湖泊沉積多指標(biāo)記錄的長(zhǎng)壽湖近60年來營(yíng)養(yǎng)化過程

朱正杰，肖 軍，龔業(yè)超，楊洪永，張 雄 雙 燕，毛玲玲

（1. 外生成礦與礦山環(huán)境重慶市重點(diǎn)實(shí)驗(yàn)室 重慶地質(zhì)礦產(chǎn)研究院，重慶 400042；2. 煤炭資源與安全開采國(guó)家重點(diǎn)實(shí)驗(yàn)室重慶研究中心，重慶 400042；3.中國(guó)科學(xué)院地球環(huán)境研究所 黃土與第四紀(jì)地質(zhì)國(guó)家重點(diǎn)實(shí)驗(yàn)室，西安 710061）

湖泊沉積多指標(biāo)記錄的長(zhǎng)壽湖近60年來營(yíng)養(yǎng)化過程

朱正杰1,2，肖 軍3，龔業(yè)超1，楊洪永1，張 雄1, 雙 燕1，毛玲玲1

（1. 外生成礦與礦山環(huán)境重慶市重點(diǎn)實(shí)驗(yàn)室 重慶地質(zhì)礦產(chǎn)研究院，重慶 400042；2. 煤炭資源與安全開采國(guó)家重點(diǎn)實(shí)驗(yàn)室重慶研究中心，重慶 400042；3.中國(guó)科學(xué)院地球環(huán)境研究所 黃土與第四紀(jì)地質(zhì)國(guó)家重點(diǎn)實(shí)驗(yàn)室，西安 710061）

長(zhǎng)壽湖位于重慶市長(zhǎng)壽區(qū)境內(nèi)，是重慶市最大的湖泊旅游風(fēng)景區(qū)和淡水魚養(yǎng)殖基地之一。近年來，其營(yíng)養(yǎng)化日趨嚴(yán)重。本文通過對(duì)有機(jī)質(zhì)碳、氮含量、碳氮比值、有機(jī)質(zhì)碳同位素等有機(jī)質(zhì)指標(biāo)的分析，結(jié)合137Cs放射性核素定年，探討了長(zhǎng)壽湖近60年來有機(jī)質(zhì)轉(zhuǎn)化及營(yíng)養(yǎng)演化過程。結(jié)果表明：長(zhǎng)壽湖有機(jī)質(zhì)主要來源于水生植物藻類，受陸源影響較?。缓喘h(huán)境演化分為兩個(gè)截然不同的階段：1980年以前，有機(jī)質(zhì)碳、氮含量較低，C / N比值較小，有機(jī)質(zhì)碳同位素值較大，暗示湖泊營(yíng)養(yǎng)環(huán)境為貧營(yíng)養(yǎng)化。1980年以后，有機(jī)質(zhì)碳、氮含量較高，C / N比值較大，有機(jī)質(zhì)碳同位素值偏負(fù)，說明湖泊已經(jīng)處于營(yíng)養(yǎng)化階段，人為活動(dòng)的增強(qiáng)及養(yǎng)殖業(yè)的發(fā)展是導(dǎo)致湖泊生產(chǎn)力增大的主要因素。長(zhǎng)壽湖沉積物多指標(biāo)忠實(shí)記錄了湖泊環(huán)境的演化過程。

長(zhǎng)壽湖；有機(jī)質(zhì)碳同位素；營(yíng)養(yǎng)化；人為活動(dòng)

