水動力條件對淺水湖泊沉積物氮磷釋放的影響

余景芝,王 烜*,蔡劍英,廖珍梅,李春暉,劉 強(qiáng)

水動力條件對淺水湖泊沉積物氮磷釋放的影響

余景芝1,2,王 烜1,2*,蔡劍英1,2,廖珍梅1,2,李春暉2,劉 強(qiáng)2

(1.北京師范大學(xué)環(huán)境學(xué)院水環(huán)境模擬國家重點(diǎn)實(shí)驗(yàn)室,北京 100875；2.北京師范大學(xué)環(huán)境學(xué)院水沙科學(xué)教育部重點(diǎn)實(shí)驗(yàn)室,北京 100875)

水動力條件是影響淺水湖泊沉積物氮磷釋放過程的主要自然因素.研究水動力條件對沉積物氮磷釋放的影響對于掌握淺水湖泊營養(yǎng)鹽遷移轉(zhuǎn)化規(guī)律?預(yù)測與防控內(nèi)源污染具有十分重要的意義.水動力因子中,流速通過影響沉積物-水界面剪切應(yīng)力改變界面氮磷的交換通量;水位和水體擾動均會影響表層沉積物理化特征?沉積物-水界面氧化還原反應(yīng)過程,通過多種效應(yīng)耦合共同對氮磷釋放產(chǎn)生影響.本文綜述了流速?水位?水體擾動等主要水動力因子對淺水湖泊沉積物中氮磷釋放的影響機(jī)制研究并提出展望:多種水動力因子耦合作用對氮磷釋放的影響機(jī)理?基于多時(shí)空尺度構(gòu)建原位監(jiān)測-耦合模擬研究體系?內(nèi)源氮磷釋放之后對生態(tài)系統(tǒng)的影響研究還有待加強(qiáng).

水動力條件；淺水湖泊；沉積物；氮磷釋放；內(nèi)源污染防治

湖泊在供給水源?調(diào)節(jié)徑流、降解污染物?保持生物多樣性等方面有極其重要的生態(tài)服務(wù)價(jià)值,在流域水循環(huán)與流域生態(tài)過程中起關(guān)鍵作用[1-2].我國淺水湖泊眾多,它們具有優(yōu)越的自然生態(tài)系統(tǒng)服務(wù)功能和社會經(jīng)濟(jì)價(jià)值.然而,這些湖泊流速較低,污染物輸移擴(kuò)散緩慢,環(huán)境容量小.水文水動力條件的微小改變會驅(qū)動湖泊沉積物懸浮和內(nèi)源營養(yǎng)鹽釋放,進(jìn)而引起湖泊生態(tài)系統(tǒng)的結(jié)構(gòu)與功能發(fā)生改變[3].當(dāng)前白洋淀等淺水湖泊的外源污染已得到有效治理,日益突顯的沉積物懸浮與內(nèi)源營養(yǎng)鹽釋放問題逐漸成為制約全局水質(zhì)改善成敗?保障湖泊生態(tài)系統(tǒng)可持續(xù)發(fā)展的熱點(diǎn)問題.

由于長期以來淺水湖泊中氮磷元素的不斷累積,沉積物成為營養(yǎng)鹽的蓄積庫[4-5],在湖泊富營養(yǎng)化的過程中成為“匯”;當(dāng)環(huán)境條件改變時(shí),沉積物中的氮、磷等營養(yǎng)物質(zhì)可通過生物分解、解吸附、礦化等作用釋放出來,成為影響上覆水體水質(zhì)的“源”.沉積物-水界面的這種源、匯轉(zhuǎn)化是決定湖泊水質(zhì)狀況的關(guān)鍵[6],而水體的pH值、溶解氧等理化特征和沉積物粒徑、孔隙度、組成分布、氮磷元素形態(tài)等理化特征決定了源、匯轉(zhuǎn)化的進(jìn)程[7-8].在河湖水體中,流速、水位、水體擾動等水動力條件的改變對水相和沉積相特征產(chǎn)生的影響是氮磷釋放的重要驅(qū)動力[9-10].因此,摸清水動力因子作用下沉積物-水界面的氮磷釋放機(jī)制及遷移轉(zhuǎn)化規(guī)律已成為近20a來淺水湖泊水動力過程及其環(huán)境效應(yīng)研究的重點(diǎn)[11-12],相關(guān)研究對于水體污染的模擬、預(yù)測和科學(xué)防控具有重要意義.

