薛亮,王輝武,韋欽勝,李勞鈺,于衛(wèi)東
(1.國(guó)家海洋局 第一海洋研究所 海洋與氣候研究中心,山東 青島 266061;2.國(guó)家海洋局 第一海洋研究所 海洋生態(tài)研究中心,山東 青島 266061)
季風(fēng)上升流對(duì)印尼爪哇島南部海域pH影響的初步研究
薛亮1,王輝武1,韋欽勝2,李勞鈺1,于衛(wèi)東1
(1.國(guó)家海洋局 第一海洋研究所 海洋與氣候研究中心,山東 青島 266061;2.國(guó)家海洋局 第一海洋研究所 海洋生態(tài)研究中心,山東 青島 266061)
基于2013年9月底至10月初在印尼爪哇島南部海域調(diào)查得到的碳酸鹽參數(shù)和相關(guān)水文數(shù)據(jù),首次報(bào)道了該海域pH的分布,并重點(diǎn)探討了南爪哇上升流(季風(fēng)上升流)對(duì)其影響。結(jié)果顯示上升流影響區(qū)表層pH低于周?chē)巧仙饔绊憛^(qū)。通過(guò)兩端元混合模型,定量討論了上升流的物理輸運(yùn)和生物活動(dòng)對(duì)pH的影響。研究表明,上升流的物理輸運(yùn)至少造成了海表層鹽度增加0.4個(gè)單位,溶解無(wú)機(jī)碳(DIC)增加110 μmol/kg,pH降低約0.2個(gè)單位;同時(shí),在上升流區(qū),強(qiáng)烈的生物活動(dòng)(葉綠素a濃度大于0.4 mg/m3)使得DIC的降低量達(dá)70 μmol/kg,pH的增加量達(dá)0.15個(gè)單位??傮w來(lái)看,該研究區(qū)域的物理輸運(yùn)作用大于生物作用,綜合效應(yīng)表現(xiàn)為DIC的增加和pH的降低。另外,同上升流的物理輸運(yùn)作用和生物作用相比,上升流引起的表層冷卻和增鹽對(duì)pH的影響較小(熱力學(xué)作用)。
pH;東邊界上升流;生物活動(dòng);物理輸運(yùn);南爪哇;印度洋
pH是海洋酸化的特征參數(shù)之一,其值由水中氫離子(H+)濃度的負(fù)對(duì)數(shù)來(lái)表示(pH=-log10[H+])。自工業(yè)革命以來(lái),由于海洋對(duì)大氣CO2的吸收,上層海洋已經(jīng)下降了大約0.1個(gè)pH單位,H+酸度增加了30%[1]。這對(duì)很多海洋生物將產(chǎn)生負(fù)面影響,尤其不利于珊瑚、有孔蟲(chóng)和貝類(lèi)等的鈣質(zhì)骨骼或外殼的形成[2—5]。除了大氣CO2含量升高引起pH降低外,近海一些典型的物理和生物地球化學(xué)過(guò)程(如上升流、河流輸入和富營(yíng)養(yǎng)化等)亦能加劇pH的降低[6—11]。近海pH變化的驅(qū)動(dòng)機(jī)制較開(kāi)闊大洋更為復(fù)雜[12]。因此,探討近海典型生態(tài)系統(tǒng)中pH的控制過(guò)程對(duì)于預(yù)測(cè)未來(lái)pH的變化趨勢(shì)具有重要的意義。
上升流過(guò)程對(duì)pH的影響具有獨(dú)特性。一方面,上升流可將富含CO2的次表層水帶至表層,引起pH的降低(物理輸運(yùn))[8,13]。除了遭受大氣CO2濃度升高引起的海洋酸化的影響外,上升流系統(tǒng)往往還經(jīng)歷了這種自然過(guò)程引起的pH降低,因而上升流區(qū)極易受到酸化的影響[8]。另一方面,上升流亦能夠?qū)⒏缓瑺I(yíng)養(yǎng)鹽的次表層水帶至海表層,由此促進(jìn)浮游植物活動(dòng)[14],這一過(guò)程則可造成海水中CO2濃度的下降,進(jìn)而增加pH值(生物作用)。在不同的上升流系統(tǒng)中,這兩方面作用的相對(duì)大小也會(huì)不同。研究表明,加利福尼亞上升流系統(tǒng)中,物理輸運(yùn)作用占主導(dǎo)[8];而在加利西亞(Galician)和俄勒岡上升流系統(tǒng)中,生物作用占主導(dǎo)[15—16]。上升流對(duì)CO2體系的凈作用取決于生物活動(dòng)對(duì)上升流帶來(lái)的營(yíng)養(yǎng)鹽的消耗程度[17—19]。更重要的是,東邊界上升流系統(tǒng)對(duì)于維持高生產(chǎn)力的海洋生態(tài)系統(tǒng)和發(fā)展?jié)O業(yè)經(jīng)濟(jì)具有重要的意義[20]。盡管它們的面積不到全球海洋的1%,但卻大約貢獻(xiàn)了全球海洋新生產(chǎn)力的11%[21],全球漁業(yè)捕撈量的20%[22]。因此,研究東邊界上升流系統(tǒng)中pH的分布水平及主要控制過(guò)程對(duì)于探討未來(lái)pH的變化趨勢(shì)及海洋酸化對(duì)海洋生態(tài)系統(tǒng)和漁業(yè)經(jīng)濟(jì)的影響極其重要[23—25]。
