosion and evaporation of the reservoir water leads an elevated DIC concentration in the reservoir,thereby resulting in a higher pCO₂.The metabolism of aquatic phototrophs acts as a secondary factor.Different water environment factors control the change of pCO₂ in different months.This indicates the interaction between different water environmental factors and pCO₂.4. The FCO₂ was characterized by pronounced temporal and spatial variations.The efflux in March was largely sustained by the release of under-ice accumulation?equivalent to average month CO₂ efflux?.The CO₂ efflux is at a low level from 9:00 to 18:00 and at a high level from 21:00 to 6:00 on the following day.The CO₂ emissions during the nighttime were greater than those during the daytime,except in August.Mean CO₂ efflux during nighttime and daytime are significantly?p<0.05?different among March,July and August.The spatial heterogeneity of FCO₂ is mainly influenced by the wind speed during the sampling periods.5. The water temperature plays a major role in affecting the seasonal variation in reservoir CO₂ emission?r=0.145,p<0.05?.Environmental factors have different correlations with FCO₂ in different months,demonstrating the complexity of FCO₂ of the the Nan-Gou reservoir.Using samples collected at noon to estimate the total CO₂ emissions would underestimate the CO₂ emissions by nearly 18 to 32%monthly.6. The range of CO₂ emission in the Nan-Gou reservoir is 101-838 g·m^-2·yr^-1.The CO₂efflux from the Nan-Gou reservoir is at an intermediate level for small reservoirs in temperate and subtropical climate,but it is lower than that from tropical reservoirs.