覃雪波,孫紅文 ,彭士濤,戴明新
(1.交通運(yùn)輸部天津水運(yùn)工程科學(xué)研究所,天津 300456;2.南開(kāi)大學(xué)環(huán)境科學(xué)與工程學(xué)院,天津 300071;3.天津自然博物館,天津 300074)
水-沉積物界面是聯(lián)系底泥與上覆水的橋梁,控制有機(jī)質(zhì)、營(yíng)養(yǎng)鹽和各種污染物的遷移轉(zhuǎn)化過(guò)程[1]。這些過(guò)程不僅受到沉積物理化性質(zhì)(如有機(jī)質(zhì)含量、滲透性)的影響,同時(shí)也受到沉積物生物性質(zhì)(主要指生活于其中的各種生物)的影響[2-3]。底棲動(dòng)物棲息于沉積物中,它們的各種活動(dòng),如攝食、避敵、排泄等行為,被稱之為生物擾動(dòng)[4]。生物擾動(dòng)是構(gòu)成河口、近岸和淺海水域關(guān)鍵生態(tài)過(guò)程的水層與底棲系統(tǒng)耦合過(guò)程的重要環(huán)節(jié)和樞紐[5]。由于生物擾動(dòng)造成的沉積物變化不明顯,過(guò)去經(jīng)常受到忽視[6]。例如,傳統(tǒng)的觀點(diǎn)認(rèn)為濱海沉積物結(jié)構(gòu)的變化主要是由于海浪等物理作用[7]。直到近十年,生物擾動(dòng)才得到重新認(rèn)識(shí),被認(rèn)為是一個(gè)非常重要的生態(tài)過(guò)程[8]。一個(gè)小尺度(μm—m)的生物擾動(dòng)同樣是一個(gè)關(guān)鍵過(guò)程,可以改變大尺度(如50 m—100 km)的沉積結(jié)構(gòu)[9]。與此同時(shí),生物擾動(dòng)還影響淺海物質(zhì)交換及生態(tài)系統(tǒng)功能[10-11]。生物擾動(dòng)因此被稱為“生態(tài)系統(tǒng)工程師”[12]。
沉積物是水生生態(tài)系統(tǒng)的重要組成部分,蓄積各種污染物。由于生物擾動(dòng)能改變沉積物的結(jié)構(gòu)和性質(zhì),因而對(duì)其中的污染物產(chǎn)生重要影響[13]。近年來(lái),生物擾動(dòng)對(duì)沉積物中污染物的環(huán)境行為的影響受到廣泛研究,本文綜述了該領(lǐng)域若干研究進(jìn)展,為進(jìn)一步了解生物擾動(dòng)對(duì)污染物的生物地球化學(xué)循環(huán)提供基礎(chǔ)資料。
生物擾動(dòng)是一個(gè)古老概念,早在達(dá)爾文時(shí)代已被提出來(lái),他注意到土壤被蚯蚓擾動(dòng)后,土壤上層的物質(zhì)被帶到下層,下層的被帶到上層,結(jié)果造成了土壤的均質(zhì)化[14]。隨后,在海洋中也發(fā)現(xiàn)類似的現(xiàn)象,如在海蚯蚓對(duì)海沙的擾動(dòng)[15]、海參對(duì)沉積物的擾動(dòng)[16]。隨著研究的深入,生物擾動(dòng)概念得到進(jìn)一步拓展,即生物對(duì)土壤和沉積物的再建過(guò)程,不僅包括陸地上,也包括水體中(圖1)[14]。由于目前生物擾動(dòng)的研究多見(jiàn)于水環(huán)境中,因此,生物擾動(dòng)一般是指底棲動(dòng)物由于攝食、建管、筑穴、爬行、避敵、分泌、排泄和遷移等行為造成沉積物結(jié)構(gòu)和性質(zhì)的改變,進(jìn)而影響到沉積物中顆粒態(tài)和溶解態(tài)物質(zhì)遷移轉(zhuǎn)化的過(guò)程。
圖1 各種動(dòng)物的生物擾動(dòng)Fig.1 Bioturbation from a range of animal
