高原湖泊溶解甲烷濃度空間異質(zhì)性研究—以陽(yáng)宗海秋季為例

鄭祥旺,陳 敏,3*,肖尚斌,3,王圣瑞,王雪竹,陳 巍,劉 佳,許浩霆

高原湖泊溶解甲烷濃度空間異質(zhì)性研究—以陽(yáng)宗海秋季為例

鄭祥旺1,2,陳 敏1,2,3*,肖尚斌1,2,3,王圣瑞4,5,王雪竹2,陳 巍2,劉 佳2,許浩霆2

(1.三峽庫(kù)區(qū)生態(tài)環(huán)境教育部工程中心,湖北 宜昌 443002；2.三峽大學(xué)水利與環(huán)境學(xué)院,湖北 宜昌 443002；3.三峽水庫(kù)生態(tài)系統(tǒng)湖北省野外科學(xué)觀測(cè)研究站,湖北 宜昌 443002；4:北京師范大學(xué)水科學(xué)研究院,北京 100875；5.云南省高原湖泊流域污染過(guò)程與管理重點(diǎn)實(shí)驗(yàn)室,云南 昆明 650034)

以西南典型高原湖泊陽(yáng)宗海為研究對(duì)象,于2020年11月應(yīng)用新型快速水-氣平衡裝置(FaRAGE)及便攜式溫室氣體分析儀,開(kāi)展了表層水體和垂向剖面溶解CH4濃度的高分辨率監(jiān)測(cè),揭示了溶解CH4濃度空間分布特征及影響因素.結(jié)果表明,陽(yáng)宗海表層水體溶解CH4濃度為0.02～0.97μmol/L,表現(xiàn)為大氣甲烷的源.空間上呈現(xiàn)出南北高、中部低的分布特征,與水生植物分布、入湖河流輸出和水深等因素密切相關(guān).湖泊不同區(qū)域垂向剖面溶解CH4濃度分布較為一致,湖心區(qū)CH4氧化消耗最為明顯.自展分析表明,基于少量樣點(diǎn)監(jiān)測(cè)的CH4排放估算具有較大不確定性,準(zhǔn)確估算陽(yáng)宗海CH4擴(kuò)散通量所需最小采樣點(diǎn)數(shù)量約為3.7個(gè)/km2,高空間分辨率的監(jiān)測(cè)對(duì)于湖庫(kù)碳排放的準(zhǔn)確估計(jì)十分必要.

溫室氣體；溶解濃度；空間分布；通量；陽(yáng)宗海

湖泊作為淡水生態(tài)系統(tǒng)的重要組成部分,是CH4的重要排放源之一[1].Bastviken等[1]估計(jì)全球湖泊CH4年排放量為71.6Tg,遠(yuǎn)高于2004年估計(jì)的6～36Tg[2].這種巨大差異不僅存在于全球尺度,單一湖泊的碳排放估計(jì)也存在較大不確定性[3-4].

目前估算單個(gè)湖泊水-氣界面CH4擴(kuò)散通量通常采用薄邊界層法[5],CH4擴(kuò)散通量主要受控于表層水體溶解CH4濃度和氣體傳輸速率,風(fēng)速作為氣體傳輸速率的主控因子較容易監(jiān)測(cè),表層水體溶解CH4濃度的準(zhǔn)確獲取是CH4通量估算的關(guān)鍵.現(xiàn)有研究多基于有限樣點(diǎn)溶解CH4濃度計(jì)算的CH4擴(kuò)散通量均值乘以水域面積估算總擴(kuò)散通量,但該方法缺乏對(duì)溶解CH4濃度分布空間異質(zhì)性的考慮[6-7]. Xiao等[8]基于太湖29個(gè)采樣點(diǎn)的監(jiān)測(cè)發(fā)現(xiàn),藻類(lèi)大量生長(zhǎng)的梅梁灣溶解甲烷濃度高達(dá)0.13μmol/L,而中心湖區(qū)僅為0.02μmol/L.Yang等[9]監(jiān)測(cè)研究則發(fā)現(xiàn),通量計(jì)算中剔除排污區(qū)域監(jiān)測(cè)點(diǎn)溶解CH4濃度數(shù)據(jù),總CH4擴(kuò)散通量將被低估69%,可見(jiàn)基于少數(shù)監(jiān)測(cè)點(diǎn)的通量估算具有很大的潛在誤差.盡管部分研究者嘗試按照水深等自然地理特征對(duì)湖泊進(jìn)行分區(qū)[10],通過(guò)分區(qū)面積加權(quán)平均估算擴(kuò)散通量,但由于觀測(cè)點(diǎn)數(shù)量有限,某一分區(qū)內(nèi)的實(shí)測(cè)均值很可能無(wú)法表征該區(qū)域的擴(kuò)散通量真實(shí)水平.事實(shí)上,為評(píng)估溶解CH4濃度分布空間異質(zhì)性對(duì)甲烷排放估算的影響,已有部分學(xué)者在水庫(kù)的研究中嘗試加密監(jiān)測(cè)點(diǎn)和革新監(jiān)測(cè)技術(shù)以獲得高空間分辨率的CH4濃度.例如,Yang等[9]基于傳統(tǒng)頂空平衡法在面積為5.2km2的文武砂水庫(kù)進(jìn)行了108個(gè)樣點(diǎn)溶解CH4濃度加密監(jiān)測(cè),Paranaíba等[11]基于膜平衡器對(duì)巴西3個(gè)水庫(kù)溶解CH4濃度進(jìn)行了高空間分辨率的走航式監(jiān)測(cè),兩項(xiàng)研究均表明,高空間分辨率的CH4濃度數(shù)據(jù)是準(zhǔn)確估算CH4擴(kuò)散通量的關(guān)鍵.