本文將分別綜述不同水動力因子對淺水湖泊沉積物中氮磷釋放的影響機(jī)制,并提出未來值得深入研究的方向,以期為淺水湖泊內(nèi)源污染控制和水資源管理提供科學(xué)依據(jù).

1 水動力因子對淺水湖泊沉積物氮磷釋放的影響機(jī)制

淺水湖泊沉積物中氮磷釋放主要通過兩種途徑.第一,由于沉積物-水濃度梯度或外力擾動下,附著在沉積物顆粒上的氮磷釋放到間隙水中,進(jìn)一步擴(kuò)散到上覆水中,這個(gè)過程簡稱為自由擴(kuò)散過程.第二,吸附氮磷的沉積物顆粒由于外力作用再懸浮至上覆水中,增大氮磷含量,即再懸浮過程[13-14].水動力因子作用下,沉積物-水界面的剪切作用發(fā)生變化,直接改變水環(huán)境理化特征、沉積物理化特征[15-16],或改變水中生物生態(tài)特征、微生物活性等,進(jìn)而對沉積物-水系統(tǒng)的理化特征產(chǎn)生影響,從而間接導(dǎo)致上覆沉積物-水界面氮磷交換過程的改變[14].水動力因子對淺水湖泊沉積物氮磷釋放的影響機(jī)制如圖1所示.不同水動力條件變化作用于沉積物氮磷釋放的機(jī)制不同,但都可以歸因?yàn)樯细菜w與沉積物之間的氮磷濃度差的改變.

2 不同水動力因子對淺水湖泊氮磷釋放的影響

2.1 流速

流速是表征水動力條件最基本的因子,流速變化引起懸浮物運(yùn)動狀態(tài)和沉積物營養(yǎng)鹽釋放速率改變,加速水質(zhì)變化的過程[17-18].目前大多數(shù)研究采用室內(nèi)水槽模擬方法(如波浪槽、生態(tài)水槽?環(huán)形水槽等)探究水體流速對沉積物氮磷釋放的影響機(jī)制.21世紀(jì)早期的研究較多關(guān)注不同流速對水相和沉積相中污染物濃度的影響.Barlow等[19]利用室內(nèi)循環(huán)水槽結(jié)合Elovich方程模擬探究流速與水深對水體TP吸附效應(yīng)的影響.Zhu等[20]、Tong等[21]也分別利用室內(nèi)波浪槽和雙向環(huán)形水槽模擬實(shí)驗(yàn)探究變化流速下沉積物的氮釋放特征.隨著環(huán)境監(jiān)測技術(shù)的迅速發(fā)展,水體流速對沉積物氮磷的質(zhì)量濃度、釋放通量、釋放速率等的影響規(guī)律逐漸成為內(nèi)源釋放研究的熱點(diǎn)問題.Peng等[22]針對淺水、不分層的混合型湖泊,通過構(gòu)建不同流速下的可溶性磷的濃度曲線,提出SRP(溶解態(tài)磷)的質(zhì)量濃度與流速滿足正相關(guān)關(guān)系:= 0.0003+0.0404,其中為流速.隨后Li等[23]、Pang等[24]和Ding等[25]在此基礎(chǔ)上建立了一定范圍內(nèi)沉積物中TN、TP釋放通量與流速的定量關(guān)系分別為=137.88e0.06x和=36.78e0.05x,其中為流速,并將此定量關(guān)系式應(yīng)用于ECOMSED模型.考慮到沉積物-水界面氮磷的釋放由水相和沉積相理化特征等共同決定,在實(shí)際研究過程中需要綜合考慮水相與沉積相的特征及相互作用,不能將二者分開孤立地進(jìn)行研究.因此一些研究建立了沉積物-水耦合模型,如Huang等[26]通過分析沉積物動力學(xué)建立水動力-沉積物-氮磷輸移的數(shù)學(xué)模型,并將其應(yīng)用于三峽大壩水庫,較好地揭示了沉積物中磷的運(yùn)移規(guī)律.當(dāng)前針對流速對沉積物氮磷釋放影響的研究以室內(nèi)水槽實(shí)驗(yàn)、數(shù)學(xué)模型、試驗(yàn)與模型相結(jié)合的方法為主,研究條件相對簡化,尚難以精確表征實(shí)際情況,因此后續(xù)的探究應(yīng)綜合室內(nèi)外的物理試驗(yàn)方法和模型技術(shù),以提高研究結(jié)果的合理性和可靠性.