生物擾動(dòng)作用類型可分為沉積物顆粒重建和洞穴通水兩大類,大類下又分為6個(gè)亞類(圖2)[17]。顆粒重建主要指底棲動(dòng)物各種行為造成的沉積物顆粒移動(dòng),洞穴通水是指底棲動(dòng)物為了呼吸和覓食而對(duì)洞穴中水和上覆水進(jìn)行交換[18]。兩種擾動(dòng)類型對(duì)沉積物產(chǎn)生不同的作用(表 1)[19-20]。
表1 生物擾動(dòng)類型及其對(duì)沉積物的影響Table 1 Bioturbation modes and their effect on the sediment matrix
圖2 生物擾動(dòng)作用類型Fig.2 The categories of bioturbation
1.3.1 底棲動(dòng)物種類
不同底棲動(dòng)物有不同的生活習(xí)性,由此產(chǎn)生不同的沉積物混合模式,導(dǎo)致不同的顆粒位移,從而對(duì)沉積物產(chǎn)生不同的擾動(dòng)強(qiáng)度。首先是覓食和攝食習(xí)性。底棲動(dòng)物的覓食和攝食的活動(dòng)控制了沉積物顆粒的位移[21]。如攝食和排糞主要發(fā)生在沉積物表層附近,因此沉積物混合模式以水平方向?yàn)橹鳎?2-23];其次是筑穴行為,主要取決于巢穴的深度,產(chǎn)生的擾動(dòng)強(qiáng)度在垂直空間上存在差異。如甲殼、雙殼類動(dòng)物的巢穴主要分布沉積物淺層,而環(huán)節(jié)動(dòng)物和其它類似蠕蟲(chóng)的類群則趨向于分布在深層沉積物,因而前者主要對(duì)沉積物淺層產(chǎn)生擾動(dòng),后者則對(duì)深層產(chǎn)生擾動(dòng)[24]。此外,底棲動(dòng)物的體積、生物量等也影響其對(duì)沉積物的擾動(dòng)強(qiáng)度[6]。實(shí)際上,動(dòng)物種類的影響可以歸納為功能群[6]。目前,可將作為生物擾動(dòng)者的底棲動(dòng)物分為5個(gè)功能群(圖3),即散布者、向下輸送者、向上輸送者、交換者和廊道散布者[25-26]。這些功能群對(duì)沉積物產(chǎn)生不同的擾動(dòng)效果。
1.3.2 底棲動(dòng)物密度
底棲動(dòng)物的密度對(duì)生物擾動(dòng)強(qiáng)度有顯著影響,兩者間呈正相關(guān)[27]。例如,高密度沙蠶(Hediste diversicolor)對(duì)沉積物的擾動(dòng)強(qiáng)度明顯高于低密度[28]。
1.3.3 沉積物有機(jī)質(zhì)含量
圖3 生物擾動(dòng)者功能群Fig.3 Functional groups of bioturbators
底棲動(dòng)物的生物擾動(dòng)強(qiáng)度受到食物供給的影響[29]。由于沉食底棲動(dòng)物主要以沉積物為食,因此沉積物中的有機(jī)質(zhì)含量直接影響了其生物擾動(dòng)強(qiáng)度[30]。在有機(jī)質(zhì)含量低的沉積物中,底棲動(dòng)物為了生存不得不攝食大量的沉積物[24],由此對(duì)沉積物產(chǎn)生強(qiáng)烈的擾動(dòng)作用。相反,在有機(jī)質(zhì)含量高的沉積物中,底棲動(dòng)物只需要取食少量的沉積物即可滿足營(yíng)養(yǎng)需求,降低了沉積物處理速度,生物擾動(dòng)強(qiáng)度隨之降低[24]。
1.3.4 沉積速率、水深和粒徑