長(zhǎng)壽湖是重慶市最大的湖泊旅游風(fēng)景區(qū)和重要的淡水魚養(yǎng)殖基地之一。自20世紀(jì)80年代以來，隨旅游業(yè)、養(yǎng)殖業(yè)蓬勃發(fā)展，長(zhǎng)壽湖已成為集飲用、灌溉、旅游、養(yǎng)殖、發(fā)電為一體的多功能水體。近年來，城市污水和工業(yè)廢水排放量漸增，大部分流入長(zhǎng)壽湖區(qū)，嚴(yán)重影響其水質(zhì)。湖區(qū)“水華”現(xiàn)象引起社會(huì)各界廣泛重視（楊鋼等，2003）。對(duì)長(zhǎng)壽湖表層沉積物氮磷研究發(fā)現(xiàn)：其TN平均含量為2255.89 mg·kg-1，TP平均含量為622.03 mg·kg-1，湖區(qū)氮磷污染嚴(yán)重。長(zhǎng)壽湖表層沉積物中有機(jī)氮是總氮的主要組成部分，占總氮的97.16 %（盧少勇等，2012）。楊鋼等（2003）對(duì)長(zhǎng)壽湖水質(zhì)的營(yíng)養(yǎng)狀況進(jìn)行了綜合評(píng)價(jià)，結(jié)果表明長(zhǎng)壽湖水質(zhì)已呈中度富營(yíng)養(yǎng)化狀態(tài)，有向富營(yíng)養(yǎng)化發(fā)展的潛在趨勢(shì)。胡鵬飛和何太蓉（2012）對(duì)長(zhǎng)壽湖表層沉積物中的氮賦存特征進(jìn)行了研究，認(rèn)為長(zhǎng)壽湖表層沉積物污染已較嚴(yán)重；同時(shí)，他們還對(duì)長(zhǎng)壽湖表層沉積物中磷的賦存狀態(tài)進(jìn)行了研究，結(jié)果顯示長(zhǎng)壽湖表層沉積物具有很強(qiáng)的釋磷潛力，長(zhǎng)壽湖沉積物呈中度污染，應(yīng)加強(qiáng)監(jiān)測(cè)和治理。目前，人們對(duì)湖泊富營(yíng)養(yǎng)化的認(rèn)識(shí)主要是通過水質(zhì)和表層沉積物的調(diào)查（楊鋼等，2003；郭海濤等，2011；胡鵬飛和何太蓉，2012；盧少勇等，2012），這種方法無法追蹤歷史時(shí)期湖泊的富營(yíng)養(yǎng)化過程，特別是在缺乏長(zhǎng)期湖泊監(jiān)測(cè)記錄的情況下，對(duì)湖泊發(fā)生富營(yíng)養(yǎng)化程度的拐點(diǎn)、富營(yíng)養(yǎng)化速率、發(fā)展趨勢(shì)并不清楚。對(duì)湖泊富營(yíng)養(yǎng)化過程的了解有利于控制和采取措施抑制湖泊的進(jìn)一步營(yíng)養(yǎng)化。湖泊沉積物，是湖泊生態(tài)系統(tǒng)的重要組成部分，是入湖物質(zhì)如有機(jī)質(zhì)、污染物等的主要儲(chǔ)蓄所，包含了豐富的生物和理化方面的信息，通過湖泊沉積剖面的環(huán)境代用指標(biāo)的研究，可以了解湖泊及流域的污染狀況，重建湖泊環(huán)境變化的歷史過程。

湖泊沉積物有機(jī)質(zhì)代用指標(biāo)如碳同位素、碳氮含量在湖泊環(huán)境演化及富營(yíng)養(yǎng)化過程示蹤研究方面的應(yīng)用日益深入。有機(jī)質(zhì)C/N 比值和碳同位素能有效判斷湖泊沉積物有機(jī)質(zhì)來源（Meyers，1997），同時(shí)湖泊有機(jī)質(zhì)碳同位素組成及有機(jī)質(zhì)碳氮含量是湖泊生產(chǎn)力變化的有效代用指標(biāo)，從而指示湖泊營(yíng)養(yǎng)狀況?；趯?duì)長(zhǎng)壽湖區(qū)域環(huán)境演變及營(yíng)養(yǎng)化過程的了解為目的，本文通過對(duì)長(zhǎng)壽湖沉積物柱芯中有機(jī)質(zhì)碳氮含量、C/N比值、有機(jī)質(zhì)碳同位素等多指標(biāo)的分析，結(jié)合精確的放射性核素137Cs定年，探討了長(zhǎng)壽湖近60年湖泊沉積物有機(jī)質(zhì)轉(zhuǎn)化及環(huán)境演化過程，以期為合理治理長(zhǎng)壽湖營(yíng)養(yǎng)化提供依據(jù)。

