筑壩河流磷素的遷移轉化及其富營養化特征

鮑林林,李敘勇,蘇靜君

1 中國科學院生態環境研究中心,城市與區域生態國家重點實驗室, 北京 1000852 中國科學院大學, 北京 100049

筑壩河流磷素的遷移轉化及其富營養化特征

鮑林林1,2,李敘勇1,*,蘇靜君1

1 中國科學院生態環境研究中心,城市與區域生態國家重點實驗室, 北京 1000852 中國科學院大學, 北京 100049

人類活動過量營養物質輸入是導致河流富營養化的主要原因,而河道過度的人為調控則進一步復雜化了河流的營養狀態變化。閘壩是河流人為調控的重要工程措施之一,提高水資源利用效率的同時嚴重干擾了河流自然的生物地球化學循環,產生諸多負面生態環境效應。磷素的遷移轉化對河流的營養限制作用受到越來越多的關注,國內外已有研究在筑壩河流磷的富營養化特征方面,已經取得了較為深刻的認識：水庫閘壩建設滯留大量磷素,導致河流水體磷含量升高、營養物質比例變化,沉積物儲存過量磷素形成的內源釋放威脅,以及進一步浮游植物和有害藻類的生長響應等,使得筑壩河流的富營養化生態風險升高；在此基礎上,也提出了根據降雨分配和閘控庫區儲水,合理設置閘壩泄流方式,以改善筑壩河流富營養化生態風險的重要管理思路。對于閘壩調控作用與水體富營養化的定量關系還有待進一步的探討,而且隨著河流資源開發和人為調控力度的增強,河流閘壩建設所產生的系列生態環境問題日益嚴峻,對此提出還需要系統研究的方向：閘壩調控作用下河流磷素的富營養化機制及其與氮、碳等元素的耦合作用,筑壩河流沉積物內源污染的綜合管理,以及閘控景觀河流的生態建設和修復等。

筑壩河流；磷滯留；富營養化；沉積物；閘壩泄流

如今,水體富營養化現象已經成為全球化的環境問題[1- 3],而過量營養物質向河流、湖泊和海岸帶等的輸送,是導致其富營養化的主要原因[4- 5]。作為生態系統重要的生源要素之一,磷素在河流和湖泊等淡水生態系統的營養限制性作用逐漸凸顯,并成為控制水體富營養化和有害藻華爆發的關鍵[6]。河流是陸地與海洋生態系統之間物質循環的橋梁,人類活動的增強加速了流域內大量資源性磷素活化和流失[7],使得水體的磷負荷和富營養化趨勢急劇增加,河流生態系統功能也逐步退化[8- 9]。然而,高強度的人為調控對河流形態的改變,如閘壩水庫修建、河道渠道化、地下水過度開發以及引調水等,進一步加劇了河流的富營養化生態風險[10- 11]。

閘壩是河流人為調控的主要工程措施,以滿足人類獲取能源、防止洪澇、提高灌溉和改善通航等各種需求,但另一方面,筑壩使河流逐漸“湖庫化”,將強水動力條件下的河流搬運作用,逐漸演變為弱水動力條件下的湖泊沉積作用；根據國際大壩委員會(ICOLD)的記錄,中國是世界上大型閘壩(壩高≥15 m)建設最多的國家,加上不計其數的小型閘壩、堰壩和橡膠壩,其對河流生境破壞和水質影響難以估量[12]。閘壩不僅阻滯了河流的水力循環,更是強烈的擾亂了河流原有的物質場、能量場、化學場和生物場,進而改變河流生態系統的物種組成、棲息地分布以及相應的生態功能[13]。對于河流磷素的遷移轉化,閘壩的攔截,使得水體滯留時間增加,磷素等營養物質積累,更加有利于浮游植物生長,進而也加劇了河流水體的富營養化趨勢[14- 15]。所以,關于閘控型河流磷素的遷移轉化和相應的水體富營養化趨勢的變化特征,以及河流生態環境改善的理論和實踐研究都具有重要意義[16- 17]。