通常,沉積速率高,為沉食的底棲動(dòng)物帶來(lái)的食物就多,從而能夠供養(yǎng)更多的底棲動(dòng)物,表現(xiàn)為較強(qiáng)的擾動(dòng)強(qiáng)度[31]。水深對(duì)生物擾動(dòng)的影響實(shí)質(zhì)上與營(yíng)養(yǎng)供給相關(guān)。水淺,營(yíng)養(yǎng)物質(zhì)高,生物擾動(dòng)強(qiáng)度大;水深,營(yíng)養(yǎng)物質(zhì)少,生物擾動(dòng)強(qiáng)度小。如在富營(yíng)養(yǎng)的近岸海域,底棲動(dòng)物對(duì)沉積物的擾動(dòng)深度可以達(dá)到100cm以下[32];而在貧營(yíng)養(yǎng)的深海,底棲動(dòng)物的擾動(dòng)深度最深只能到20cm左右[33]。沉積物粒徑同樣對(duì)生物擾動(dòng)強(qiáng)度產(chǎn)生影響。由于沉食底棲動(dòng)物對(duì)細(xì)顆粒沉積物有優(yōu)先攝食和向下輸送的習(xí)性,因此生物擾動(dòng)強(qiáng)度表現(xiàn)明顯的粒徑相關(guān)[34],如在加拿的Fundy灣,生物擾動(dòng)強(qiáng)度隨沉積物粒徑增大而減弱[35]。
在生物擾動(dòng)的作用下,沉積物的結(jié)構(gòu)和性質(zhì)發(fā)生了改變。由此影響蓄積于沉積物中的污染物的各種環(huán)境行為。其中一個(gè)非常重要的影響是促進(jìn)沉積物中的污染物向水體釋放(圖4)[36];與此同時(shí),沉積物中的污染物還會(huì)被生物擾動(dòng)者所富集;此外,還引起沉積物中含氧量和微生物等變化,從而間接影響污染物的環(huán)境行為。因此,生物擾動(dòng)使得蓄積于沉積物中的污染物的環(huán)境行為更加復(fù)雜。
圖4 生物擾動(dòng)導(dǎo)致沉積物向水體釋放污染物Fig.4 Processes of chemical release from a sediment bed contaminant source
2.1.1 氮
在生物擾動(dòng)作用下,沉積物一個(gè)重要的變化特征是含氧量增加[37-38]。例如,在搖蚊(Chironomid larvae)擾動(dòng)沉積物的實(shí)驗(yàn)中,對(duì)照組沉積物含氧量隨深度而減少,在4mm處含氧量降低至0,而生物擾動(dòng)處理的沉積物,在7mm處,含氧量仍可達(dá)到5mg/L[39]。沉積物含氧量的變化對(duì)其中氮的硝化與反硝化作用產(chǎn)生重要影響[40]。因此,生物擾動(dòng)影響硝化與反硝化作用。通常,生物擾動(dòng)顯著提高硝化作用[41]。這種促進(jìn)作用受到多種因子影響。Pelegri和Blackburn發(fā)現(xiàn),在較低密度時(shí)(20000個(gè)/m2),正顫蚓(Tubifex tubifex)的生物擾動(dòng)提高氮的硝化,而在高密度時(shí)(70000個(gè)/m2)卻降低氮的硝化[42],表明底棲動(dòng)物密度影響硝化作用。Nogaro和Mermillod-Blondin的研究表明,顫蚓的生物擾動(dòng)對(duì)不同來(lái)源的沉積物中的氮的硝化作用影響完全不同,對(duì)于來(lái)自大學(xué)校園的沉積物,生物擾動(dòng)顯著提高了氮的硝化,而對(duì)來(lái)自工業(yè)區(qū)的沉積物,氮的硝化并沒(méi)有受到生物擾動(dòng)的影響[43],表明沉積物的性質(zhì)也影響到氮的硝化。當(dāng)沉積物中含有硫化物時(shí),由于生物擾動(dòng)促進(jìn)硫化物釋放,這些硫化物抑制微生物生長(zhǎng),從而降低了硝化作用[38]??梢?jiàn),生物擾動(dòng)對(duì)硝化用影響不同,受多種因素影響。生物擾動(dòng)也能影響沉積物中氮的反硝化作用。