1 湖區(qū)概況

長(zhǎng)壽湖位于重慶市長(zhǎng)壽區(qū)境內(nèi)，地理坐標(biāo)為北緯29°50′ — 30°04′，東經(jīng)107°15′ — 107°25′，是獅子灘水電站攔河大壩建成以后形成的人工淡水湖，是我國(guó)西南地區(qū)最大的人工淡水湖，也是重慶市最大的湖泊旅游風(fēng)景區(qū)和國(guó)家級(jí)生態(tài)旅游休閑度假區(qū)。長(zhǎng)壽湖水域面積約65.5 km2，平均水深15 m，最大水深40 m，是重慶市重要的淡水養(yǎng)殖基地。

2 樣品采集和實(shí)驗(yàn)方法

使用重力采樣器于2013年10月在長(zhǎng)壽湖深水區(qū)采得兩根沉積物柱芯，編號(hào)為CS1和CS2，長(zhǎng)度分別為50 cm和49 cm，采樣點(diǎn)遠(yuǎn)離湖岸，覆水深25 m。所采沉積物柱芯保存完好，懸浮層未受擾動(dòng)，界面水清晰。沉積物柱芯現(xiàn)場(chǎng)按1 cm間隔分截樣品裝入塑料自封袋中密閉保存。CS1和CS2柱芯樣品運(yùn)回實(shí)驗(yàn)室，經(jīng)真空冷凍干燥器（型號(hào)：FD-IA-50）干燥后，用瑪瑙研缽研磨至200目以下備用。CS1柱芯樣品用于有機(jī)質(zhì)指標(biāo)測(cè)定，包括有機(jī)碳、氮含量和有機(jī)質(zhì)碳同位素，CS2柱芯樣品用于放射性核素137Cs比活度測(cè)定。

2.1 放射性核素137Cs測(cè)定

137Cs的比活度采用Caberra公司生產(chǎn)的S-100多道能譜儀進(jìn)行γ-譜測(cè)定，根據(jù)樣品量情況，分別用GC5019同軸鍺探測(cè)器（探測(cè)器效率50%）或GCW井型鍺探測(cè)器（探測(cè)器效率30%）。137Cs計(jì)數(shù)峰的位置為661.6 k eV，測(cè)量誤差＜5%。

2.2 元素含量分析方法

有機(jī)質(zhì)碳、氮含量及C/N比值測(cè)定采用前處理方法如下：稱取樣品3 g，加入0.5 M的HCl溶液25 mL，在水浴鍋60℃中反應(yīng)2 h，去除碳酸鹽，后用氧水洗凈，冷凍干燥后利用元素分析儀（PE2400Ⅱ型）測(cè)定有機(jī)質(zhì)碳、氮含量及C/N比值，分析誤差小于5 %。

2.3 有機(jī)質(zhì)碳同位素分析方法

將沉積物0.5 g放入50 mL離心管中，加入0.5 M的HCl溶液25 mL，在水浴鍋60℃中反應(yīng)2 h，去除碳酸鹽，后用氧水洗凈，冷凍干燥后以備有機(jī)質(zhì)碳同位素測(cè)定。有機(jī)質(zhì)碳同位素測(cè)定采用連續(xù)流質(zhì)譜，儀器型號(hào)為EA IsoPrime，以國(guó)際纖維素標(biāo)樣IAEA-C3（δ13C = -24.91‰）為參考標(biāo)準(zhǔn)校正分析結(jié)果。測(cè)量誤差小于0.1‰，結(jié)果采用PDB標(biāo)準(zhǔn)。

圖1 長(zhǎng)壽湖位置示意圖及采樣點(diǎn)位圖Fig.1 Location of Lake Changshou and sampling site

δ13C計(jì)算公式為：

以上所有實(shí)驗(yàn)均在中國(guó)科學(xué)院地球化學(xué)研究所環(huán)境地球化學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室完成。