由此,本文主要針對國內外筑壩河流磷素遷移轉化及其富營養化的相關研究,總結了閘壩調控下河流對磷素的滯留特征,水體的營養物質分布、浮游植物生長響應和沉積物蓄積磷素的內源污染,以及閘壩泄流水力調控作用下的富營養化生態風險改善等。較為全面的梳理了目前對筑壩河流磷循環和閘壩調控干擾的認識,并提出了應著重開展閘控河流的富營養化機制、合理控制沉積物內源負荷、解決閘控景觀河道富營養化所產生的系列生態問題的研究展望和應用,為我國普遍存在的筑壩河流的生態建設和管理提供參考。

1 河流磷素來源和遷移轉化特征

磷是生命活動必須的營養物質之一,河流生態系統的磷素包括自然來源與人為來源,但是由于經濟發展和城市化的不斷深入,人為來源磷素顯著的增加了河流的磷負荷和污染程度[7- 8]。其中,由人類活動所產生的磷素,主要來自于生活污水和工業廢水為主的點源排放,以及農業和城市產生的非點源輸入[18- 20],而且,降雨徑流沖刷驅使的非點源磷流失常以顆粒態磷為主,高度集中的點源排放則主要為溶解態活性磷(SRP),點源溶解態磷對河流磷濃度、沉積物磷負荷的貢獻,也常常是導致水體富營養化的主要原因[21]。可見,流域內氣候環境和人類活動特征的影響是河流磷素的來源、形態和分布的關鍵因素,特別是SRP的輸入,其具有較高的生物可利用性[22- 23],容易誘發浮游植物爆發性生長并逐步惡化水生態環境[24- 25]。

圖1 河流內部磷素遷移轉化的主要過程 Fig.1 In-stream processes of phosphorus cycling and delivery

磷素在河流內部的遷移轉化,主要包括生物循環、沉積物吸附釋放和懸浮物遷移等過程(圖1)[8]。不同的循環過程決定了磷的形態、分布和生態環境效應。這些物理、化學、生物過程的相互作用對流域磷素的輸入輸出也具有一定調節作用。研究表明,每年大概有30%的SRP滯留在Walker Branch流域(美國)的河道內[26],并主要由生物吸收所去除[8]。多樣化生物群落(包括浮游植物、大型植物、動物和微生物等)的吸收利用和釋放能夠有效削減和緩沖水體磷負荷,并維持河流良好的生態系統服務功能,但是單一種類的生物爆發性生長則適得其反。沉積物的吸附和釋放是緩沖上覆水體中磷含量和形態分布的主要過程。沉積物吸附沉積,能夠降低水體磷濃度,而沉積物的磷釋放、礦化則能夠繼續維持水體磷需求,甚至提高水體富營養化程度。懸浮物對磷素的吸附、釋放和遷移作用,也關系到河流水體及其下游受納水體的磷濃度和營養水平。據估算,英國有26%—75%的磷素是由河流懸浮物輸送到海洋中[27],Seine River(法國)河口的磷通量結果也表明懸浮物的遷移作用約占總磷(TP)的44%[28]。可見,懸浮物是陸源磷素輸出的重要介質[29],當然,懸浮物沉降將磷素轉移到河床沉積物中時,也可以一定程度的降低水體磷含量[30]。總之,河流磷素來源和遷移轉化規律,關系到水環境中磷素的最終歸宿和受納水體的營養狀態,對于掌握河流生態系統磷循環、控制流域富營養化至關重要。而閘壩建設等人為調控對河流磷循環的干擾作用,對深入理解現如今河流的磷素遷移轉化特征和水體富營養化響應更加具有現實意義。