搖蚊的生物擾動(dòng)能將總反硝化速率從(0.76 ±0.34)mmol N m-2d-1提高到(5.50 ±1.30)mmol N m-2d-1,極大提高了反硝作用[39]。最近的研究表,生物擾動(dòng)對(duì)不同深度的沉積物反硝化作用影響不同,在海姑蝦(Neotrypaea californiensis)的擾動(dòng)下,沉積物深度在13cm以上的反硝化速度低于沒(méi)有生物擾動(dòng)處理,而在13 cm以下卻是生物擾動(dòng)處理高于無(wú)生物擾動(dòng)處理[40]。這一方面表明生物擾動(dòng)促進(jìn)深層沉積物中的氮發(fā)生反硝化作用,另一方面也說(shuō)明生物擾動(dòng)影響反硝化作用也受到其它因素的影響。
生物擾動(dòng)促進(jìn)沉積物向水體釋放氮營(yíng)養(yǎng)鹽。Fanjul等在河口濕地發(fā)現(xiàn),在有無(wú)穴居蟹Neohelice granulate的沉積物間隙水中的氮營(yíng)養(yǎng)鹽含量存在明顯差異,無(wú)論是硝酸鹽還是氨氮,都呈現(xiàn)有蟹居住的洞穴>無(wú)蟹居住洞穴>無(wú)蟹分布區(qū)的分布特征[44]。表明穴居蟹的生物擾動(dòng)促進(jìn)沉積物向水體釋放氮營(yíng)養(yǎng)鹽。Mermillod-Blondin的室內(nèi)微宇宙研究證明了這一點(diǎn),顫蚓的生物擾動(dòng)提高氨氮從沉積物向水體的釋放達(dá)到200%[45];其它底棲動(dòng)物,如環(huán)節(jié)動(dòng)物Marenzelleria sp.的生物擾動(dòng)也顯著提高沉積物中的氨氮向水體釋放量[46]。生物擾動(dòng)造成沉積物中氨氮釋放主要通過(guò)生物淋洗(Bioirrigation)作用完成[40]。例如在淺海灣,76%的氨氮從沉積物向水體的釋放歸結(jié)于生物淋洗作用[47]。生物擾動(dòng)促進(jìn)沉積物中的氮向水體釋放的主要原因是生物擾動(dòng)造成了沉積物的搬運(yùn)和混合,使沉積物吸附的營(yíng)養(yǎng)鹽得以釋放,加快間隙水中物質(zhì)的擴(kuò)散速率和溶解速率,從而增加氮在沉積物-水界面上的交換通量[48]。
2.1.2 磷
磷在沉積物中的環(huán)境行為也受到生物擾動(dòng)的影響。一方面,生物擾動(dòng)促進(jìn)沉積物中溶解性磷(SRP)向水體釋放。Mermillod-Blondin的研究表明,顫蚓的生物擾動(dòng)提高了190%的SRP從沉積物向水體釋放[45]。Swan等發(fā)現(xiàn)在溶解氧較低的水平下,Neanthes succinea的生物擾動(dòng)提高SRP的釋放速率達(dá)到60%—70%[49]。然而,有些底棲動(dòng)物的生物擾動(dòng)卻抑制了SRP向水體釋放[50]。如水絲蚓的生物擾動(dòng)不但沒(méi)有促進(jìn)沉積物中SRP向水體釋放,相反,還抑制了沉積物中SRP的釋放[51]。造成這種現(xiàn)象的原因是沉積物中的一些礦物質(zhì)在生物擾動(dòng)的作用下增強(qiáng)對(duì)間隙水SRP吸附所導(dǎo)致[50]。間隙水SRP濃度的降低會(huì)減小SRP向水體擴(kuò)散的濃度梯度,從而減小SRP的釋放通量,甚至產(chǎn)生沉積物對(duì)水體中SRP的吸附[52-53]。由此可見(jiàn),生物擾動(dòng)對(duì)沉積物中SRP的促進(jìn)/抑制釋放作用不僅取決于生物擾動(dòng)者,還與沉積物特點(diǎn)相關(guān)。
另一方面,生物擾動(dòng)還能改變沉積物中磷的化學(xué)形態(tài)。河蜆的生物擾動(dòng)增加了沉積物中鐵結(jié)合態(tài)磷含量[54]。這是由于生物擾動(dòng)提高沉積物中的氧含量,氧化了間隙水中Fe2+,氧化反應(yīng)所生成的水合鐵氧化物對(duì)SRP具有良好的吸附作用[50],從而形成鐵結(jié)合態(tài)磷。