3 分析結(jié)果

3.1 沉積物柱芯年代的確定

由于長(zhǎng)壽湖是人工湖泊，其形成年代較短，因此利用137Cs測(cè)年是比較合理的方法。137Cs是人類活動(dòng)（核試驗(yàn)）釋放后通過大氣擴(kuò)散而沉降到地表的放射性核素，廣泛用于湖泊現(xiàn)代沉積計(jì)年的研究（白占國(guó)和萬國(guó)江，1998）。從長(zhǎng)壽湖沉積物137Cs比活度圖上可以看出，剖面上出現(xiàn)一個(gè)明顯峰值（圖2），深度為36 cm，這個(gè)峰值被認(rèn)為是1963年全球性核試驗(yàn)高峰期（Wan et al，1987）。全球核試驗(yàn)的初始時(shí)間是20世紀(jì)50年代初期，而散落的高峰期是1963 — 1964年（Wan et al，1987）。因此利用137Cs散落峰的時(shí)間和沉積深度可以獲得沉積物堆積速率，平均堆積速率為0.85 cm·a-1，依據(jù)此速率從而建立起長(zhǎng)壽湖沉積物年代標(biāo)尺。此外，利用137Cs測(cè)年所獲得沉積物底部年齡為1943 AD，與長(zhǎng)壽湖建于20世紀(jì)40年代比較吻合。

圖2 長(zhǎng)壽湖沉積物CS2柱芯137Cs比活度剖面變化圖Fig.2 Diagram of137Cs activities in Lake Changshou sediments

3.2 有機(jī)質(zhì)碳、氮含量特征

長(zhǎng)壽湖沉積物柱芯TOC含量在0.85% — 4.30%波動(dòng)，平均值為1.79‰，最大變幅為3.45%（圖3）；從有機(jī)碳含量曲線變化特征來看，1980年以前TOC平均值為1.25%，1980年以后TOC平均值為2.43%，表現(xiàn)出顯著增高的特征；TN含量變化范圍為0.13% — 0.49%，平均值為0.23%，最大變幅為0.36%（圖3），其變化特征與TOC含量變化特征極為相似，以1980年為時(shí)間拐點(diǎn)，1980年以后TN含量顯著增高。此外，TOC與TN呈顯著相關(guān)性變化（圖4），相關(guān)系數(shù)接近1，一方面表明這兩者均與有機(jī)質(zhì)有關(guān)，同時(shí)也說明長(zhǎng)壽湖沉積物中N的含量主要以有機(jī)氮為主，沉積物有機(jī)質(zhì)來源單一，與前人研究結(jié)果（盧少勇等，2012）一致。盧少勇等（2012）的研究認(rèn)為長(zhǎng)壽湖沉積物中有機(jī)氮是總氮的主要組成部分，占總氮97.16%。TOC/N值變化范圍為6.61 — 11.62，平均值為8.92 （圖3），TOC/N顯示了與TOC、TN相同的變化特征，在1980年以后TOC/N含量顯著增大。

3.3 有機(jī)質(zhì)碳同位素變化特征

長(zhǎng)壽湖沉積物柱芯有機(jī)質(zhì)碳同位素值的變化范圍為-22.48‰ — -28.23‰，平均值為-26.66%（圖3）。其變化特征顯示了與TOC、N、TOC/N截然相反的變化特征，在1980年以后有機(jī)質(zhì)碳同位素值顯著偏負(fù)。

圖3 長(zhǎng)壽湖沉積物CS1柱芯有機(jī)質(zhì)碳、氮含量、C/N比值及有機(jī)質(zhì)碳同位素值隨年代變化特征Fig.3 Evolution of organic carbon, nitrogen content, C/N ratios and organic carbon isotope values in sediment core CS1 at Lake Changshou

4 討論

4.1 長(zhǎng)壽湖有機(jī)質(zhì)來源

一般認(rèn)為，湖泊水生植物含有較多的蛋白質(zhì)，其C/N值小于10；陸生植物富含腐殖質(zhì)，其C/ N值較大，在20 — 200（Krishnamurthy et al，1986；Lamb et al，2004）。

湖泊沉積物有機(jī)質(zhì)的陸源和內(nèi)源具有完全不同的δ13C值。陸源植物按其光合作用途徑分為C3、C4和CAM三種類型。如果光合作用的最初產(chǎn)物為四碳二羥酸，則相應(yīng)植物稱為C4植物，C4植物δ13C值為-9‰ — -21‰，平均為-14‰；如果光合作用的最初產(chǎn)物為三磷酸甘油酯，則相應(yīng)植物稱為C3植物，C3植物δ13C值較低，一般為-21‰ — -33‰，平均為-27‰。CAM植物類型較少（例如仙人掌科），其典型生境為干旱環(huán)境，δ13C值變化范圍較大。浮游植物由于其吸收水中溶解CO2，與大氣CO2處于同位素平衡，所以其同位素組成與C3植物相似。