2 閘壩建設對河流磷素遷移轉化的影響

2.1 閘壩攔截下河流磷滯留特征

人類活動極大的改變了全球磷循環,閘壩的建設使得河流磷素的遷移轉化、滯留特征和相應的生態效應變得更為復雜[31]。筑壩河流具有大面積穩定水域(如水庫),減少了河流水量輸出的同時截留大量營養物質[32],所以大大提高了磷滯留能力,而且,從全球范圍看來,河流的磷滯留量會隨著庫壩建設的擴張而持續升高[31]。研究表明,Seine River(法國)上游流域庫壩滯留了60%的入庫磷酸鹽,主要被沉積物和底棲生物所吸收[33]；美國中西部河網閘控河段和自然河段長年監測數據的對比分析表明,水庫出水比無閘壩攔截河流出水的TP年輸出量減少約20%,TP輸出的年內變化也因閘壩的調控而變小[11]。磷素的大量滯留,使得筑壩河流的磷含量顯著升高[34],閘壩出水磷濃度相對較低,這對農業流域排水河道來說,筑壩可以有效地減少磷素等污染物質的輸出,具有流域污染最佳管理措施的功效[35]。此外,對于營養物質含量本身較低的河流來說,閘壩攔截使得滯留水體營養水平升高的同時可能還會引起下游水體的貧營養化。比如,Lule?lven River(瑞典,歐洲閘壩調控最嚴重的河流)沿程眾多閘壩水庫的建立導致該流域磷的輸出逐漸減少[36- 38],威脅到海岸帶的初級生產力[39- 40]。

河流梯級開發的閘壩建設對水量和營養物質的截留強度更甚。由于高強度的梯級開發,黃河流域的斷流和水質惡化現象愈加嚴重,TP的輸出減少了84%左右[41];長江上游流域也建設有大量的閘壩和水庫,尤其是三峽大壩建設后,其向中下游流域輸送的TP減少了77%[42];貓跳河是中國第一條被完整開發的河流,是全國流域梯級開發最早和程度最高的河流,全流域水能資源控制近90%[43],其系列水庫對磷的逐級滯留,使得磷酸鹽濃度降低了約90%[17,44]。總之,閘壩這一強有力的人為調控方式,改變了河流營養物質的循環和分布,導致滯留水體的磷含量升高,提高筑壩河流的營養水平,引發其富營養化,而其下游水體的營養狀況因磷輸出減少而有所降低,甚至加劇貧營養化從而限制初級生產力。可見,閘壩的磷滯留能力關系到筑壩河流及其下游水體營養狀況,對整個生態系統都具有深遠的影響。

2.2 筑壩河流磷素的富營養化特征

2.2.1 磷含量升高和營養物質比例變化

大量磷素有效攔截和水體較長的滯留時間都為浮游植物生長提供了有利的條件,筑壩河流的富營養化問題更加嚴峻[5,45]。Han River是韓國最重要的河流,其主干和支流建立有各種水庫大壩、堤壩、浸沒式閘壩、小型堰壩等,導致河道磷素大量滯留,浮游植物常爆發性生長[46- 48]。澳大利亞西南部的Canning River和Lower Vasse River,春末到秋初整個生長季節河流處于攔截蓄積狀態,水體的TP濃度升高,大型水生植物逐漸消失,浮游植物成為優勢類群并大量生長[49]。進一步水質惡化會使筑壩河流的生物多樣性降低,生態系統功能急劇退化。另一方面,閘壩內滯留水體營養物質的攔截和浮游植物吸收利用,可以一定程度降低下游水體浮游植物的生物量和爆發風險[44,50],但是,當遇到大規模降雨或洪水事件,儲存在壩內的營養物質和浮游植物被沖刷流出時,下游水體將受到嚴重污染[50]。