南爪哇上升流由季風(fēng)主導(dǎo)[26—28],同其他東邊界上升流類(lèi)似,該系統(tǒng)擁有較高的初級(jí)生產(chǎn)力[29—31],是金槍魚(yú)等的重要產(chǎn)地[32]。另外,在爪哇島沿岸還分布著大量珊瑚[33],而珊瑚對(duì)海洋酸化很敏感[34]?;谛l(wèi)星遙感資料(例如海表層溫度和Chla)或早期零星的現(xiàn)場(chǎng)觀測(cè)數(shù)據(jù),關(guān)于南爪哇上升流對(duì)營(yíng)養(yǎng)鹽、溶解氧、Chla和漁業(yè)的影響已有些許研究[27,29—32,35]。然而,迄今關(guān)于南爪哇上升流系統(tǒng)中pH的分布及上升流如何影響pH尚未見(jiàn)報(bào)道。基于2013年9月底至10月初的現(xiàn)場(chǎng)調(diào)查資料,本文報(bào)道和分析了印尼爪哇島南部海域的pH分布,并使用兩端元混合模型定量探討了上升流過(guò)程的兩種相反效應(yīng)(物理輸運(yùn)和生物作用)對(duì)pH的影響。
本文研究區(qū)域位于東南印度洋、印尼爪哇島的南部海域(圖1)。該區(qū)域由季風(fēng)系統(tǒng)主導(dǎo),6月-10月盛行東南季風(fēng),12月-2月盛行西北季風(fēng)[27]。上升流主要存在于東南季風(fēng)期間,止于西北季風(fēng)的爆發(fā)。另外,該區(qū)域還受到南爪哇流和印尼貫穿流的影響[36]。
圖1 研究區(qū)域及調(diào)查斷面A、B和C。其中由于CTD采水器故障,在斷面C只有一個(gè)調(diào)查站位Fig.1 Study site and transects A,B and C. Only one station was completed along transect C because of malfunction of the CTD sampler
本文所用數(shù)據(jù)于2013年9月22日-10月2日期間獲取,調(diào)查船為印度尼西亞科學(xué)院考察船BJ-8。研究區(qū)域總共有16個(gè)調(diào)查站位(圖1)。水體溫度和鹽度由海鳥(niǎo)911CTD測(cè)得。水樣由CTD攜帶的10 L Niskin采樣瓶收集,采樣層次為3 m、10 m、30 m、50 m、75 m、100 m、150 m、200 m 和300 m。本研究中的pH、溶解無(wú)機(jī)碳(DIC)和總堿度(TA)等碳酸鹽參數(shù)樣品的采集與分析均參照國(guó)際標(biāo)準(zhǔn)方法[37],按照溶解性氣體采樣法進(jìn)行收集。具體分析時(shí),pH采用總氫離子標(biāo)準(zhǔn),在25°C下用奧立龍8102BN Ross 復(fù)合電極和精密pH 計(jì)測(cè)定;DIC和TA樣品在加入0.02%體積的飽和HgCl2固定后于4°C冷藏保存,然后分別用溶解無(wú)機(jī)碳分析儀和海水總堿度分析儀測(cè)定。DIC和TA的精密度為0.1%[38]?,F(xiàn)場(chǎng)溫度下的pH (pHinsitu)由25°C下的pH(pH@25)和總堿度并結(jié)合現(xiàn)場(chǎng)溫度和鹽度通過(guò)CO2系統(tǒng)計(jì)算軟件來(lái)計(jì)算(CO2sys)[39]。另外,Chla由熒光法測(cè)定,詳見(jiàn)《海洋監(jiān)測(cè)規(guī)范》(2007)。