圖4 長(zhǎng)壽沉積物柱芯有機(jī)質(zhì)碳、氮含量的相關(guān)性圖Fig.4 The correlation between organic carbon and nitrogen content at Lake Changshou

因此，湖泊沉積物有機(jī)質(zhì)C/N比結(jié)合其碳同位素組成（δ13Corg）是判斷有機(jī)質(zhì)來源的有效手段（Meyers and Ishiwatari，1993；Meyers，1994，1997）。長(zhǎng)壽湖沉積物有機(jī)質(zhì)C/N比和δ13Corg值顯示其主要來源于水生植物和藻類（圖5），基本不受或受陸源物質(zhì)影響較小。

4.2 指標(biāo)的環(huán)境指示意義

自從Stuiver（1975）首次使用湖泊沉積物有機(jī)質(zhì)碳同位素（δ13Corg）恢復(fù)湖泊生產(chǎn)力變化以來，有機(jī)質(zhì)碳同位素被廣泛應(yīng)用于古氣候和古環(huán)境重建（Stuiver，1975； Mckenzie，1985；Schelske and Hodell，1991；Mayer and Schwark，1999；Leng and Marshall，2004；Xu et al，2006；Henderson and Holmes，2009；趙艷等，2013；Zhu et al，2013）。湖泊沉積物有機(jī)質(zhì)碳同位素的變化可能受以下過程的影響：（1）不同來源碳的相對(duì)含量變化（陸源植物和水生植物具有不同的碳同位素組成）；（2）內(nèi)源為主的湖泊，受湖水溶解無機(jī)碳（DIC）的碳同位素組成控制（Hodell and Smith，2001）。在內(nèi)源有機(jī)質(zhì)為主的湖泊中，沉積物有機(jī)質(zhì)碳同位素主要用來判斷歷史時(shí)期湖泊生產(chǎn)力變化。眾多研究者建立了湖泊沉積物有機(jī)質(zhì)δ13C隨湖泊初級(jí)生產(chǎn)力變化的響應(yīng)模式（Schelske and Hodell，1991；McKenzie and Hollander，1993；Meyers，1997；Teranes et al，1999；Teranes and McKenzie，1999；Teranes and Bernasconi，2005），即隨著湖泊初級(jí)生產(chǎn)力的逐漸增大，使得水體中可利用CO2減少，水生植物傾向于吸收湖水溶解無機(jī)碳（DIC）中作為碳源。由于的δ13C值比可溶解CO2的δ13C偏正（Meyers，1997；Leng and Marshall，2004；Xu et al，2006），從而導(dǎo)致有機(jī)質(zhì)δ13C值偏正；反之，隨著湖泊初級(jí)生產(chǎn)力的逐漸減小，有機(jī)質(zhì)δ13C偏負(fù)。

圖5 長(zhǎng)壽湖沉積物C/N比值和δ13Corg示意圖不同來源有機(jī)質(zhì)C/N比和δ13Corg值引自Mayers and Ishiwatari，1993；Meyers，1994，1997。Fig.5 C / N ratios andδ13Corgvalues of sedimentary organic matter from Lake Changshou General C/N ratios andδ13Corgranges for aquatic plants and C3/C4land plants cited from Mayers and Ishiwatari, 1993; Meyers, 1994, 1997.

4.3 多指標(biāo)記錄的環(huán)境演化

根據(jù)長(zhǎng)壽湖沉積物柱芯有機(jī)質(zhì)C、N含量、C/N比值及有機(jī)質(zhì)碳同位素綜合來看（圖3），長(zhǎng)壽湖近60年來的演化過程可以分為兩個(gè)階段，以1980年為時(shí)間拐點(diǎn)：（1）1980年以前的湖泊自然演化的貧營(yíng)養(yǎng)特征。有機(jī)質(zhì)碳、氮含量較低，C/N比值較小，δ13Corg值較大且變化穩(wěn)定，說明長(zhǎng)壽湖從湖泊形成到1980年這段時(shí)間內(nèi)屬于自然演化過程，各有機(jī)質(zhì)指標(biāo)均比較穩(wěn)定，變化較??；（2）1980年以后的營(yíng)養(yǎng)化階段。有機(jī)質(zhì)C、N含量、C/N比值及無機(jī)碳含量均呈現(xiàn)顯著增大過程，C/N比值增大說明長(zhǎng)壽湖在1980年以后有少量陸源植物的輸入，δ13Corg值的顯著偏負(fù)暗示湖泊有機(jī)質(zhì)來源仍然以藻類為主。