圖1 水動力條件對淺水湖泊沉積物氮磷釋放的影響

總體而言,在某一范圍內(nèi),流速的增大能夠促進(jìn)沉積物中氮磷的釋放,當(dāng)流速達(dá)到臨界流速之后,氮磷釋放驅(qū)動機(jī)制更加復(fù)雜,此時(shí)流速將不再成為主要的影響因素.當(dāng)前研究對臨界流速的界定尚未提出較為統(tǒng)一的計(jì)算范式,需要深入探究流速對沉積物氮磷釋放產(chǎn)生影響的動力學(xué)過程,明確不同水體臨界流速的界定方法.

2.2 水位

水位是表征水文水動力條件的關(guān)鍵因子,淺水湖泊水位的變化與湖泊生態(tài)系統(tǒng)功能密切相關(guān),直接作用于沉積物的理化特性,并影響營養(yǎng)鹽的遷移和循環(huán)過程[35].

通常水位調(diào)節(jié)會引起上覆水環(huán)境及表層沉積物理化特征的改變,大多數(shù)研究集中于氧化還原條件[36-37]、沉積物顆粒特征[38]、微生物生長[39-40]等因素,通過這些因素的變化進(jìn)而間接研究水位對沉積物氮磷釋放的影響.水位調(diào)節(jié)也會直接影響氮磷的形態(tài)轉(zhuǎn)化過程.Yu等[41]提出水位調(diào)節(jié)過程中沉積物-水界面的氮循環(huán)模式發(fā)生了明顯的轉(zhuǎn)變,NH4+向NO3-的轉(zhuǎn)化明顯增強(qiáng),削弱了水體中氮的去除.水位變化的范圍、變化頻度、持續(xù)時(shí)間等對沉積物中氮磷的釋放具有重要影響[35].Tong等[42]模擬不同水位和流速下沉積物磷的釋放特性,發(fā)現(xiàn)磷釋放速率與水位符合第二拋物線方程,當(dāng)流速保持0.3m/s時(shí),上覆水總磷濃度在25cm的水位處達(dá)到最大值,磷釋放速率隨水位升高而增加.Tang等[38]對水位調(diào)節(jié)速度與周期進(jìn)行動態(tài)模擬,提出6cm/d的快速水位調(diào)節(jié)促進(jìn)了氮的釋放,3cm/d的緩慢水位調(diào)節(jié)有利于磷的淋溶.此外,水位亦為水生植物群落分布和生長發(fā)育的重要限制因子,而水生植物根系的吸收作用會間接影響沉積物與間隙水中TN(總氮)?TP(總磷)交換過程[43].Bai等[44]的研究發(fā)現(xiàn),狐尾藻的株高和生物量隨著水深的增加而呈現(xiàn)下降趨勢,相對較淺的水深條件促進(jìn)狐尾藻對氮磷的根系吸收,一定程度上減少了沉積物與上覆水之間的氮磷交換.

圖2 流速對沉積物氮磷釋放過程的作用機(jī)制

水位的變化會導(dǎo)致不同的水文景觀格局和水文連通性的變化[45],這也是近年來湖泊濕地水文研究的熱點(diǎn)問題[46-47].水文連通性變化對淺水湖泊生態(tài)環(huán)境的影響極為復(fù)雜,不僅會影響水體的水文節(jié)律、水動力特征及沉積物的組成、結(jié)構(gòu),同時(shí)還會聯(lián)動影響污染物質(zhì)的遷移、水生物遷移擴(kuò)散等過程,進(jìn)而對湖泊水質(zhì)和水生態(tài)系統(tǒng)產(chǎn)生直接或間接的影響[48].因而在水文連通性變化背景下,湖泊的諸多特征如水量的交換、流速、水齡等都會受到影響,進(jìn)而影響沉積物-水界面氮磷的釋放.