中國(guó)西南云貴高原湖泊眾多,在全球氣候變化的背景下,其碳排放問(wèn)題成為國(guó)內(nèi)外關(guān)注的焦點(diǎn)[12].本文以云貴高原湖區(qū)典型湖泊陽(yáng)宗海為研究對(duì)象,應(yīng)用新型快速水-氣平衡裝置(FaRAGE)[13-14]及便攜式溫室氣體分析儀原位高頻監(jiān)測(cè)表層水體及垂向剖面中的溶解CH4濃度,并估算擴(kuò)散通量,探討陽(yáng)宗海溶解CH4濃度的分布規(guī)律和影響因子,通過(guò)將空間異質(zhì)性因素納入考慮,以期為湖庫(kù)水域CH4排放更準(zhǔn)確的估算提供參考.

1 材料與方法

1.1 研究區(qū)概況

陽(yáng)宗海(102°59'～03°02' E、24°51'～24°58' N)地處云南省昆明市東南,湖面面積31.9km2(水位1770m時(shí)),平均水深19.5m,蓄水量6.04×108m3,換水周期13a[15].流域內(nèi)有陽(yáng)宗大河、七星河、擺衣河等河流匯水入湖,出水河道僅有湯池河.陽(yáng)宗海流域地處亞熱帶高原季風(fēng)氣候區(qū)域,多年平均降水量824.7mm,多年平均湖面蒸發(fā)量1161.7mm,多年平均溫度16.2℃[16].研究區(qū)域及監(jiān)測(cè)點(diǎn)位置如圖1所示.

1.2 樣品采集與分析

于2020年11月9日白天在陽(yáng)宗海進(jìn)行走航監(jiān)測(cè),監(jiān)測(cè)船以10km/h的速度航行并采用自吸泵連續(xù)取水,取水口位于水面以下20cm處,共設(shè)3個(gè)監(jiān)測(cè)點(diǎn)進(jìn)行垂向剖面監(jiān)測(cè).

圖1 走航軌跡及監(jiān)測(cè)點(diǎn)分布

利用手持式氣象站(YGY-QXY,中國(guó))監(jiān)測(cè)風(fēng)速和氣溫等氣象要素.溶解氧飽和度、電導(dǎo)率、葉綠素a、水溫、pH值和水深采用多參數(shù)水質(zhì)分析儀(YSI- EXO,美國(guó))進(jìn)行測(cè)定.走航水深使用聲學(xué)多普勒流速剖面儀(RDI RiverRay ADCP,美國(guó))測(cè)定,并同步記錄船只移動(dòng)過(guò)程的地理坐標(biāo).

水體溶解CH4濃度及其穩(wěn)定碳同位素監(jiān)測(cè)分為表層水體和垂向剖面2個(gè)部分.水體溶解CH4濃度及其穩(wěn)定碳同位素采用新型快速水-氣平衡裝置(FaRAGE)[13-14]及便攜式溫室氣體分析儀(Picarro G2301,美國(guó))連續(xù)測(cè)定,待測(cè)水體和載氣(氮?dú)?以恒定流量進(jìn)入水-氣平衡裝置,載氣形成的微氣泡與待測(cè)水體發(fā)生水氣交換并達(dá)到動(dòng)態(tài)頂空平衡,該裝置具有響應(yīng)時(shí)間短(12s)、測(cè)量精度高(誤差小于0.5%)的優(yōu)點(diǎn).選取S1、S2和S3共3個(gè)監(jiān)測(cè)點(diǎn)進(jìn)行垂向剖面監(jiān)測(cè),垂向剖面溶解CH4濃度及其穩(wěn)定碳同位素監(jiān)測(cè)使用潛水泵從水體表層到底層分層抽取水,采用相同方法測(cè)定不同深度水體溶解CH4濃度,且與多參數(shù)水質(zhì)分析儀監(jiān)測(cè)同步.