本研究在該區(qū)域觀測(cè)到南爪哇上升流的明顯信號(hào)。從斷面A可以看出,等溫線(xiàn)和等鹽線(xiàn)有著明顯的向岸抬升的趨勢(shì); 50 m水深的水甚至可以直接涌升到海表面(見(jiàn)圖2)。相對(duì)于周?chē)巧仙鲄^(qū),上升流區(qū)(A0、A1和A2站)的表層水展現(xiàn)出低溫、高鹽的特征。這同以往的研究結(jié)果相吻合[26,28,31,35]。然而,從溫度和鹽度的垂直分布來(lái)看,上升流似乎對(duì)斷面B的影響不明顯(見(jiàn)圖2),其實(shí)不然。通過(guò)比較斷面A和斷面B溫度和鹽度的垂直分布,并結(jié)合先前調(diào)查結(jié)果,可以看出整個(gè)斷面B都受到了上升流的強(qiáng)烈影響,而且上升流對(duì)斷面B的影響比斷面A還要?jiǎng)×襕26]。之所以斷面B的溫度和鹽度的垂直分布顯示其上升流特征不明顯主要與調(diào)查站位的設(shè)置有關(guān),因?yàn)檎麄€(gè)斷面B都處于上升流影響區(qū),倘若將此斷面的調(diào)查站位再向深水區(qū)外延,近岸端底層冷水的涌升現(xiàn)象將會(huì)凸顯出來(lái)(在后續(xù)的調(diào)查中應(yīng)考量此問(wèn)題)。另外,從本航次所有調(diào)查站位的溫鹽關(guān)系圖來(lái)看,斷面B的溫鹽特征與斷面A上升流影響區(qū)的溫鹽特征基本一致(見(jiàn)圖3),這也說(shuō)明斷面B受到了上升流的影響??紤]到斷面A的近岸區(qū)受到上升流的顯著影響,而離岸區(qū)未受影響,便于對(duì)比研究,在下文具體的分析過(guò)程中,本研究將重點(diǎn)討論斷面A。
圖2 斷面A和斷面B溫度(℃)、鹽度和Chl a濃度(mg/m3)的垂直分布(其中距離表示離岸距離)Fig.2 Vertical profiles of temperature,salinity and chlorophyll a along transects A and B(The distance from the shore was shown)
圖3 150 m 以淺水柱的溫鹽散點(diǎn)圖。其中黑色三角表示斷面B和C的數(shù)據(jù),紅色實(shí)心圈表示斷面A的A0至A2站(上升流區(qū)),紅色空心圈表示斷面A的A3至A6站(非上升流區(qū))Fig.3 Scatterplots of temperature and salinity in the upper 150 m of the water column. The black triangles denote the data from transects B and C,red filled circles the data from station A0 to A2 (upwelling zone),and red empty circles the data from station A3 to A6 (nonupwelling zone)
圖4 25℃ (pH@25)和現(xiàn)場(chǎng)溫度pH (pHin situ)的垂直分布(其中距離表示離岸距離)Fig.4 Vertical profiles of pH at 25℃ and in situ temperatures(The distance from the shore was shown)
圖5 斷面A 150 m以淺總堿度(TA)、溶解無(wú)機(jī)碳(DIC)和25℃時(shí)的pH (pH@25)與鹽度的散點(diǎn)圖Fig.5 Scatterplots of TA vs. salinity (a),DIC vs. salinity (b),and pH@25 vs salinity (c) in the upper 150 m of the water column along transect A其中圖a中虛線(xiàn)表示TA與鹽度的線(xiàn)性回歸線(xiàn);圖b和c中的虛線(xiàn)表示兩個(gè)端元值之間的理論混合線(xiàn)。另外,還給出了上升流的物理輸運(yùn)作用和生物生產(chǎn)作用對(duì)DIC和pH@25的影響及相應(yīng)的葉綠素濃度(色標(biāo))In (a),the dashed line was the linear regression line; in (b) and (c),the theoretical mixing line (dashed line) between two end-members and influences of physical transport of upwelling and biological production were shown. Also Chl a concentration was shown with color bar