近10a來我國通過實(shí)施水文連通工程增加了近50個(gè)湖泊的湖水流動性[49],其水文水動力特征也隨之發(fā)生改變.如太湖水體經(jīng)過引江濟(jì)太工程后調(diào)水周期從原來的300d縮短至250d[50],現(xiàn)階段太湖沉積物對TP的吸附和沉積作用要遠(yuǎn)大于釋放[51].對于形狀復(fù)雜?面積較大的湖泊而言,不同湖區(qū)和湖區(qū)的不同位置水文連通性不同,造成的氮磷滯留與釋放效果差異大,Sun等[52]通過二維水動力模型分析后發(fā)現(xiàn)湯遜湖主要區(qū)域的NH3-N、TN、TP濃度下降,而邊界區(qū)域則無明顯改善情況.同時(shí),水文連通性的變化也會改變淺水湖泊的風(fēng)生流場,Zheng等[53]在利用Delft3D軟件構(gòu)建大東湖水動力學(xué)模型過程中發(fā)現(xiàn),連通之后水系流速大于0.01m/s的水面面積比連通前擴(kuò)大了一倍以上,而通過水動力-水質(zhì)耦合數(shù)學(xué)模型模擬分析發(fā)現(xiàn)主流線附近水域流速增加,其TN?TP濃度降低,在偏離主線較遠(yuǎn)水域,流速變化與TN、TP濃度變化均不明顯[54].綜合來看,增強(qiáng)水文連通性能夠通過影響水文水動力特征,增強(qiáng)湖泊內(nèi)部污染物的稀釋降解過程,促進(jìn)沉積物從營養(yǎng)物質(zhì)的“源”向“匯”的轉(zhuǎn)化.同時(shí),水文連通性增強(qiáng)還可能造成污染物的輸入和水體擾動,容易發(fā)生沉積物再懸浮和增加氮磷內(nèi)源釋放的風(fēng)險(xiǎn).

以往研究表明一定范圍內(nèi)淺水湖泊水位的增加促進(jìn)氮磷的釋放,而水位變化的范圍、頻度、持續(xù)時(shí)間及引起水文連通性的改變,均可導(dǎo)致水位-沉積物氮磷釋放這一響應(yīng)過程受到影響,而目前針對這一過程的機(jī)制研究尚為單一.未來研究中尤其需要關(guān)注水位-水文連通性-氮磷釋放風(fēng)險(xiǎn)這一影響路徑,定量評估水位變化下的沉積物水界面氮磷釋放風(fēng)險(xiǎn).同時(shí),綜合考慮湖泊形狀?地形地貌特征等其他因素,使研究結(jié)果更加準(zhǔn)確可靠.

2.3 水體擾動

水體擾動是水動力條件的重要因子,擾動發(fā)生時(shí),大量沉積物再懸浮至上覆水中,導(dǎo)致間隙水中的溶解態(tài)氮磷也隨之向上遷移擴(kuò)散.需要注意的是,水體擾動與流速關(guān)系密切,二者相互聯(lián)系和影響.水流的流速直接決定流態(tài)的變化,進(jìn)而改變水流剪切應(yīng)力,使得氮磷的釋放受到影響;而擾動會引起淺水湖泊的流場和動能變化,進(jìn)而引起上覆水體和沉積物顆粒的狀態(tài)變化.為便于描述,本文水體擾動特指水體外部的作用力(如風(fēng)浪等)引起的流場變化,與“2.1流速”中水體內(nèi)部的流速變化相區(qū)別,以更加針對性地研究淺水湖泊氮磷釋放的關(guān)鍵因子.