1.3 數(shù)據(jù)處理與統(tǒng)計(jì)分析

淡水中溶解氧飽和度計(jì)算公式為[17]:

式中:DO為溶解氧飽和度,%;DO為實(shí)測(cè)溶解氧質(zhì)量濃度,mg/L;c為觀測(cè)溫度下的飽和溶解氧質(zhì)量濃度,mg/L.計(jì)算公式為:

c=477.8/(+32.36) (2)

式中:為實(shí)測(cè)水溫,℃.

穩(wěn)定碳同位素的組成利用公式(3)計(jì)算.

式中:sa和s1分別表示待測(cè)樣品和標(biāo)準(zhǔn)樣的同位素比值,13C/12C對(duì)應(yīng)于國(guó)際標(biāo)準(zhǔn)ViennaPDB.

薄邊界層法計(jì)算水-氣界面氣體通量利用公式(4)計(jì)算:

式中:為水-氣界面擴(kuò)散通量,mol/(m2·h);為氣體交換系數(shù),cm/h;sat為該氣體相對(duì)于上方空氣而言平衡時(shí)表層水體中的濃度,mol/L;W為該氣體在表層水體中的濃度,mol/L.

為便于不同氣體傳輸速率的比較,采用式(5)對(duì)進(jìn)行標(biāo)準(zhǔn)化[18]:

式中:c為給定氣體在相應(yīng)溫度下的CH4的Schmidt數(shù),本研究利用Wanninkhof等[19]提出的方法計(jì)算c;600是當(dāng)Schmidt數(shù)為600時(shí)的氣體傳輸速率;是與風(fēng)速相關(guān)的系數(shù),由于所有10m高度的風(fēng)速均小于<3.7m/s,本文中取值為2/3[20].

氣體傳輸速率600使用風(fēng)速模型[21]計(jì)算:

式中:10為水面10m高處的風(fēng)速,m/s,可由實(shí)測(cè)風(fēng)速和觀測(cè)高度計(jì)算[22].

采用Spearman相關(guān)性分析探求表層溶解CH4濃度與各理化因子的相關(guān)性,使用自展分析進(jìn)行重抽樣,評(píng)估數(shù)據(jù)樣本量大小對(duì)通量估算的影響.

2 結(jié)果與分析

2.1 環(huán)境因子

如表1和圖2所示,表層水體溶解氧飽和度為52.17%～75.93%,表現(xiàn)為南部湖區(qū)大于北部湖區(qū);電導(dǎo)率為421.82～427.56mS/cm,湖泊北部大于南部;表層水體水溫為18.56～20.18℃,湖泊北部高于南部;葉綠素a濃度為1.82～12.37mg/L,空間上呈現(xiàn)南北高、中部低的特征;pH值為8.23～8.55,呈現(xiàn)弱堿性,南部湖區(qū)大于北部湖區(qū),整個(gè)湖泊差異較小.

表1 層水體理化性質(zhì)及溶解甲烷濃度(n=1554)

如圖3所示,受秋季氣溫、光照等氣象因素的影響,垂向上水溫均存在分層現(xiàn)象,但溫躍層深度和厚度有所差異.3個(gè)監(jiān)測(cè)點(diǎn)溶解氧飽和度在0～6.0m先增大后減小,S1和S2監(jiān)測(cè)點(diǎn)溶解氧飽和度在2.5m水深處達(dá)到最大值,S1監(jiān)測(cè)點(diǎn)為71.71%,S2監(jiān)測(cè)點(diǎn)為75.92%,而S3監(jiān)測(cè)點(diǎn)在3.8m水深處達(dá)到最大值(73.50%),各監(jiān)測(cè)點(diǎn)氧躍層以下溶解氧飽和度均維持一個(gè)穩(wěn)定的低值(<15.01%).3個(gè)監(jiān)測(cè)點(diǎn)垂向剖面中葉綠素a濃度最大值均位于3m水深附近,底部葉綠素a濃度均接近0,S2監(jiān)測(cè)點(diǎn)葉綠素a濃度表現(xiàn)出雙峰現(xiàn)象.3處監(jiān)測(cè)剖面電導(dǎo)率變化趨勢(shì)一致,在0～18.0m水深處保持穩(wěn)定,而在18.0～20.0m快速上升.