圖6 溫度(a)和鹽度(b)對(duì)pH的熱力學(xué)效應(yīng)Fig.6 Thermodynamic effects of temperature (a) and salinity (b) on pH在評(píng)估溫度的熱力學(xué)效應(yīng)時(shí),保持DIC、TA和鹽度不變(圖a);在評(píng)估鹽度的熱力學(xué)效應(yīng)時(shí),保持DIC、TA和溫度不變(pH@25) (圖b)In (a),we kept salinity,DIC,and TA constant; in (b) we kept temperature,DIC,and TA constant
相應(yīng)的,Chla和pH的分布亦受到上升流的明顯影響(圖2和圖4)。較高的Chla濃度位于上升流影響區(qū),大于0.4 mg/m3。在75 m以淺水域,斷面B的Chla濃度高于斷面A,這也說(shuō)明斷面B受上升流影響的程度可能較高。pH的分布同溫度類(lèi)似(圖2和圖4)。從斷面A可以看出,低pH的次表層水明顯向近岸抬升,50 m水深的水甚至直接涌上升至海表面。因此,同周?chē)巧仙鲄^(qū)影響區(qū)相比,上升流區(qū)的表層pH較低(見(jiàn)圖4)。另外,斷面B的表層pH值同斷面A 50 m水深處的pH值基本一致(見(jiàn)圖4)。
4.1 上升流的物理輸運(yùn)和生物活動(dòng)對(duì)pH的影響
以斷面A為例來(lái)討論上升流過(guò)程的兩種相反效應(yīng)(物理輸運(yùn)和生物活動(dòng))對(duì)pH的影響。圖2和圖4顯示,上升流影響區(qū)(A0、A1和A2站)明顯受到物理輸運(yùn)和生物活動(dòng)的雙重影響。同周?chē)姆巧仙饔绊憛^(qū)相比,此處具有低溫、高鹽和高Chla特征(圖2)。盡管上升流區(qū)具有較高的生物活動(dòng),但這里的pH仍然低于非上升流影響區(qū)(見(jiàn)圖4),由此表明物理輸運(yùn)可能占主導(dǎo)作用。
下面探討物理輸運(yùn)和生物活動(dòng)對(duì)pH變化的各自貢獻(xiàn)。盡管該研究區(qū)域受到若干海流或水團(tuán)的影響,但從TA的保守行為可以看出(見(jiàn)圖5),該海域上層150 m水柱可以近似看作兩端元混合。據(jù)此,本研究使用兩端元混合模型來(lái)剝離物理輸運(yùn)和生物活動(dòng)對(duì)pH的影響[22,40-41]。分別選擇A6站的10 m水和A5站的150 m水作為表層和深層的端元值(分別對(duì)應(yīng)著圖5b所示散點(diǎn)圖中的最低鹽度和最高鹽度)。這主要基于以下考慮:(1)A6站遠(yuǎn)離爪哇島,不受上升流過(guò)程的影響,而且10 m水不受降雨等過(guò)程的干擾;(2)已有報(bào)道150 m水深的水能夠到達(dá)表層[28,31];(3)從Chla濃度來(lái)看,這兩個(gè)端元的生物作用水平較低。
考慮到pH變化是非線(xiàn)性的,因此我們首先利用兩端元混合模型計(jì)算保守混合條件下的DIC(DICmix)和TA(TAmix),然后利用保守混合條件下的DIC和TA及CO2系統(tǒng)計(jì)算軟件(CO2sys)計(jì)算25℃下的pH(pH@25)。注意:此處在討論上升流對(duì)pH的影響時(shí)(25℃),未考慮溫度的作用,這將在4.2部分討論。
(1a)
(1b)
式中,DIC10(DIC150)、TA10(TA150)和S10(S150) 分別表示表層(深層)端元值的DIC、TA和鹽度;S為表層到150 m水深之間的鹽度。其中S10=33.71,DIC10=1 751 μmol/kg,TA10=2 047 μmol/kg,S150=34.72,TA150=2 141 μmol/kg,DIC150=2 029 μmol/kg。
如果上升流發(fā)生前的表層水性質(zhì)跟非上升流區(qū)一致(例如A6站),從圖5可以發(fā)現(xiàn),上升流的物理輸運(yùn)至少造成了鹽度增加0.4個(gè)單位,DIC增加110 μmol/kg,pH降低約0.2個(gè)單位。另外,從圖5可以看出,當(dāng)Chla濃度小于0.2 mg/m3時(shí),DIC和pH的變化基本上可以由鹽度變化(混合)來(lái)解釋?zhuān)蝗欢?dāng)Chla濃度大于0.2 mg/m3時(shí),DIC虧損(低于理論混合線(xiàn)),pH盈余(高于理論混合線(xiàn)),這表明生物作用在Chla濃度較高時(shí)發(fā)揮著重要作用。在上升流區(qū),生物作用較高,Chla濃度大于0.4 mg/m3,生物活動(dòng)使得DIC的降低量可達(dá)70 μmol/kg,pH的增加量可達(dá)0.15個(gè)單位。由此可見(jiàn),該研究區(qū)域的物理輸運(yùn)作用大于生物作用,總體效應(yīng)表現(xiàn)為DIC的增加和pH的降低。