對水體擾動的研究常采用室外監(jiān)測和室內(nèi)模擬的方式進(jìn)行,其中室外監(jiān)測主要包括野外觀測和原位圍隔實(shí)驗(yàn)法等,如Qin等[55]、Zhu等[56]通過野外觀測太湖大風(fēng)浪過程和靜風(fēng)期間水體TP、TDP等含量的變化,以探究風(fēng)浪擾動對于水體內(nèi)源磷釋放的影響.這類方法可以直接利用原位監(jiān)測或?qū)嶒?yàn)分析得到流速對沉積物氮磷釋放的影響規(guī)律和機(jī)制.然而,由于受地理環(huán)境、氣象水文等多種因素的綜合影響,野外觀測實(shí)驗(yàn)受到一定限制,因此目前對水體擾動的研究大多采用室內(nèi)模擬方式進(jìn)行,且多關(guān)注于擾動引起的沉積物再懸浮行為.以往研究所采用的室內(nèi)模擬擾動的方式各異,對擾動引起的沉積物再懸浮及營養(yǎng)鹽釋放的研究視角也有所不同.如Wang等[57]和Jiang等[58]采用六聯(lián)攪拌儀模擬巢湖、東平湖中不同擾動強(qiáng)度.隨后的研究開始逐漸采用水槽試驗(yàn)對擾動進(jìn)行模擬,如Hu等[59]、Sun等[60]采用矩形波浪水槽進(jìn)行水動力試驗(yàn),在水槽中模擬不同波浪擾動對沉積物再懸浮及營養(yǎng)鹽釋放速率的影響;在此基礎(chǔ)之上也有研究提出采用改良的自制U型水槽,以不同水頭差來模擬波浪的循環(huán)載荷作用, Zhang等[61]提出沉積物液化狀態(tài)下的沉積物釋放速率的擬合方程,從沉積物自身狀態(tài)模擬的角度對以往研究進(jìn)行了補(bǔ)充.

隨著室內(nèi)模擬試驗(yàn)的日益成熟,專家學(xué)者開始嘗試將其與數(shù)值模擬進(jìn)行耦合.Huang等[26-62]提出了綜合水動力、風(fēng)浪和泥沙輸運(yùn)的磷動態(tài)模型,并結(jié)合水槽試驗(yàn)定量估計(jì)風(fēng)浪和湖流對磷釋放和分布的影響,精確化、定量化地模擬了風(fēng)浪湖流作用下的磷釋放過程.水體擾動對沉積物氮磷釋放影響的室內(nèi)模擬試驗(yàn)可總結(jié)為表1.

表1 水體擾動對氮磷釋放影響的室內(nèi)模擬實(shí)驗(yàn)

一般說來,持續(xù)的擾動會直接改變沉積物的組成和結(jié)構(gòu)特征從而影響沉積物氮磷的釋放.如Li等[68]、孫小靜等[69-70]提出指出持續(xù)的風(fēng)浪擾動下懸浮物細(xì)顆粒組分的百分含量明顯增加,溶解態(tài)磷更易被吸附沉降從而抑制磷的釋放.同時(shí)擾動過程中大量的溶解性磷從水中釋放出來,導(dǎo)致溶解氧含量升高,在一定程度上制約磷的釋放[71].研究表明好氧條件下沉積物氮和磷的釋放量減小,厭氧條件更有利于沉積物釋放氮磷[72-73].同時(shí),溶解氧含量的增加還將使得沉積物中大量的鐵錳硫化物被氧化,生成鐵錳氫氧化物,強(qiáng)烈吸附水中的溶解態(tài)磷,因而使得擾動后期磷釋放減弱[74-77].

此外,擾動還可以促進(jìn)微生物對磷的分解以及水體中顆粒態(tài)磷的酶解,這也是內(nèi)源磷釋放的重要途徑.Huang等[66]和Chao等[78]提出低至中度的擾動會使得DAPA(溶解性堿性磷酸酶)活性隨時(shí)間增加,有利于各類有機(jī)磷化合物從沉積物中水解.Fan等[79]定量估算了風(fēng)浪擾動下太湖懸浮顆粒物中磷的生物轉(zhuǎn)化量,因生物分解導(dǎo)致的上覆水磷負(fù)荷的增量約為425.8t/a,并認(rèn)為在水動力作用的促進(jìn)下,附著于懸浮顆粒上的磷的生物分解對上覆水SRP負(fù)荷量的增加做出了重要貢獻(xiàn).