圖3 垂向剖面中各水環(huán)境因子分布

2.2 CH4擴(kuò)散通量和水體溶解CH4濃度分布

CH4擴(kuò)散通量為0.61～4.85μmol/(m2×h),平均值為(1.38±1.03)μmol/(m2×h).表層水體溶解CH4濃度為0.02～0.97μmol/L,平均值為(0.14±0.15)μmol/L.CH4擴(kuò)散通量和表層水體CH4濃度空間分布十分相似,均表現(xiàn)為南北高、中間低的特點(diǎn)(圖2a、2d).如圖4所示,各監(jiān)測(cè)點(diǎn)垂向剖面溶解CH4濃度分布特征相似,但數(shù)值上存在較大差異.S1監(jiān)測(cè)點(diǎn)表層水體溶解CH4濃度為0.43μmol/L,在0～18m水深處表現(xiàn)為先降低后增加,7m水深處達(dá)到最低值0.13μmol/L,18m水深以下溶解CH4濃度快速增加,在底層達(dá)到最大值104.98μmol/L.相較于其他監(jiān)測(cè)點(diǎn),S2監(jiān)測(cè)點(diǎn)表層水體溶解CH4濃度最低(0.04μmol/L),在0～18m水深處溶解CH4濃度變化較小(0.04～0.03μmol/L),20m水深以下溶解CH4濃度快速增加,在底層達(dá)到最大值94.71μmol/L.S3監(jiān)測(cè)點(diǎn)表層到底層溶解CH4濃度先降低后增加,表層水體溶解CH4濃度為0.12μmol/L, 0～3.0m水深處快速下降,最低值出現(xiàn)在5m水深處(0.07μmol/L),15m水深以下溶解CH4濃度快速增加,在底層達(dá)到最大值107.67μmol/L.

2.3 表層水體和垂向剖面中δ13C-CH4分布

表層水體13C-CH4為-60.75‰～-20.12‰,平均值為(-37.99±10.70)‰, 如圖2b所示,空間分布表現(xiàn)為南端和北端偏負(fù),中部偏正.如圖4所示,3個(gè)監(jiān)測(cè)點(diǎn)垂向剖面中13C-CH4表現(xiàn)出相似的變化趨勢(shì).S1監(jiān)測(cè)點(diǎn)13C-CH4為-31.07‰～-68.49‰,0～16m水深處13C-CH4變化較小(-31.07‰～-41.64‰),16～21m水深處13C-CH4快速下降,21m水深處13C-CH4為-67.28‰,底部21～22m水深處變化較小.S2監(jiān)測(cè)點(diǎn)13C-CH4為-31.27‰～-66.73‰,0～15m水深處13C-CH4變化較小.15m水深以下13C-CH4快速下降,21m水深處13C-CH4為-66.67‰,21～25m水深處變化較小.S3監(jiān)測(cè)點(diǎn)0～16m水深處13C-CH4變化較小(-31.27‰～-39.03‰),16～20m水深處13C-CH4快速下降,底部20～25m水深處13C-CH4保持穩(wěn)定((-66.78±0.87)‰)

圖4 垂向剖面中溶解CH4濃度及13C-CH4分布

3 討論

3.1 溶解CH4濃度分布規(guī)律及主要影響因素

如圖2所示,溶解CH4濃度空間分布表現(xiàn)為南北高、中間低的特點(diǎn).表層水體溶解CH4濃度與水深表現(xiàn)出極顯著負(fù)相關(guān)關(guān)系,因?yàn)樗钍怯绊懞碈H4氧化消耗和傳輸?shù)闹匾蛩?淺水區(qū)CH4濃度更高,原因在于風(fēng)浪擾動(dòng)更容易傳遞到湖泊底部.一方面,波浪引起強(qiáng)烈的壓力振蕩[23],增強(qiáng)沉積物孔隙水-上覆水的交換[24],減小沉積物-水界面擴(kuò)散邊界層的厚度,導(dǎo)致沉積物擴(kuò)散通量增加[25].另一方面,波浪引起顆粒再懸浮,導(dǎo)致孔隙水中的CH4直接向上覆水中釋放[26].

