丹江口水库浪河库湾夏季溶存CH_4特征

The characteristics of dissolved CH4 in the Langhe Bay of the Danjiangkou Reservoir in summer

针对丹江口水库溶存CH_4浓度(C_(CH4))和水-气界面CH_4通量(F_(CH4))及其理化性质,本研究采用高分辨率装置(FaRAGE)于2023年8月20~22日在浪河库湾开展为期2d的定点昼夜监测和多点位垂向断面监测,结果表明:库湾表层C_(CH4)、F_(CH4)具有显著的空间差异性.表层C_(CH4)、F_(CH4)分别为(0.54±0.16)μmol/L、(89.35±27.23)μg/(m^(2)·h),其F_(CH4)与其它水源地水库相近,但总体低于非水源地水库.垂向上,C_(CH4)具有表层低、底层高的分层现象.时间上,夜间表层C_(CH4)、F_(CH4)均高于白天.结构方程模型结果显示:库湾表层C_(CH4)与pH值变化较大有关(解释度为39.17%).垂向上,水温与溶解氧(DO)是C_(CH4)变化的主要原因,对其解释度分别为56.14%和35.25%.昼夜C_(CH4)变化的主导因素是DO(解释度为52.32%).根据水体综合营养指数(TLI(Σ)),丹江口水库为贫营养水库.库湾表层TLI(Σ)(47.36±8.28)从上游(LH00)到下游(LH05)逐渐下降.垂向上,TLI(Σ)(均小于50)总体呈表、底层高而中层低分布趋势.与其它研究不同的是,丹江口水库C_(CH4)与TLI(Σ)并无显著正相关关系.研究表明中营养水源地水库水体降低了水体藻华爆发风险,进而保持水源地水库水质和减少CH_4排放,但在中营养水库中并不能用TLI(Σ)评估CH_4排放水平.

This study focuses on the dissolved CH4 concentration(C_(CH4)),CH4 diffusive flux at the water-air interface(FCH4)and the physicochemical properties of the water body in Danjiangkou Reservoir.Using high-resolution equipment(FaRAGE),a two-day monitoring campaign was conducted from August 20th to 22nd,2023 in the Langhe Bay,including continuous monitoring at a certain site and vertical profile monitoring at multiple sites.The results showed that the surface C_(CH4)and FCH4 in the reservoir bay exhibited significant spatial variability,with mean values of(0.54±0.16)μmol/L and(89.35±27.23)μg/(m^(2)·h),respectively,and FCH4 was comparable to other water source reservoirs but generally lower than non-water-source reservoirs.Vertically,C_(CH4)was stratified with low levels in the surface water and high levels in the bottom water.Temporally,the nocturnal surface C_(CH4)and FCH4 were higher than those during the day.The results of the structural equation model indicated that spatial variations of surface C_(CH4)in the reservoir bay could be attributed to pH with an explanation degree of 39.17%.Vertically,water temperature and dissolved oxygen(DO)mainly drove C_(CH4),explaining 56.14%and 35.25%of the variance of C_(CH4),respectively,while 52.32%of variance of diurnal C_(CH4)resulted dominantly from DO.Water quality in the reservoir bay was classified as the oligotrophic status based on the comprehensive trophic level index(TLI(Σ)),and TLI(Σ)of the surface water gradually decreased longitudinally from site LH00 to LH05,with an average of(47.36±8.28).Vertically,TLI(Σ)(the mean was less than 50)was lower in the middle layer than both at the surface and bottom layers of the water column.There was no significant positive correlation between C_(CH4)and TLI(Σ)in the Danjiangkou Reservoir,which was different from the findings in other studies.The mesotrophic water in source reservoirs reduced the risk of algal blooms,and thereby maintained the water quality and low CH4 emissions;however,evaluating CH4 emissions using TLI(Σ)was not suitable for mesotrophic reservoirs.