長(zhǎng)壽湖近60來的環(huán)境演化過程的轉(zhuǎn)換期為1980年。1980年以前長(zhǎng)壽湖為自然演化過程，湖泊生產(chǎn)力水平低下，湖泊處于穩(wěn)定狀態(tài)。1980年以后所有指標(biāo)均呈現(xiàn)出顯著變化特征，說明湖泊已經(jīng)處于營(yíng)養(yǎng)化狀態(tài)。事實(shí)上，自20世紀(jì)80年代以來，隨旅游業(yè)、養(yǎng)殖業(yè)蓬勃發(fā)展，長(zhǎng)壽湖已成為集飲用、灌溉、旅游、養(yǎng)殖、發(fā)電為一體的多功能水體。伴隨著旅游業(yè)和養(yǎng)殖業(yè)的快速發(fā)展，嚴(yán)重影響了長(zhǎng)壽湖的區(qū)域環(huán)境，內(nèi)源物質(zhì)的逐漸積累，導(dǎo)致湖泊生產(chǎn)力的增大；湖區(qū)偶爾爆發(fā)的“水華”現(xiàn)象說明了其湖泊生態(tài)環(huán)境已遭到破壞（楊鋼等，2003）。近60年來長(zhǎng)壽湖沉積物有機(jī)質(zhì)多指標(biāo)反映的環(huán)境演化特征與人類活動(dòng)顯示了一致的變化過程，忠實(shí)記錄了長(zhǎng)壽湖從自然演化環(huán)境逐步過渡到富營(yíng)養(yǎng)化的過程。

從圖3a、3c、3d可以看出，長(zhǎng)壽湖沉積物有機(jī)質(zhì)碳同位素值隨著有機(jī)質(zhì)碳、氮含量的增加而減?。欢鴤鹘y(tǒng)的研究認(rèn)為湖泊初級(jí)生產(chǎn)力的逐漸增大，導(dǎo)致有機(jī)質(zhì)δ13C值增大。隨著湖泊生產(chǎn)力的提高，導(dǎo)致有機(jī)質(zhì)碳同位素值偏負(fù)的原因可能有：（1）同位素的非平衡沉淀。有研究結(jié)果認(rèn)為湖泊營(yíng)養(yǎng)化過程的同位素非平衡沉淀導(dǎo)致有機(jī)質(zhì)δ13C值偏負(fù)（Fronval et al，1995；Teranes and McKenzie，1999；吳敬祿等，2002；Wu et al，2004；Teranes and Bernasconi，2005）；（2）藻類吸收的CO2來源會(huì)受到湖泊環(huán)境制約而改變。營(yíng)養(yǎng)化初級(jí)階段湖泊生產(chǎn)力得到提高，藻類吸收的CO2部分來源于大氣CO2，隨著營(yíng)養(yǎng)化加劇，藻類發(fā)育，水中溶解的CO2與大氣CO2的交換平衡被打破，水體中溶解的CO2已經(jīng)不能滿足藻類生長(zhǎng)的需要，有機(jī)質(zhì)降解作用產(chǎn)生的CO2逐漸被藻類吸收，導(dǎo)致有機(jī)質(zhì)碳同位素組成偏負(fù)（周志華等，2007；Zhu et al，2011）。一般認(rèn)為同位素的非平衡沉淀發(fā)生在特別富營(yíng)養(yǎng)化的湖泊中（Wu et al，2004），而長(zhǎng)壽湖顯然沒有達(dá)到超級(jí)富營(yíng)養(yǎng)化階段。因此，可以認(rèn)為長(zhǎng)壽湖沉積物有機(jī)質(zhì)碳同位素值隨著湖泊生產(chǎn)力的增大而偏負(fù)的原因主要是藻類吸收偏負(fù)的同位素所致。周志華等（2007）在研究巢湖時(shí)也發(fā)現(xiàn)隨著巢湖富營(yíng)養(yǎng)化過程的進(jìn)一步加劇，其有機(jī)質(zhì)碳同位素逐漸偏負(fù)。Zhu et al（2011）研究了不同水生植物類型湖泊，沉積物有機(jī)質(zhì)δ13C值與湖泊初級(jí)生產(chǎn)力變化的響應(yīng)過程，認(rèn)為大型水草為主的湖泊（如草海），其沉積物有機(jī)質(zhì)δ13C值隨湖泊生產(chǎn)力的增大呈現(xiàn)增加的變化趨勢(shì)；藻類為主的湖泊（巢湖、程海、長(zhǎng)壽湖等），其沉積物有機(jī)質(zhì)δ13C隨湖泊生產(chǎn)力的增大呈現(xiàn)減小的變化趨勢(shì)，藻類易降解是導(dǎo)致δ13C值隨湖泊生產(chǎn)力的增大呈現(xiàn)減小變化趨勢(shì)的主要原因（周志華等，2007）。因此，長(zhǎng)壽湖富營(yíng)養(yǎng)化過程中藻類發(fā)育及藻類易降解是導(dǎo)致其沉積物有機(jī)質(zhì)δ13C值隨湖泊生產(chǎn)力的增大呈現(xiàn)減小趨勢(shì)的主要原因。