水溫是影響湖泊CH4分布的重要因素之一.一方面,水溫升高通過(guò)增加產(chǎn)甲烷菌和甲烷氧化菌活性提高甲烷產(chǎn)率和消耗速率[27-29],例如張成[30]的室內(nèi)培養(yǎng)試驗(yàn)表明,25℃時(shí)甲烷產(chǎn)率為15℃時(shí)的近10倍,15℃甲烷消耗速率為0℃時(shí)的1.77倍.另一方面,水溫升高會(huì)降低CH4在水體中的溶解度.由觀測(cè)結(jié)果可知, S1～S3 3個(gè)監(jiān)測(cè)點(diǎn)垂向剖面中水溫分布基本一致(圖2),湖域表層水體水溫變化范圍為18.56～20.18℃(圖2f),而表層水體CH4濃度為0.02～0.97μmol/L(圖2a),雖然表層水體水溫與溶解CH4濃度表現(xiàn)出負(fù)相關(guān)(表2),但較小的水溫差異對(duì)CH4溶解度及CH4產(chǎn)生和消耗速率的影響十分有限,水溫并不是造成陽(yáng)宗海溶解CH4濃度空間異質(zhì)性的主要原因.

室內(nèi)培養(yǎng)試驗(yàn)表明,沉積物生成的CH4穩(wěn)定碳同位素13C-CH4約為-70‰,CH4氧化伴隨著強(qiáng)烈的同位素分餾,較輕的12CH4優(yōu)先被氧化,導(dǎo)致13C-CH4偏正[31].本次監(jiān)測(cè)結(jié)果顯示,13C-CH4空間分布表現(xiàn)為南端和北端淺水區(qū)偏負(fù),中部深水區(qū)偏正,表明中部深水區(qū)產(chǎn)自沉積物的CH4向表層水體傳輸路徑長(zhǎng),更多的CH4被氧化消耗,到達(dá)表層水體CH4較少[32],降低了表層水體溶解CH4濃度.由圖4可知, S1～S3三個(gè)監(jiān)測(cè)點(diǎn)底層水體溶解CH4濃度遠(yuǎn)高于中上層,且底層水體13C-CH4明顯偏負(fù),表明大量產(chǎn)生于沉積物的CH4積累在底層.溫躍層附近CH4濃度快速下降,13C-CH4快速上升,表明CH4在溫躍層附近發(fā)生氧化,消耗了大量CH4并導(dǎo)致13C-CH4偏正.雖然S2和S3監(jiān)測(cè)點(diǎn)水深相差較小,但S3監(jiān)測(cè)點(diǎn)更靠近湖泊北部的淺水區(qū),S2監(jiān)測(cè)點(diǎn)則位于中心湖區(qū),高濃度CH4由淺水區(qū)向中心湖區(qū)側(cè)向擴(kuò)散傳輸?shù)木嚯x遠(yuǎn),CH4氧化消耗路徑加長(zhǎng)使得更多的CH4被氧化,導(dǎo)致S2監(jiān)測(cè)點(diǎn)中上層水體溶解CH4濃度遠(yuǎn)低S3監(jiān)測(cè)點(diǎn).

湖泊水體分層是影響水體溶解CH4濃度的重要因素.湖泊水體分層結(jié)構(gòu)的打破往往出現(xiàn)在季節(jié)性混合期.此期間積累在溫躍層以下的高濃度CH4由于水體的快速摻混而向上遷移,形成甲烷的快速排放期.Ferna?ndez等[33]在德國(guó)南部Mindelsee湖的研究發(fā)現(xiàn),夏季水溫分層期間水體底部存儲(chǔ)的甲烷約46%在秋季湖水混合期被釋放,占到全年CH4擴(kuò)散通量的80%.大風(fēng)和降雨事件對(duì)水溫分層的擾動(dòng)更為迅速[34],Mindelsee湖在秋季大風(fēng)時(shí)期,淺水區(qū)底層儲(chǔ)存CH4快速進(jìn)入混合層,而深水區(qū)受影響較小.由圖4可知,溫躍層之上垂向剖面中溶解CH4濃度雖然遠(yuǎn)低于底層,但依然為過(guò)飽和,表現(xiàn)為S1>S3>S2監(jiān)測(cè)點(diǎn),其原因在于水體的垂向摻混導(dǎo)致底層高濃度CH4向中上層水體的遷移[23,35].本次監(jiān)測(cè)正值湖泊水溫分層的消退期,隨著氣溫的降低,表層水溫持續(xù)下降,水體垂向摻混加劇,底層積累的部分CH4通過(guò)冷卻對(duì)流進(jìn)入混合層,混合層內(nèi)CH4的消耗量低于深層CH4的補(bǔ)給量,導(dǎo)致了混合層內(nèi)CH4的過(guò)飽和.相較于S2和S3監(jiān)測(cè)點(diǎn),S1監(jiān)測(cè)點(diǎn)水深小,表層擾動(dòng)可傳遞到有著高濃度CH4的底層,促進(jìn)底層儲(chǔ)存的CH4進(jìn)入上層水體,提高混合層內(nèi)溶解CH4濃度.