研究表明擾動使得底泥氮磷釋放速率增大,Huang等[67]提出低至中度的擾動促使沉積物中氮磷及其他養(yǎng)分的釋放.Jiang等[58]的研究指出,擾動強(qiáng)度為25,50,100r/min時(shí),TP、TN的釋放速率比靜態(tài)條件下分別增加了36.2%?41.7%?127.6%,呈現(xiàn)顯著增加的趨勢.同時(shí),擾動帶來的沉積物-水界面的壓力差也會促進(jìn)氮的釋放,Wu等[80]提出強(qiáng)風(fēng)浪作用導(dǎo)致孔隙水運(yùn)動大大增加了氮的擴(kuò)散速率,在4和10cm高的風(fēng)浪作用下,上覆水體NH4+濃度增加了0.016mg/L,是沉積物間隙水中NH4+降幅的近1倍.

在此基礎(chǔ)上,學(xué)者們深入探究了引起底泥懸浮的臨界擾動強(qiáng)度.Wang等[81]認(rèn)為臨界風(fēng)速可用于表征沉積物所受擾動大小,并提出引起太湖梅良灣沉積物再懸浮的臨界風(fēng)速約為7m/s,此條件下的平均再懸浮率為1000g/(m2d).Li等[82]以湍流強(qiáng)度表述水體擾動的大小,提出在湍流強(qiáng)度較小時(shí)(3.6×10-3m2/ s3),水體中磷酸鹽的釋放較初始水平增加了36.36%;而當(dāng)湍流強(qiáng)度增大至7.4×10-2m2/s3時(shí),沉積物大量懸浮,從而使得磷被懸浮物吸附固定,抑制其釋放.值得注意的是,擾動作用過程常表現(xiàn)為最初釋放劇烈,隨后逐漸減緩并最終達(dá)到平衡.Yu等[83]通過水槽實(shí)驗(yàn)?zāi)M太湖沉積物釋放特性發(fā)現(xiàn),TDP和TDN的釋放在最初30min內(nèi)最劇烈, 30～60min輕度釋放,最后達(dá)到平衡,TDP和TDN的總釋放量及其平衡濃度的增長速率在達(dá)到一定值后減慢,Zhang等[84]的研究也得出類似的結(jié)論.

目前大多研究采用湖面風(fēng)速、湍流強(qiáng)度、紊動強(qiáng)度來表征水體擾動強(qiáng)度,尚未形成統(tǒng)一的表征因子.同時(shí),各研究以不同形態(tài)的氮磷為研究對象,如水體總磷總氮濃度、膠體氮(CN)?膠體磷(CP)濃度以及真溶解態(tài)氮(UDN)、真溶解態(tài)磷(UDP)等,結(jié)果之間缺乏可比性.因此,未來對水體擾動的研究中有必要提出通用的擾動強(qiáng)度表征因子和代表性氮磷元素標(biāo)的,以便各研究之間的相互借鑒.此外,水體擾動的強(qiáng)度與持續(xù)時(shí)間等特征參數(shù)對氮磷釋放進(jìn)程的定量化影響機(jī)制、再懸浮過程沉積物釋放氮磷的動力學(xué)過程及生態(tài)效應(yīng)尚缺乏系統(tǒng)性的研究成果,如擾動之后沉積物氮磷再分配行為的響應(yīng)機(jī)制,以及有多少氮磷重新吸附沉淀回到沉積物中,有多少被生物利用等問題需深入探究.

2.4 其他因子

水動力條件不僅體現(xiàn)在流速?水位?水體擾動這幾種因子上,國內(nèi)外的很多學(xué)者從不同的角度對其他水動力表征因子進(jìn)行了研究.