此外,生物擾動(dòng)還促進(jìn)有機(jī)磷降解。Hietanen等的研究表明,加入底棲動(dòng)物12d后,沉積物中的有機(jī)磷發(fā)生了降解[46]。由于有機(jī)磷包括多種物質(zhì),生物擾動(dòng)對(duì)有機(jī)磷不同組分影響不同。目前,磷脂和DNA在生物擾動(dòng)下能發(fā)生快速分解或轉(zhuǎn)化得到證實(shí)[55]。對(duì)于其它有機(jī)磷,生物擾動(dòng)是否能促進(jìn)其降解并不清楚。
2.2.1 改變重金屬在沉積物中的分布
Benoit等發(fā)現(xiàn)美國(guó)波士頓港的沉積物中甲基汞的垂直分布受片腳類動(dòng)物生物擾動(dòng)的影響,隨著生物擾動(dòng)強(qiáng)度的增大,甲基汞含量最高值點(diǎn)向下遷移[56]。Klerks等對(duì)海姑蝦的生物擾動(dòng)對(duì)美國(guó)Tampa灣沉積物中鋅與鎘的分布研究表明,在有海姑蝦活動(dòng)區(qū)域的沉積物中鋅與鎘的含量是沒(méi)有海姑蝦活動(dòng)區(qū)域的3倍,說(shuō)明生物擾動(dòng)顯著改變了重金屬在沉積物中的水平分布[57]。室內(nèi)的實(shí)驗(yàn)研究表明,顫蚓的生物擾動(dòng)能促進(jìn)水體中的Pb向沉積物深層擴(kuò)散[58]??梢?jiàn),生物擾動(dòng)不僅改變重金屬在沉積物中的垂直分布,也改變其在沉積物中的水平分布。
2.2.2 促進(jìn)重金屬?gòu)某练e物向水體釋放
Simpson等對(duì)悉尼港的5個(gè)河口沉積物鋅釋放研究表明,5周后,沒(méi)有生物的對(duì)照組中鋅的釋放速率為27 mg m-2d-1,而有生物的釋放速率為71 mg m-2d-1,說(shuō)明生物擾動(dòng)顯著增強(qiáng)沉積物向水體釋放重金屬[59]。Ciutata等研究表明,顫蚓生物擾動(dòng)促進(jìn)沉積物中的鎘向水體釋放,但以顆粒態(tài)為主,與生物擾動(dòng)導(dǎo)致沉積物的再懸浮相關(guān)[60]。同樣的結(jié)果見(jiàn)路永正和閻的研究,他們發(fā)現(xiàn)顫蚓的生物擾動(dòng)顯著提高了上覆水中顆粒態(tài)鎘的濃度[58]。生物擾動(dòng)還通過(guò)改變沉積物的理化性質(zhì),從而促進(jìn)沉積物向水體釋放重金屬。例如,生物擾動(dòng)增加沉積物的氧含量,增強(qiáng)生物擾動(dòng)強(qiáng)度,促進(jìn)沉積物中甲基汞向水體遷移[61-62]。
2.2.3 改變重金屬化學(xué)形態(tài)
在水環(huán)境中,硫酸鹽還原菌是控制甲基汞生成的重要因子[63]。由于生物擾動(dòng)能為間隙水提供氧化劑,降低二價(jià)硫離子的生成,從而減少了甲基汞形成[56]。然而,Nogaro等的研究卻得到相反的結(jié)論,他們發(fā)現(xiàn)生物擾動(dòng)增強(qiáng)沉積物中微生物的有氧吸吸,促進(jìn)甲基汞形成[64]。兩個(gè)截然不同的結(jié)論,表明生物擾動(dòng)對(duì)沉積物中甲基汞的影響錯(cuò)綜復(fù)雜。