靠近岸邊的淺水區(qū)通常直接接收污水排放、降雨徑流和河流匯入,新鮮有機(jī)物的增加可能提高沉積物CH4產(chǎn)量[8].事實(shí)上污染物中新鮮有機(jī)碳的輸入可刺激表層沉積物中的CH4生成,也通過(guò)溶解的不穩(wěn)定有機(jī)物(如乙酸鹽、丙酸鹽)的擴(kuò)散刺激深層沉積物中的CH4生成[36].陽(yáng)宗海北部入流河流為擺衣河,南部入流為陽(yáng)宗大河和七星河(圖1),入流河流可能攜帶居民區(qū)的大量污水進(jìn)入陽(yáng)宗海.梅涵一等[37]對(duì)陽(yáng)宗海流域農(nóng)村水污染的調(diào)查研究表明,農(nóng)村污水對(duì)陽(yáng)宗海造成很大的污染風(fēng)險(xiǎn).李楠[38]研究發(fā)現(xiàn),陽(yáng)宗海南部湖區(qū)表層沉積物總有機(jī)碳含量最高,指示南部入流可能攜帶大量有機(jī)物進(jìn)入陽(yáng)宗海,在南部湖區(qū)沉積,成為促進(jìn)CH4產(chǎn)生的重要因素.

水生植物的生長(zhǎng)對(duì)CH4的生成和傳輸有重要影響:一方面,水生植物氣孔組織可促進(jìn)CH4從沉積物向水體的釋放[8];另一方面,水生植物生長(zhǎng)和死亡帶來(lái)豐富的碳積累,促進(jìn)沉積物CH4的產(chǎn)生[39].潘義宏[40]在陽(yáng)宗海南部和北部電廠附近采集到金魚(yú)藻、黑藻、小葉眼子菜等水生植物,本次監(jiān)測(cè)過(guò)程中在淺水區(qū)可觀察到大量沉水植物和水藻.陽(yáng)宗海表層水體溶解CH4濃度與葉綠素a濃度呈現(xiàn)極顯著正相關(guān)關(guān)系(表2),與Wang等[41]在太湖的研究結(jié)論相似.新近研究表明,CH4可在湖泊有氧水體中產(chǎn)生,有氧水體缺氧微環(huán)境中的產(chǎn)甲烷古生菌產(chǎn)甲烷[42]、藻類(lèi)的直接產(chǎn)生[43]和甲基膦酸酯脫甲基化[44]可能是產(chǎn)生此現(xiàn)象的原因.Gunthel等[45]在Stechlin湖進(jìn)行的野外監(jiān)測(cè)和室內(nèi)培養(yǎng)試驗(yàn)表明,藍(lán)細(xì)菌和硅藻等浮游植物可促進(jìn)甲烷的生成,并伴隨著13C-CH4的偏正.本次監(jiān)測(cè)結(jié)果顯示,中上層水體葉綠素a濃度的高值出現(xiàn)在3m水深處,但上層水體溶解CH4濃度的高值并未出現(xiàn)在這一水深處.S3監(jiān)測(cè)點(diǎn)表層水體13C-CH4偏正,與上層水體溶解CH4濃度的高值位于同一水深處,但S1和S3監(jiān)測(cè)點(diǎn)并未表現(xiàn)出相似特征.溶解CH4濃度、葉綠素a和13C-CH4的分布特征表明,陽(yáng)宗海表層水體溶解CH4過(guò)飽和是否與有氧CH4產(chǎn)生有關(guān),尚需要進(jìn)一步的研究.

表2 陽(yáng)宗海表層溶解CH4濃度與理化參數(shù)皮爾遜相關(guān)系數(shù)(N=1554)

注:*為<0.05,**為<0.01.