湖泊水庫的換水周期直接影響水體中營養(yǎng)物的濃度與停留時(shí)間,以及水體中發(fā)生的生物和化學(xué)反應(yīng)過程時(shí)間長短.Hatcher和Frith[85]的研究表明,水體內(nèi)銨濃度的長期均值與湖泊換水周期具有很好的相關(guān)性,在湖泊水質(zhì)惡劣的情況下,換水周期過長會導(dǎo)致水質(zhì)在空間上存在較大的差異性,減弱湖泊水動力;而換水周期過短則不利于水生生物的生長.因而換水周期也存在閾值問題,Wang等[50]通過構(gòu)建湖泊水動力?沉積物-水營養(yǎng)鹽轉(zhuǎn)化?生物生長代謝及種群競爭等過程耦合的EcoTaihu模型,模擬了各湖區(qū)營養(yǎng)鹽狀況,得到太湖最適宜換水周期為150～160d.還有的研究將換水周期與其他因子耦合分析其對于營養(yǎng)鹽釋放的影響,Gao和Zong[86]采用鹽度、水齡和營養(yǎng)鹽之間的雙變量回歸分析,提出在一定程度上水齡參數(shù)顯著決定了研究區(qū)內(nèi)營養(yǎng)鹽濃度的變化,明顯改變了氮磷釋放通量.

水體流態(tài)發(fā)生變化時(shí),新的形態(tài)特征會改變水體物質(zhì)分布特征.Xu等[87]在研究黃河河口營養(yǎng)鹽通量時(shí)發(fā)現(xiàn),河口水流形態(tài)由于水沙調(diào)節(jié)而從原來的單一羽流變?yōu)殡p羽流.羽流形態(tài)的改變使得營養(yǎng)鹽的混合更加均勻,也導(dǎo)致水體鹽度更低,水齡更小,從而影響水體氮磷的遷移輸送過程.除此之外水動力條件的改變還會影響水體混合過程,可能會使下層水體出現(xiàn)躍溫層現(xiàn)象,增大下層水體水層間粘滯力,從而抑制氮磷的大量釋放過程[88].除此之外,近年來還有研究對沉積物液化狀態(tài)進(jìn)行探究,提出液化狀態(tài)的沉積物比固結(jié)狀態(tài)時(shí)釋放出更多的磷進(jìn)入上覆水體中,Xu等[89]通過對比研究發(fā)現(xiàn)液化階段的總磷、總?cè)芙饬追謩e是固結(jié)階段的59倍和25倍;Zhang等[61]采用不同水頭差來模擬波浪擾動作用,提出液化狀態(tài)下總氮和總?cè)芙鈶B(tài)氮的釋放速率會隨著水動力的增強(qiáng)而增加,且顯著高于固結(jié)狀態(tài)情境.

在考慮其他因子對于氮磷釋放造成的影響時(shí),部分研究從單一變量角度出發(fā),而忽視了其他因素的耦合作用,從而使得研究結(jié)果存在較大偏差.因此,對氮磷釋放的影響需要進(jìn)一步探究多因子的耦合效應(yīng),以更準(zhǔn)確的為淺水湖泊的管理提供科學(xué)依據(jù).

3 結(jié)論與展望

水動力條件對淺水湖泊沉積物氮磷釋放的影響機(jī)制復(fù)雜,流速、水位、水體擾動等是影響沉積物氮磷釋放的主要水動力因子.水動力因子通過改變水體理化環(huán)境特征與沉積物特征而影響沉積物氮磷的釋放過程,還能通過影響水生生物、微生物等生長代謝過程而間接影響沉積物對氮磷的釋放.流速、擾動等因子均存在臨界值,高于或低于臨界值會對氮磷釋放產(chǎn)生不同的效果.同時(shí),各水動力條件對沉積物界面氮磷釋放的影響不是線性關(guān)系,而是多個(gè)因子相互耦合作用,互相影響和聯(lián)系,存在顯著的不確定性特征.

目前水動力條件對沉積物氮磷釋放影響的定量關(guān)系并不明確,并且多種效應(yīng)耦合下產(chǎn)生的綜合影響還有待研究,在研究方法、技術(shù)手段方面仍有待完善.針對當(dāng)前研究中存在的不足,提出以下幾點(diǎn)展望.