鎘是沉積物中常的污染物,在不同的條件下具有不同的化學(xué)形態(tài)。由于生物擾動(dòng)改變沉積物的pH值、氧化還原電位等,因而對(duì)蓄積于沉積物中的鎘的化學(xué)形態(tài)產(chǎn)生重要影響。顫蚓的生物擾動(dòng)提高鎘從沉積物內(nèi)部微孔向溶液中擴(kuò)散的速率,進(jìn)而提高沉積物中鎘的遷移能力,促進(jìn)鎘從鐵錳氧化物結(jié)合態(tài)向可交換態(tài)轉(zhuǎn)移[65]。由于生物擾動(dòng)增大沉積物比表面積,不斷更新沉積物顆粒上的吸附點(diǎn)位[60],有利于碳酸鹽礦物顆粒對(duì)鎘離子的再吸附,形成碳酸鹽結(jié)合態(tài)鎘;與此同時(shí),呼吸作用產(chǎn)生的CO2能增加其中CO-32濃度,也有利于增加碳酸鹽結(jié)合態(tài)鎘[65]??梢?jiàn),生物擾動(dòng)增強(qiáng)沉積物碳酸鹽礦物對(duì)鎘的吸附能力,對(duì)于控制水-沉積物界面鎘的交換具有重要作用。
2.3.1 促進(jìn)向水體釋放
生物擾動(dòng)促進(jìn)沉積物中的疏水性有機(jī)污染物(Hydrophobic organic contaminants,HOCs)向水體釋放已成為共識(shí)[66]。Schaanning等發(fā)現(xiàn),大型底棲動(dòng)物的生物擾動(dòng)可以使挪威奧斯陸港沉積物的PAHs、PCBs和DDT等污染物分別以每天243、19.6 pmol/m2和 13.6 pmol/m2的速度向水體釋放[67]。Menone等野外調(diào)查了穴居蟹Chasmagnathus granulate的生物擾動(dòng)對(duì)阿根廷的Bahía Blanca河口沉積物中的有機(jī)氯農(nóng)藥影響,結(jié)果表明,生物擾動(dòng)顯著提高沉積物向水體釋放有機(jī)氯農(nóng)藥[68-69]。Granberg等研究發(fā)現(xiàn),生物擾動(dòng)促進(jìn)波羅的海沉積物中的PCBs向水體釋放[70]。類似的結(jié)果在Josefsson等的研究中也得到,他們發(fā)現(xiàn),在利用活性炭對(duì)受二苯并二惡英和二苯并呋喃(PCDD/Fs)污染的沉積物進(jìn)行原位修復(fù)時(shí),由于底棲動(dòng)物的存在,它們的生物擾動(dòng)促進(jìn)沉積物中的PCDD/Fs向水體遷移,影響了修復(fù)效果[71],因此,在對(duì)沉積物進(jìn)行修復(fù)時(shí),應(yīng)該考慮底棲動(dòng)物生物擾動(dòng)的影響[72-73]。研究表明,底棲動(dòng)物的排泄物中含有許多溶解性有機(jī)質(zhì)(DOC)及小分子的物質(zhì),提高間隙水中的DOC含量,由于DOC對(duì)有機(jī)物具有很強(qiáng)的吸附能力,由此引發(fā)PAHs在沉積物-間隙水的重新分配,從而提高了水相中的 PAHs含量[74]。即,生物擾動(dòng)打破了有機(jī)污染物在沉積物-水界面的動(dòng)力學(xué)平衡,使沉積物中的 HOCs發(fā)生解吸[74-76]。其它一些 HOCs,如多溴聯(lián)苯醚(PBDEs)[77]、醫(yī)藥品和個(gè)人護(hù)理品(PPCPs)[75],生物擾動(dòng)也能促進(jìn)它們向水體釋放。然而,HOCs的釋放還受到其化學(xué)性質(zhì)影響。通常,疏水性高的有機(jī)污染物,不易發(fā)生解吸。例如,底棲動(dòng)物 Chironomus dilutus和 Hyalella azteca的生物擾動(dòng)并不能顯著提高乙炔基雌二醇(EE2)從沉積物向水體釋放,與EE2具有較高的疏水性特點(diǎn)相關(guān)[75]。此外,沉積物的特點(diǎn)也影響生物擾動(dòng)促進(jìn)HOCs釋放的效果。如沉積物中的黑炭由于具有較強(qiáng)的吸附能力,抑制生物擾動(dòng)促進(jìn)沉積物中 HOCs解吸[78]。