3.2 溶解CH4高空間分辨率監(jiān)測(cè)的意義

薄邊界層法是一種被廣泛使用的水-氣界面擴(kuò)散通量估算方法[46],而精確的表層水體溶解CH4濃度是進(jìn)行通量估算的重要前提.現(xiàn)有研究中,多采用少數(shù)點(diǎn)位的監(jiān)測(cè)代表湖泊整體水平[6-7],但是湖泊表層水體溶解CH4濃度往往具有很大的空間異質(zhì)性[47].例如本次高空間分辨率的走航式監(jiān)測(cè)發(fā)現(xiàn),陽(yáng)宗海表層水體溶解CH4濃度為0.02～0.97μmol/L,相差近48倍,這種空間分布上的巨大差異給水-氣界面擴(kuò)散通量的估算帶來(lái)了巨大的潛在誤差.為進(jìn)一步評(píng)估這種誤差,本研究采用自展分析進(jìn)行了模擬采樣,探究采樣點(diǎn)數(shù)量的大小對(duì)通量估算的影響.本次走航式監(jiān)測(cè)共獲得1554個(gè)表層水體溶解CH4濃度數(shù)據(jù),采用薄邊界層法求得1554個(gè)擴(kuò)散通量數(shù)據(jù),自展分析即基于1554個(gè)通量值進(jìn)行.以采樣點(diǎn)數(shù)量為5的第一組為例,從1554個(gè)通量數(shù)據(jù)中隨機(jī)提取5個(gè)不重復(fù)的數(shù)據(jù)并計(jì)算其平均值5,重復(fù)此過(guò)程1000次,得到1000個(gè)5,第2～6組的采樣點(diǎn)數(shù)量分別為10, 50, 100, 500和1000,計(jì)算方法與第1組一致.模擬采樣結(jié)果表明,平均CH4擴(kuò)散通量的標(biāo)準(zhǔn)差從5個(gè)采樣點(diǎn)數(shù)量時(shí)的0.24μmol/(m2·h)持續(xù)下降到1000個(gè)采樣點(diǎn)數(shù)量時(shí)的0.01μmol/(m2·h)(圖5).由此可見(jiàn),增加采樣點(diǎn)數(shù)量可以有效降低整個(gè)湖泊排放量估算的不確定性,高空間分辨率的表層水體溶解CH4濃度監(jiān)測(cè)對(duì)于準(zhǔn)確估算CH4擴(kuò)散通量十分重要.

基于少數(shù)采樣點(diǎn)濃度監(jiān)測(cè)的擴(kuò)散通量估算在未來(lái)很長(zhǎng)一段時(shí)間內(nèi)仍然是不可缺少的,考慮到溶解CH4濃度的空間異質(zhì)性,通量的準(zhǔn)確估算往往需要大量采樣點(diǎn)的濃度數(shù)據(jù),時(shí)間和人力等因素限制了采樣點(diǎn)數(shù)量的增加,如何通過(guò)最少的采樣點(diǎn)數(shù)量得到相對(duì)準(zhǔn)確的通量估算成為亟待解決的問(wèn)題.Paranaíba等[11]在大量監(jiān)測(cè)數(shù)據(jù)的基礎(chǔ)上對(duì)巴西3個(gè)熱帶水庫(kù)進(jìn)行了模擬采樣,發(fā)現(xiàn)水庫(kù)面積是影響單位面積所需最小采樣點(diǎn)數(shù)量的重要因素,這是因?yàn)槌跫?jí)生產(chǎn)力、營(yíng)養(yǎng)狀態(tài)、匯流和淺水區(qū)沉積物CH4輸入的異質(zhì)性對(duì)小型水庫(kù)擴(kuò)散通量影響更大.然而針對(duì)湖泊最小采樣點(diǎn)數(shù)量的定量分析尚未見(jiàn)報(bào)道,本研究進(jìn)一步采用自展分析進(jìn)行模擬采樣,探討單位面積所需最小采樣量.自展分析數(shù)據(jù)為本次走航式監(jiān)測(cè)結(jié)合薄邊界層法計(jì)算獲得的1554個(gè)擴(kuò)散通量數(shù)據(jù),且假定1554個(gè)擴(kuò)散通量數(shù)據(jù)的均值為真實(shí)通量,模擬采樣將每平方公里所需最小采樣點(diǎn)數(shù)量設(shè)置為0.1～5公差為0.1的50個(gè)數(shù),若模擬采樣計(jì)算所得95%通量值落在真實(shí)通量±20%范圍內(nèi),則認(rèn)為滿(mǎn)足通量估算的精度要求.結(jié)果表明,相對(duì)準(zhǔn)確估算陽(yáng)宗海擴(kuò)散通量所需最小采樣點(diǎn)數(shù)量約為3.7個(gè)/km2(圖6).雖然不同湖泊的空間異質(zhì)性特征存在差異,但該結(jié)果能夠?yàn)楹?kù)水域甲烷濃度監(jiān)測(cè)及通量估算提供科學(xué)參考.