剝離物理輸運(yùn)和生物活動(dòng)對(duì)pH的具體貢獻(xiàn)有助于定量理解上升流過(guò)程對(duì)pH的影響。然而,這種方法也存在一定的不確定性。首先,上升流影響區(qū)比非上升流影響區(qū)更靠近岸邊,而我們假設(shè)上升流發(fā)生前的條件跟非上升流區(qū)域一致,這將低估物理輸運(yùn)的作用。其次,本文通過(guò)混合端元的TA和DIC,使用兩端元混合模型來(lái)剝離物理輸運(yùn)和生物活動(dòng)對(duì)pH的具體貢獻(xiàn)時(shí),未能考慮海-氣CO2交換的影響(盡管其作用較小經(jīng)常被忽略掉[41]),這將高估生物活動(dòng)對(duì)pH的貢獻(xiàn)(該區(qū)域向大氣釋放CO2)。再次,上升流把低溫、高鹽的次表層水帶到表層,也將影響pH(見(jiàn)4.2部分的討論)。最后,陸源輸入也可能把富含CO2的水帶到上升流區(qū)[42],引起pH的降低。然而,同非上升流區(qū)相比,上升流區(qū)較高的表層鹽度(見(jiàn)圖2)說(shuō)明陸源輸入的貢獻(xiàn)較小。
4.2 溫度和鹽度對(duì)pH的熱力學(xué)效應(yīng)
上升流不但將富含CO2的次表層水帶到表層,同時(shí)也導(dǎo)致了上升流區(qū)的表層溫度較低、鹽度較高(見(jiàn)圖2)。因此,上升流可以通過(guò)改變上層海洋的溫度和鹽度來(lái)影響pH。如果保持DIC、TA和鹽度不變,當(dāng)溫度從27.8℃(表層溫度)降低到13.8℃(150 m水深的溫度),pH將升高0.20個(gè)單位。假定研究區(qū)域表層溫度的空間差異完全由上升流引起,那么A6站和C1站的最大表層溫度差2.8℃最多將引起0.04個(gè)pH單位的差別。因此,上升流的冷卻作用所引起的pH增加不會(huì)超過(guò)0.04個(gè)單位。另外,25℃時(shí)和現(xiàn)場(chǎng)溫度下pH較為相似的空間分布(見(jiàn)圖4)也反映出溫度所起作用較小。類(lèi)似的,如果保持DIC、 TA和溫度不變,鹽度從33.7(表層)增加到34.7(150 m水深),pH將降低0.01個(gè)單位。本研究中A6站和C1站的最大表層鹽度差0.6最多將引起0.006個(gè)pH單位的差別。也就是說(shuō),上升流的增鹽作用最多使pH降低0.006個(gè)單位。由此可知,同上升流的物理輸運(yùn)作用和生物作用相比,上升流引起的表層冷卻和增鹽對(duì)pH的影響較小。
調(diào)查結(jié)果顯示,爪哇島南部海域上升流影響區(qū)表層pH低于周?chē)巧仙饔绊憛^(qū)。上升流的物理輸運(yùn)可至少造成海表層鹽度增加0.4個(gè)單位,DIC增加110 μmol/kg,pH降低約0.2個(gè)單位;同時(shí),在上升流區(qū),生物作用較高,Chla濃度大于0.4 mg/m3,生物活動(dòng)使得DIC的降低量可達(dá)70 μmol/kg,pH的增加量可達(dá)0.15個(gè)單位。該研究區(qū)域的物理輸運(yùn)作用大于生物作用,總體效應(yīng)表現(xiàn)為DIC的增加和pH的降低。同上升流的物理輸運(yùn)作用和生物作用相比,上升流引起的表層冷卻和增鹽對(duì)pH的影響較小。在后續(xù)的研究中,尚需進(jìn)一步調(diào)查南爪哇上升流的消長(zhǎng)及氣候變化引起的其強(qiáng)度的變化對(duì)pH等碳酸鹽參數(shù)的影響。
致謝:感謝印尼科考船“BJ-8”的全體船員在調(diào)查中的幫助。
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Influence of monsoon upwelling on pH off the southern coast of Java Island,Indonesia: a preliminary study
Xue Liang1,Wang Huiwu1,Wei Qinsheng2,Li Laoyu1,Yu Weidong1