3.1 由于淺水湖泊生態(tài)系統(tǒng)是一個(gè)要素高度關(guān)聯(lián)和互饋的多過程非線性系統(tǒng),變化環(huán)境下影響沉積物-水界面氮磷釋放過程的環(huán)境因子較多,因子的作用強(qiáng)度及互饋關(guān)系具有不確定性.因此未來的研究需要結(jié)合野外監(jiān)測?室內(nèi)氮磷靜態(tài)模擬和釋放動力學(xué)實(shí)驗(yàn),厘清多因子共同作用下的釋放機(jī)制,更全面的預(yù)測評估水動力學(xué)條件改變帶來的影響.

3.2 當(dāng)前對沉積物釋放的水動力學(xué)過程的研究手段多集中室內(nèi)模擬實(shí)驗(yàn),對其動力學(xué)過程及機(jī)制研究還不夠深入,未來研究需要擴(kuò)展時(shí)空尺度,結(jié)合原位監(jiān)測、原型觀測,耦合水動力學(xué)模型、生態(tài)模型、系統(tǒng)動力學(xué)理論等多種研究手段,建立原位監(jiān)測-耦合模擬綜合研究體系,從優(yōu)化研究手段出發(fā)精確模擬水動力條件對于沉積物氮磷釋放的影響.

3.3 當(dāng)前研究主要針對沉積物氮磷的吸附-釋放過程,缺少釋放之后對生態(tài)系統(tǒng)所產(chǎn)生影響的系統(tǒng)性探究,因此需要闡明釋放后的氮磷歸趨、再分配行為,明確釋放出的氮磷被重新吸附回到沉淀物中的比例、被生物利用的比例以及對于生物產(chǎn)生的影響,以便進(jìn)一步明晰內(nèi)源氮磷釋放對生態(tài)系統(tǒng)產(chǎn)生的綜合影響.

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Effects of hydrodynamic conditions on nitrogen and phosphorus release from sediments in shallow lakes.

YU Jing-zhi1,2, WANG Xuan1,2*, CAI Jian-ying1,2, LIAO Zhen-mei1,2, LI Chun-hui2, LIU Qiang2

(1.State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, China；2.Key Laboratory for Water and Sediment Sciences of Ministry of Education, School of Environment, Beijing Normal University, Beijing 100875, China)., 2023,43(8)：4219～4228

Hydrodynamic conditions are the main natural factors affecting the release of nitrogen and phosphorus from sediments in shallow lakes. It is meaningful to study the effects of hydrodynamic conditions on the release of nitrogen and phosphorus in sediments and to grasp the migration and transformation of water nutrients for preventing lake endogenous pollution in lakes. Among the hydrodynamic factors, the flow velocity can change the exchange flux of nitrogen and phosphorus of the sediment-water interface by affecting the shear stress. Both water level and water disturbance affect the physical and chemical characteristics of surface sediments and the REDOX reaction process of sediment-water interface, and thus jointly affect the release of nitrogen and phosphorus through multiple coupling effects. The mechanism of major hydrodynamic factors such as velocity, water level, water disturbance on nitrogen and phosphorus release in shallow lake sediments was reviewed and the prospect was put forward: The mechanism of coupling effects of various hydrodynamic factors on nitrogen and phosphorus release, the construction of in-situ monitoring and coupling simulation research system based on multi-temporal and spatial scales, and the impact of endogenous nitrogen and phosphorus release on the ecosystem need to be further strengthened.

hydrodynamic condition；shallow lake；sediment；nitrogen and phosphorus release；prevention and control of endogenous

X524

A

1000-6923(2023)08-4219-10

余景芝(2000-),女,江西九江人,北京師范大學(xué)碩士研究生,主要研究方向?yàn)榱饔蛩h(huán)境過程.yujz2000@mail.bnu.edu.cn.

余景芝,王 烜,蔡劍英,等.水動力條件對淺水湖泊沉積物氮磷釋放的影響 [J]. 中國環(huán)境科學(xué), 2023,43(8):4219-4228.

Yu J Z, Wang X, Cai J Y, et al. Effects of hydrodynamic conditions on nitrogen and phosphorus release from sediments in shallow lakes [J]. China Environmental Science, 2023,43(8):4219-4228.

2023-01-17

國家自然科學(xué)基金資助項(xiàng)目(52270194,52070024)

* 責(zé)任作者, 教授, wangx@bnu.edu.cn