5 結(jié)論

（1）長(zhǎng)壽湖沉積物有機(jī)質(zhì)C/N比和δ13Corg值表明長(zhǎng)壽湖沉積物有機(jī)質(zhì)主要源于水生植物和藻類，基本不受或受陸源物質(zhì)的影響較小。

（2）近60年來長(zhǎng)壽湖沉積物有機(jī)質(zhì)多指標(biāo)反映的環(huán)境演化特征與人類活動(dòng)影響一致，忠實(shí)記錄了長(zhǎng)壽湖從自然演化環(huán)境逐步過渡到富營(yíng)養(yǎng)化的過程。1980年以前長(zhǎng)壽湖為自然演化過程，湖泊生產(chǎn)力水平低下，湖泊處于穩(wěn)定狀態(tài)；1980年以后所有指標(biāo)均呈現(xiàn)了顯著的變化特征，說明湖泊已經(jīng)處于營(yíng)養(yǎng)化狀態(tài)。長(zhǎng)壽湖沉積物有機(jī)質(zhì)碳同位素值隨著有機(jī)質(zhì)碳、氮含量的增加而減小與傳統(tǒng)的研究認(rèn)為湖泊初級(jí)生產(chǎn)力的逐漸增大導(dǎo)致有機(jī)質(zhì)δ13C值增大不一致，湖泊營(yíng)養(yǎng)化過程中藻類發(fā)育及藻類易降解是導(dǎo)致長(zhǎng)壽湖有機(jī)質(zhì)δ13C值隨湖泊生產(chǎn)力增大呈現(xiàn)減小趨勢(shì)的主要原因。長(zhǎng)壽湖營(yíng)養(yǎng)化過程的逐漸加劇提醒人們需要對(duì)其進(jìn)行監(jiān)管和采取有效防治措施。

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Trophication evolution recorded by multi-proxy evidences in Lake Changshou, Chongqing during the recent 60 years

ZHU Zhengjie1,2, XIAO Jun3, GONG Yechao1, YANG Hongyong1, ZHANG Xiong1, SHUANG Yan1, MAO Lingling1
(1. Chongqing Key Laboratory of Exogenic Mineralization and Mine Environment, Chongqing Institute of Geology and Mineral Resources, Chongqing 400042, China; 2. Chongqing Research Center of State Key Laboratory of Coal Resources and Safe Mining, Chongqing 400042, China; 3. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710061, China)