傳統(tǒng)的觀點(diǎn)認(rèn)為,生物擾動(dòng)主要是增加水體中顆粒態(tài)污染物[70]。筆者的研究得到類似的結(jié)論,在天津厚蟹(Helice tientsinensis)的生物擾動(dòng)下,水體中PAHs的顆粒態(tài)是其溶解態(tài)的4倍,表明生物擾動(dòng)以促進(jìn)顆粒態(tài)污染物釋放為主[12,60]。生物擾動(dòng)主要促進(jìn)顆粒態(tài)HOCs釋放也得到Josefsson等的研究結(jié)果支持[77]。然而,近年的研究表明,生物擾動(dòng)也增加水體中溶解態(tài)的污染物[70,79]。Granberg等研究發(fā)現(xiàn),Marenzelleria neglecta的生物擾動(dòng)釋放的溶解態(tài)PCBs要高于其顆粒態(tài)一個(gè)數(shù)量級(jí),表明生物擾動(dòng)促進(jìn)沉積物PCBs釋放以溶解態(tài)為主[70]。這種差異可能與動(dòng)物種類、密度、沉積物性質(zhì)以及水化學(xué)條件相關(guān)[13]。
生物擾動(dòng)無(wú)論是促進(jìn)溶解態(tài)還是顆粒態(tài)HOCs釋放,都造成二次污染。特別是溶解態(tài)的HOCs,生物擾動(dòng)提高了它們的生物有效性[76]。如夾雜帶絲蚓(Lumbriculus variegatus)的生物擾動(dòng)顯著提高了美洲鉤蝦(Hyalella azteca)對(duì) PAHs的富集[80]。通常認(rèn)為顆粒態(tài)HOCs不易被生物利用,但最近的研究表明,顆粒態(tài)的多溴聯(lián)苯醚也能被鯉魚(yú)(Cyprinus carpio)所富集[81]。由此可見(jiàn),生物擾動(dòng)促進(jìn)沉積物的HOCs釋放,這些HOCs能被水生生物所利用,最終通過(guò)食物鏈傳遞到人類,對(duì)人類健康造成潛在的風(fēng)險(xiǎn)。
2.3.2 加強(qiáng)富集和代謝
由于底棲動(dòng)物幾乎終生生活于沉積物中,因此通過(guò)體表吸收和攝食沉積物,使得HOCs更容易富集于其體內(nèi)。一項(xiàng)對(duì)日本Ariake海濱的生物體PCBs和PAHs的調(diào)查研究表明,PCBs在比目魚(yú)體內(nèi)的含量為3.6 ng/g(濕重)、蛤?yàn)?8 ng/g、彈涂魚(yú)為1700 ng/g脂肪;PAHs在沙躅屬動(dòng)物體內(nèi)含量為24 ng/g(濕重)、牡蠣為 6.3 ng/g、蛤?yàn)?6.3 ng/g、蟹為4.2ng/g、彈涂魚(yú)為 2.9 ng/g[82]。在渤海灣的北塘河口附近的天津厚蟹,總 PAHs含量達(dá)到(8816±2885)ng/g 脂肪[74]。水絲蚓[81]、沙蠶[83]對(duì)多溴聯(lián)苯醚也具有不同的富集能力。
某些底棲動(dòng)物對(duì)HOCs還具有代謝能力。穴居蟹[68-69]、藍(lán)蟹[84]對(duì)有機(jī)機(jī)氯農(nóng)藥具有代謝能力。一些環(huán)節(jié)動(dòng)物(如 Clymenella torquata,Nereis virens,N.succinea,Nephtys incisa,Spio setosa,Cirriformia grandis)、雙殼類(如 Macoma balthica,Mya arenaria,Mulinia lateralis)以及一些片腳類動(dòng)物(如Ampelisca abdita,Leptocheirus plumulosus)可以代謝 PAHs[85]。此外,一些蝦類可以代謝PCBs[86]。沙蠶對(duì)多溴聯(lián)苯醚也具有一定的代謝能力[83]。