營養物質含量升高導致浮游植物大量生長是河流富營養化最直觀的表現,所以,營養物質含量與浮游植物群落特征的響應也能反映水體營養狀況的變化特征。研究表明,水體浮游植物爆發性生長,主要是少數藻類優勢生長的結果,營養物質的過量輸入更加有利于單一類群的有害藻類大量生長[10]。所以,閘壩滯留水體中的藻密度通常高于自然河流段,而藻群結構的豐富度較低[51],其優勢類群也主要為喜營養豐富和靜水環境的種類(如綠藻)[52]。閘壩的攔截作用還會引起主要營養物質,如氮(N)、磷(P)和硅(Si)比例的變化,對水體富營養化和藻群結構變化也具有顯著影響作用。由于硅的沉降效率較高而磷素的循環速度相對較快,筑壩使得河流Si∶P比例明顯降低,Danube River(法國)即因系列閘壩調控，河流湖庫化營養物質比例失衡,甲藻類有害藻類大量生長,導致海岸帶漁業受到嚴重影響[10]。貓跳河的梯級水庫中上游庫區磷素大量滯留時,為綠藻和藍藻等優勢種類所利用[53],下游水庫磷含量降低時,Si∶P比例有所升高,所以浮游植物中硅藻的比重又有所增加[44]。相對于水體流動性和連續性較好的自然河流,筑壩河流的營養物質比例和藻群結構常發生顯著變化[54- 55],從而改變其富營養化特征。已有研究表明,由于河流閘壩滯留相當量的營養物質(特別是Si和顆粒態的N、P),而人類活動產生的溶解態N、P卻有增無減,河流輸出Si∶N∶P比值持續下降,這將會導致我國主要海域非硅質浮游植物和有害藻類的持續爆發性生長,危害整個水環境安全和人類健康[2,40,56]。總之,閘壩的水力攔截使得河流的富營養化生態風險急劇升高,從生態恢復的角度來解決這一問題將具有非常大的挑戰。

2.2.2 沉積物內源磷釋放的污染

筑壩河流更容易淤積大量沉積物,成為磷素主要儲存庫,特別是顆粒態磷和懸浮物攜帶的磷素更容易儲存在壩內沉積物中[42],使其成為筑壩河流富營養化的主要污染源之一。已有研究表明,城市河流Tanchun Stream(韓國)攔蓄河段內沉積物中的TP含量和沉積物的磷釋放潛力均顯著高于其下游河段,對滯留水體的富營養化具有很大貢獻[46]；1997—1998年夏季,由于Canning River(澳大利亞)的攔蓄滯留,沉積物釋放大量磷素,導致河流爆發藍藻而不得不停止向公眾開放[57]；Lot River(法國)系列閘壩庫區的沉積物中曾儲存有近10 000 t的生物可利用磷素[58],瀾滄江梯級水庫內沉積物的生物可利用磷含量也非常高,具有很高的釋放風險和生態威脅[59- 60]；九龍江北溪西坡電站庫區的沉積物則已經處于磷釋放狀態[61]。而且,洪水爆發等所致河流水動力增強時,筑壩河流沉積物中的磷素也會因沖刷再懸浮作用而釋放出來,對水體造成二次污染[50,62]。可見,沉積物對磷的吸附與釋放,是控制筑壩河流水體富營養化和維持滯留水體自凈能力的關鍵[63],河流的閘壩建設也需要關注沉積物內源磷釋放的污染問題。

3 閘壩水力調控的富營養化生態風險改善

控制人類活動過量營養物質的輸入,能夠有效降低河流生態系統的磷負荷和富營養化趨勢,而對于筑壩河流來說,改善磷素的富營養化生態風險還需要從優化閘壩調控等方面著手,因為河流水體滯留容易引發富營養化,而一定強度的水動力擾動則不利于營養物質滯留和浮游植物爆發性生長[64-65]。