Background, aim, and scopeLake Changshou, located in Changshou District, is one of the largest lake scenic spots and most important freshwater fish breeding bases in Chongqing. Recently, the eutrophication of this lake is increasing seriously. The aim of this study is to evaluate the process of eutrophication of Lake Changshou by using organic proxies, including carbon and nitrogen content of organic matter, C/N ratios, and carbon isotope composition of organic matter.Materials and methodsSediment core named CS1 and CS2 were retrieved from the central part of Lake Changshou in 2013 using a self-designed gravitational sediment sampler. Sediment samples were sectioned at 1 cm interval and put into plastic bags in the field. Sediment samples were dried using a vacuum freeze drier. Core CS1 samples were used for geochemical analysis, and CS2 samples were used for analyzing radionuclide137Cs activities. Element analyses of total organic carbon (TOC) and nitrogen were measured by the element analyzer. Samples for organic carbon analysis were pretreated with HCl (1 mol · L-1), then placed in water bath for two hours at 60℃ to remove carbonates, and rinsed repeatedly with distilled water for four times. After that,δ13C values of organic matter were measured on the Finnigan Delta Plus isotope ratio mass spectrometer. A number of duplicate samples were also measured to monitor the measurement accuracy. The activity of137Cs was measured at 661.6 k eV byγ-spectrometry using low-background germanium detector.ResultsData of137Cs demonstrate that the average sedimentation rate of Lake Changshou is 0.85 cm · a-1. Total organic carbon contents vary between 0.85% and 4.3%, with the mean value of 1.75%. Before 1980 AD, the average TOC is 1.25%, and after 1980 AD, the average TOC is 2.43%. Nitrogen contents and C/N ratios show similar variations with TOC. However, carbon isotope composition of organic matter shows contrast variations. The results indicate that the source of organic matter in Lake Changshou is derived from aquatic plants and algae, rather than terrestrial plants, or little affected. The environmental evolution of Lake Changshou can be classifi ed into two periods with the contrasting characteristics during the past 60 years. The change time of lake environment happened approximately in 1980 AD.DiscussionsC/N ratios can be used to identify the source of organic matter. In general, C/N ratios in algae are less than 10, while C/N ratios in terrestrial plants are high, varying between 20 and 200. In addition,δ13C values are signifi cantly different between aquatic and terrestrial plants. Therefore, C/N ratios andδ13Corgare effective proxies to distinguish the source of organic matter from lake sediments. In Lake Changshou, data plot within the fi eld of aquatic plants, rather than C3/C4plants, implies that the source of organic matter is derived from aquatic plant. While TOC and TN are indicators of lake productivity and eutrophication. The increase ofδ13Corgvalue is interpreted as the increase of lake productivity. Based on the organic indicators, the environment evolution of Lake Changshou can be divided into two periods. Before 1980 AD, stable values of carbon isotope of organic matter, the low carbon and nitrogen content of organic matter, and low C/N ratios suggested that the lake environment was oligotrophication. After 1980 AD, notable increase of carbon and nitrogen content of organic matter, C/N ratios, and decrease of carbon isotope values of organic matter, demonstrated that the increase of lacustrine productivity and the eutrophication was the result of strong human activities and the development of fi sh breeding. Therefore, multiproxy has reliably recorded the environment evolution of Lake Changshou. It is obvious that there is a negative correlation betweenδ13Corgand TOC, inconsistent with the previous views that such an increase in productivity would produce higherδ13C values of organic matter. We believe that the degradation of aquatic algae during the process of eutrophication is the primary factor resulting in the decrease ofδ13C values of organic matter with the increase of productivity in Lake Changshou.ConclusionsC/ N ratios and carbon isotope composition of organic matter reveal that the origin of organic matter in Lake Changshou is aquatic plant, rather than terrestrial plant. The trophication state of Lake Changshou can be divided into two periods. Before 1980 AD, the lake was oligotrophication and was little affected by human activity. After 1980 AD, strong human activities and the development of fi sh breeding led to the serious trophication state. The eutrophication of Lake Changshou after 1980 AD produced the decrease ofδ13C values of organic matter. It is proposed that degradation of aquatic plant likely controls the carbon isotopic composition of organic matter.Recommendations and perspectivesThus, multi-proxy have reliably recorded the environmental evolution of Lake Changshou. The eutrophication of Lake Changshou should be paid much attention, especially on the controls of the human impacts.

Lake Changshou; organic carbon isotope; eutrophication; human activity

ZHU Zhengjie, E-mail: zhuzhjie@163.com

10.7515/JEE201603007

2016-01-22；錄用日期：2016-04-01

Received Date:2016-01-22;Accepted Date:2016-04-01

重慶市基礎(chǔ)與前沿研究計(jì)劃項(xiàng)目（cstc2013jcyjA20001）

Foundation Item:Project of Natural Science Foundation of Chongqing, China (cstc2013jcyjA20001)

朱正杰，E-mail: zhuzhjie@163.com