(1.CenterforOceanandClimateResearch,F(xiàn)irstInstituteofOceanography,StateOceanicAdministration,Qingdao266061,China; 2.MarineEcologyResearchCenter,F(xiàn)irstInstituteofOceanography,StateOceanicAdministration,Qingdao266061,China)
pH and its influence by upwelling process off the southern coast of Java Island,Indonesia were examined for the first time using carbonate and related hydrographic data collected from late September to early October 2013. We found that sea surface pH was lower in the upwelling area than in the nearby nonupwelling area. A two end-member mixing model was used to separate the two opposite effects of the upwelling process on pH: physical transport of CO2rich waters from subsurface layers vs. biological production. Results showed that physical transport at least led to a dissolved inorganic carbon (DIC) increase of 110 μmol/kg,and a pH decrease of about 0.2 units,while strong biological production (chlorophyllalevel above 0.4 mg/m3) induced a DIC decrease by up to 70 μmol/kg,and a pH increase by up to 0.15 units. Overall,the physical transport of CO2rich waters was dominant,leading to a net decrease in pH. Also,we found that surface cooling and salinity increases induced by the upwelling played a relatively minor role in affecting pH (thermodynamically),compared with effects of physical transport and subsequent biological production associated with the upwelling.
pH; eastern boundary upwelling; biological activity; physical transport; South Java;Indian Ocean
10.3969/j.issn.0253-4193.2015.08.001
2015-01-29;
2015-05-14。
國(guó)家自然科學(xué)基金(U1406404);國(guó)家海洋局專(zhuān)項(xiàng)(DC0315011,YZ0115004,QY0115003)。
薛亮(1981—),男,山東省膠州市人,博士,助理研究員,主要從事海洋碳循環(huán)研究。E-mail:xueliang@fio.org.cn
P734.2
A
0253-4193(2015)08-0001-08
薛亮,王輝武,韋欽勝,等. 季風(fēng)上升流對(duì)印尼爪哇島南部海域pH影響的初步研究[J].海洋學(xué)報(bào),2015,37(8):1—8,
Xue Liang,Wang Huiwu,Wei Qinsheng,et al. Influence of monsoon upwelling on pH off the southern coast of Java Island,Indonesia: a preliminary study[J]. Haiyang Xuebao,2015,37(8):1—8,doi:10.3969/j.issn.0253-4193.2015.08.001