位于韓國東南地區的Nakdong River(韓國第二大河)建設有4個多功能水壩和1個河口大壩以調控整個流域的流量,水體富營養化嚴重,常有浮游植物爆發性生長,而且季風和臺風所產生的降雨年際分配不均也是導致河流水量、水質年際變化的原因之一。所以,Jeong等[15]根據降雨、流量和閘控泄流等與水體葉綠素含量的響應關系,提出了通過多年自然降雨狀況,調控閘壩泄流方式和泄流流量,以緩解河流的富營養化趨勢并改善水質。其調控方式如圖2所示,如果當年和上一年雨季都有足夠降雨,庫壩內水量充足,則增加閘壩泄流量,如果兩年中只有一年降雨充沛,則需要謹慎管理壩內儲水,采用脈沖式泄流,這兩種情景都能改善筑壩河流的富營養化狀況,抑制浮游植物生長；但是,如果連續兩年沒有充沛降雨,蓄水不足,閘壩泄流將減少,浮游植物爆發性生長則不可避免。閘壩泄流的沖刷和稀釋作用,能夠顯著降低下游河段的氮磷濃度,并且減少水體中藻群的生物量[66],這種根據水量分配以控制閘壩泄流的方式,對于河流年際的水質改善和枯水期的水質改善都具有積極作用[67]。

圖2 根據氣候變化和浮游植物繁殖特征的筑壩河流水文管理示意圖[15]Fig.2 Diagram depicting the river regulation mechanism regarding climate changes and phytoplankton population dynamics

閘壩泄流調控的作用在于：適當的泄流量可以防止水體長期滯留形成溫度分層,而且較高的流速能夠增加濁度以減少光的可利用性,同時稀釋和轉移藻類細胞,從而有效地抑制單一藻類和有害藻類的大量生長,以達到改善水質的目的[68- 69]。Webster等[70]也認為,從優化閘壩泄流方式出發,采用閘壩最小基本泄流流量、脈沖式泄流、高落差泄流、虹吸式泄流等水力調控措施,均能降低閘控滯留水體和下游水體的富營養化生態風險。此外,Alrajoula等[71]的研究表明,閘壩調控的水動力變化在汛期具有積極作用,但是枯水期的流量干預對下游河流生態系統具有一定負面影響(如岸邊棲息地的擾動),可見,閘壩泄流的有效管理和實踐應用還有賴于對流域降雨、閘壩儲水、泄流量的時空分配和水質變化、生態效益之間響應關系的定量化研究[72]。當然,如今人類活動對生態系統的干擾有增無減,氣候變化所致降雨分配變異也越來越大,筑壩河流的富營養化生態風險改善和生態恢復將是成效緩慢且需要持續關注和巨大投入的漫長過程。

4 結論與展望

綜合看來,閘壩建設對河流磷素分布和富營養化的影響已經取得了較為充分的認識,閘壩建設使得磷素大量滯留加快了水體的富營養化趨勢,由此導致河流營養物質的遷移輸出比例失調,浮游植物爆發風險升高,而沉積物富集磷素的內源釋放污染也具有很大威脅。如何有效地改善閘壩滯留磷素的富營養化生態風險,以及合理科學的控制閘壩運營方式,是目前筑壩河流生態環境建設和管理亟待解決的問題。在此基礎上,筑壩河流的富營養化還需要進一步關注以下幾個問題：

(1)河流磷素的富營養化機制及其與氮、碳等其他元素的耦合作用 磷的形態決定了其生物可利用性和滯留方式,及其與浮游植物生長的響應關系,筑壩河流的富營養化還受到流量、水體滯留時間和閘壩泄流等水力條件的影響作用,此外,水體富營養化是包括了碳源和氮、磷等營養物質供給和環境變化響應的綜合過程[45],所以,筑壩河流人為調控作用下不同形態磷的遷移轉化和相應浮游植物生長的營養物質閾值的定量分析,以及磷與碳、氮耦合對水體富營養化的影響作用,對闡明筑壩河流的富營養化機制具有重要意義,有待進一步的研究解答。