2.3.3 提高生物降解
底棲動(dòng)物的生物擾動(dòng)可以間歇性地向深層的無(wú)氧區(qū)輸送氧、直接或者間接影響微生物菌群變化,使顆粒物縱向遷移[87-88];同時(shí),底棲動(dòng)物的排泄物有助于 HOCs從沉積物中解吸[74-76],促進(jìn) HOCs與微生物接觸;此外,底棲動(dòng)物消化液中的助溶劑對(duì)生物降解有很好的促進(jìn)作用[89];這些因素都有利于促進(jìn)沉積物中HOCs發(fā)生生物降解。Granberg等對(duì)Amphiura filiformis和N.diversicolor的生物擾動(dòng)對(duì)微生物降解芘的研究發(fā)現(xiàn),相對(duì)于深層的沉積物,在洞穴中的沉積物中的芘的含量明顯減少,表明生物擾動(dòng)促進(jìn)微生物降解芘[90];Timmermann等通過(guò)室內(nèi)研究發(fā)現(xiàn),有生物擾動(dòng)的裝置中,芘的降解提高了180%—200%倍[91]。物擾動(dòng)促進(jìn)沉積物中HOCs生物降解是由于生物擾動(dòng)增加沉積物中的含氧量,從而提高了微生物降解 HOCs的能力[89]。生物擾動(dòng)促進(jìn)HOCs生物降解取決于生物擾動(dòng)者和污染物在沉積物中的深度。通常,降解主要發(fā)生在沉積物淺層,與生物擾動(dòng)者主要活動(dòng)于該區(qū)域相關(guān)[89]。
生物擾動(dòng)是沉積物中污染物動(dòng)態(tài)變化的一個(gè)關(guān)鍵因子。目前,生物擾動(dòng)對(duì)沉積物中的污染物的環(huán)境行為的影響開(kāi)展了一系列的研究,了解一些影響機(jī)制。底棲動(dòng)物通過(guò)生物泵、再懸浮、分泌物排泄等過(guò)程影響沉積物中的污染物并造成釋放[92]。生物擾動(dòng)影響沉積物中的污染物環(huán)境行為受控于多種環(huán)境因子,包括溫度[93]、有機(jī)質(zhì)[42]、水動(dòng)力[10]、污染等[94]。特別是污染的影響,最近的研究表明,污染影響生物擾動(dòng)者的行為,進(jìn)而影響沉積物的地球化學(xué)循環(huán)[95]??梢?jiàn),生物擾動(dòng)和沉積物中的污染物相互關(guān)系錯(cuò)綜復(fù)雜,兩者之間的關(guān)系需要進(jìn)一步研究。
數(shù)學(xué)模型是了解生物擾動(dòng)和沉積物中污染物相互關(guān)系的重要研究手段。在生物擾動(dòng)對(duì)沉積物結(jié)構(gòu)的影響研究中,許多模型如箱式模型、平流模型、信號(hào)處理模型、擴(kuò)散-反應(yīng)模型和顆粒物輸運(yùn)擴(kuò)散模型等已被廣泛運(yùn)用[96],取得良好效果。但在生物擾動(dòng)對(duì)沉積物中污染物的影響研究中,鮮有運(yùn)用數(shù)學(xué)模型,這方面研究急需加強(qiáng)。
總體而言,生物擾動(dòng)對(duì)沉積物中在氮、磷和重金屬的環(huán)境行為影響研究相對(duì)較多,而對(duì)于HOCs研究較少,特別是生物擾動(dòng)對(duì)HOCs的生物降解研究更少。因此,今后有必要開(kāi)展這方面的研究,同時(shí)應(yīng)該對(duì)生物擾動(dòng)對(duì)促進(jìn)沉積物中污染物的二次釋放進(jìn)行風(fēng)險(xiǎn)評(píng)估,正解認(rèn)識(shí)其潛在的生態(tài)風(fēng)險(xiǎn)。
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