圖6 相對(duì)準(zhǔn)確估算陽(yáng)宗海擴(kuò)散通量所需的最小采樣點(diǎn)數(shù)量

4 結(jié)論

4.1 2020年11月10日陽(yáng)宗海表層水體溶解CH4濃度為0.02～0.97μmol/L,表現(xiàn)為大氣甲烷的源.空間上呈現(xiàn)出南北部高,中部低的分布特征,與水生植物分布、入湖河流輸出和水深等因素密切相關(guān).

4.2 監(jiān)測(cè)期間存在水溫分層現(xiàn)象,湖泊不同區(qū)域垂向剖面溶解CH4濃度分布形式一致,高濃度的CH4積累在缺氧的底層水體中,底層CH4在向上擴(kuò)散的過(guò)程中,氧化消耗作用明顯,上部混合層溶解CH4濃度受垂向摻混和側(cè)向輸移影響顯著.

4.3 基于1554個(gè)擴(kuò)散通量數(shù)據(jù)進(jìn)行的模擬采樣表明,采樣點(diǎn)數(shù)量的增加可以有效降低整個(gè)湖泊排放量估算的不確定性,基于少量采樣點(diǎn)監(jiān)測(cè)數(shù)據(jù)進(jìn)行CH4排放估算具有較大不確定性,相對(duì)準(zhǔn)確估算陽(yáng)宗海CH4擴(kuò)散通量所需最小采樣點(diǎn)數(shù)量約為3.7個(gè)/km2,高空間分辨率的監(jiān)測(cè)對(duì)于湖庫(kù)碳排放的準(zhǔn)確估計(jì)是必要的.

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Spatial heterogeneity of dissolved methane in a plateau lake: a case study in Yangzonghai Lake in autumn.

ZHENG Xiang-wang1,2, CHEN Min1,2,3*, XIAO Shang-bin1,2,3, WANG Sheng-rui4,5, WANG Xue-zhu2, CHEN Wei2, LIU Jia2, XU Hao-ting2

(1.Engineering Research Center of Eco-environment in Three Gorges Reservoir Region, Yichang 443002, China；2.College of Hydraulic and Environmental Engineering, China Three Gorges University, Yichang 443002, China；3.Hubei Field Observation and Scientific Research Stations for Water Ecosystem in Three Gorges Reservoir,Yichang 443002, China；4.College of Water Sciences, Beijing Normal University, Beijing 100875, China；5.Yunnan Key Laboratory of Pollution Process and Management of Plateau Lake-Watershed, Kunming 650034, China)., 2022,42(2)：834～842

Near-surface and vertical profiles of dissolved CH4concentrations were measured at a high resolution using the new fast-response automated gas equilibrator (FaRAGE) connected to a greenhouse gas analyzer at Yangzonghai Lake in November 2020, to investigate the spatial pattern of dissolved CH4and its driving factors. Results showed that dissolved CH4concentrations of surface water ranged between 0.02 and 0.97μmol/L, indicating a net source of atmosphere CH4. The dissolved CH4was relatively higher in the north and south parts of the lake, but lower in the middle area, which was mainly influenced by aquatic plants distribution, discharge from inflow rivers and topographical conditions. Vertical profiles of dissolved CH4at different spots of the lake exhibited relatively consistent pattern, while CH4was more significantly oxidized in the middle section. The bootstrap analysis suggested that estimates of CH4emission derived from fewer measurements were subjected to potentially large biases, and the minimum number of sampling sites that guaranteed an accurate estimation of diffusive CH4flux at Yangzonghai Lake was 3.7per km2. It was necessary to perform high-resolution observations of CH4concentration for an accurate estimation of carbon emission from lakes or reservoirs.

greenhouse gas；dissolved concentration；spatial distribution；flux；Yangzonghai Lake

X524

A

1000-6923(2022)02-0834-09

鄭祥旺(1996-),男,湖北宜昌人,三峽大學(xué)碩士研究生,主要從事淡水生態(tài)系統(tǒng)碳循環(huán)研究.發(fā)表論文2篇.

2021-06-21

國(guó)家自然科學(xué)基金資助項(xiàng)目(41807513,51979148);湖北省自然科學(xué)基金創(chuàng)新群體項(xiàng)目(2019CFA032)

* 責(zé)任作者, 副教授, minchen@ctgu.edu.cn