(2)沉積物內源污染的綜合管理 閘壩攔截有效滯留大量磷素和沉積物,導致相當部分的磷素蓄積在壩內沉積物中,而沉積物的磷吸附釋放直接關系到滯留水體的營養狀況和水質安全。控制外源磷污染的同時,內源磷釋放的管理是筑壩河流需要特別重視的問題,清淤、鎖磷劑固持以及沉水植物種植等方法,都對沉積物內源釋放具有一定的改善效果,但是具體控制管理措施的實施和評估,還需要結合筑壩河流沉積物的吸附釋放特征不斷地嘗試和改進。

(3)閘控景觀河流的生態修復 河道景觀化是目前我國河流綜合整治和管理的主要方式之一,特別是在城市黑臭河道的整治方面。但是,硬質化河床和護坡以及一系列長期處于閉蓄狀態的閘壩或橡膠壩的設置,導致景觀河流水質依舊不容樂觀,水體富營養化嚴重、河道自凈能力低、河流生態功能簡單化等。景觀河流的建設和管理更應該關注筑壩所帶來的水體富營養化等負面生態效應,以達到真正的河流景觀建設和生態環境恢復。

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Phosphorus cycling and the associated ecological effects of eutrophication in dam-regulated rivers

BAO Linlin1,2, LI Xuyong1,*, SU Jingjun1

1StateKeyLaboratoryofUrbanandRegionalEcology,ResearchCenterforEco-EnvironmentalSciences,ChineseAcademyofSciences,Beijing100085,China2UniversityofChineseAcademyofSciences,Beijing100049,China

The anthropogenic input of nutrients to rivers is the main cause of eutrophication, and both the excessive control and engineering of river channels further complicate the issue. To meet the demand for energy and water resources, thousands of dams, weirs, and sluices have been constructed and affected almost every river, which deeply interferes with the biogeochemistry cycles of nutrients and the ecological function of river systems. Previous research has investigated the ecological effects of eutrophication caused by phosphorus in dammed rivers, since phosphorus limitation of primary production is more predominant in river systems. Dammed rivers can reduce outflow and, hence, sequester a significant amount of phosphorus within the impoundments. As a consequence, river water becomes eutrophic, and the ratios of main nutrients change drastically, owing to the different responses of the nutrients to retention by dams. In addition, sediments in the dammed rivers, which containing high contents of phosphorus, can easily become a potential pollution source, especially under intensive scouring events. At the same time, the abundance of algae communities increases, as certain species bloom in the water column, which aggravates the ecologic effects of river eutrophication and threatens the biodiversity of river systems. Therefore, researchers have found that an efficient management strategy based on annual rainfall storage and dam discharge control can be used to improve the ecological effects of eutrophication in dammed rivers. However, as anthropogenic impacts continue to increase, further studies of dammed rivers are needed to clarify the relationship between artificial control in rivers and river eutrophication, in order to reduce the ecological effects of eutrophication, as well as to quantify the threshold of phosphorus, in order to further understand the eutrophication mechanisms of dammed rivers and the combined effects of nitrogen and carbon, to manage phosphorus within sediments, in order to prevent its release and resuspension, and to address concerns regarding the construction and restoration of landscape rivers with dams throughout China.

dammed rivers; phosphorus retention; eutrophication; sediment; reservoir discharge

京津冀城市群地區生態安全關鍵技術綜合示范區建設(2016YFC0503007)；國家水體污染控制與治理科技重大專項(2015ZX07203-005)；中國科學院重點部署項目(KZZD-EW-10-02)；國家自然科學基金青年基金(41401590)

2016- 03- 31; 網絡出版日期:2017- 03- 02

10.5846/stxb201603310588

*通訊作者Corresponding author.E-mail: xyli@rcees.ac.cn

鮑林林,李敘勇,蘇靜君.筑壩河流磷素的遷移轉化及其富營養化特征.生態學報,2017,37(14):4663- 4670.

Bao L L, Li X Y, Su J J.Phosphorus cycling and the associated ecological effects of eutrophication in dam-regulated rivers.Acta Ecologica Sinica,2017,37(14):4663- 4670.