張武昌 陳 雪, 李海波, 豐美萍, 于 瑩, 趙 苑 肖 天

(1.中國科學(xué)院海洋研究所海洋生態(tài)與環(huán)境重點(diǎn)實(shí)驗(yàn)室 青島 266071;2.中國科學(xué)院大學(xué) 北京 100049)

20世紀(jì)70年代,表面熒光顯微鏡的應(yīng)用使得人們可以計(jì)數(shù)海水中的浮游細(xì)菌(Hobbieet al,1977;Sieburth,1978),隨后人們發(fā)現(xiàn)了104—105cell/mL的藍(lán)細(xì)菌(Johnsonet al,1979;Waterburyet al,1979)在水體中廣泛分布。由于體積微小,不能被橈足類攝食,這些微微級生物的生產(chǎn)力能否被傳遞到高層營養(yǎng)級直至魚類是海洋浮游生態(tài)學(xué)關(guān)注的一個問題,尋找他們的攝食者也成為隨后研究的熱點(diǎn)。

Azam 等(1983)設(shè)想細(xì)菌等生物被鞭毛蟲和纖毛蟲利用,提出了微食物環(huán)的概念。Sherr等(1988)把微微級自養(yǎng)生物加入到微食物環(huán),一起組成了微食物網(wǎng)。在這些設(shè)想的微型浮游生物營養(yǎng)關(guān)系中,浮游纖毛蟲與異養(yǎng)腰鞭毛蟲(dinoflagellates)、異養(yǎng)鞭毛蟲(flagellates)同屬于海洋生態(tài)系統(tǒng)微型浮游動物(microzooplankton),它們以微微級浮游生物為食物,同時也被中型浮游動物(mesozooplankton)攝食。

海洋浮游纖毛蟲是一類具纖毛的單細(xì)胞原生動物,屬于纖毛門(Ciliophora),多數(shù)屬于旋毛綱(Spirotrichea)下的寡毛亞綱(Oligotrichia)及環(huán)毛亞綱(Choreotrichia)。浮游纖毛蟲的粒級在 10—200μm之間,根據(jù)殼的有無分為具殼的砂殼纖毛蟲(900余種)和無殼纖毛蟲(約 150種),浮游纖毛蟲在自然水體中的豐度約為 1×103ind./L,在大多數(shù)情況下,無殼纖毛蟲的豐度大于砂殼纖毛蟲。在微食物網(wǎng)中,浮游纖毛蟲由于粒級比異養(yǎng)鞭毛蟲(2—20μm)大,易于被橈足類等中型浮游動物攝食,成為聯(lián)系海洋浮游微食物網(wǎng)和經(jīng)典食物鏈(大型浮游植物-橈足類等中型浮游動物-魚)的重要中間環(huán)節(jié)。

在20世紀(jì)80年代,纖毛蟲被上層營養(yǎng)級生物利用的研究受到廣泛關(guān)注,如Porter等(1979),Conover(1982),Sherr等(1986),Stoecker等(1990),Gifford(1991),Sanders等(1993)等。橈足類是中型浮游動物(＞200μm)的優(yōu)勢類群,大約占浮游動物生物量的80%(Verityet al,1996),其餌料通常包括藻類、浮游纖毛蟲等微型浮游生物,其中藻類等浮游植物被認(rèn)為是其主要餌料來源(Calbetet al,2005)。但是近來的研究表明,浮游植物很難提供橈足類自身生長代謝所需的全部營養(yǎng)物質(zhì)(Damet al,1994;Carlottiet al,1996;Petersonet al,1996),需要有其它的餌料補(bǔ)充,有實(shí)驗(yàn)表明在藻類和纖毛蟲共存的情況下橈足類優(yōu)先攝食纖毛蟲(Stoeckeret al,1985,1987)。因此大多數(shù)關(guān)于纖毛蟲被攝食的研究都是橈足類對纖毛蟲的攝食。

橈足類對纖毛蟲的攝食在實(shí)驗(yàn)室(Berket al,1977;Robertson,1983)和自然海區(qū)(Giffordet al,1988;Tiselius,1989)的研究均已開展。我國目前還尚未開展實(shí)驗(yàn)室內(nèi)橈足類攝食浮游纖毛蟲的研究,在自然海區(qū)的相關(guān)研究也很少(Huoet al,2008)。本文闡述了海洋浮游橈足類攝食纖毛蟲的研究歷史和現(xiàn)狀,為我國橈足類攝食纖毛蟲的研究提供借鑒。

1 浮游橈足類攝食纖毛蟲的早期證據(jù)

人們在無意中積累了一些纖毛蟲被橈足類攝食的證據(jù)。間接的證據(jù)包括:1.Sheldon等(1986)和Smetacek(1981)分別發(fā)現(xiàn)圍隔和自然海區(qū)中纖毛蟲的生物量與橈足類的生物量常出現(xiàn)負(fù)相關(guān)關(guān)系;2.Heinle等(1977)用碎屑產(chǎn)生的纖毛蟲(無種類信息)來培養(yǎng)橈足類Eurytemora affinis,產(chǎn)卵率和用浮游植物喂養(yǎng)的效果一樣,所以推斷橈足類是攝食纖毛蟲的。

直接的證據(jù)是在一些橈足類的腸道(Mullin,1966;Harding,1974)和糞便(Turneret al,1983;Turner,1984)中發(fā)現(xiàn)砂殼纖毛蟲的殼。Berk等(1977)用盾纖類海洋纖毛蟲尾絲蟲Uronema nigricans(直徑為 8.88—17.30μm)喂養(yǎng)橈足類E.affinis,餌料豐度為 4.17—7.44×103mL–1,E.affinis的清濾率為 0.068—0.934 mL/(ind.·h),攝食率為 717—5688ind.纖毛蟲/(ind.橈足類·h)。

由于無殼纖毛蟲很脆弱,容易解體,所以無法確認(rèn)培養(yǎng)過程中無殼纖毛蟲的消失是被攝食所引起的,還是由于其他原因造成無殼纖毛蟲解體引起的。直到Merrell等(1998)用 CMFDA(5-chloromethylfluorescein diacetate)染料為無殼纖毛蟲染色,喂養(yǎng)橈足類E.affinis,并用熒光顯微鏡觀察到橈足類體內(nèi)有熒光,確認(rèn)無殼纖毛蟲確實(shí)被橈足類E.affinis攝食。

2 橈足類攝食纖毛蟲的實(shí)驗(yàn)室內(nèi)研究

2.1 橈足類攝食纖毛蟲的清濾率和功能反應(yīng)

實(shí)驗(yàn)室內(nèi)研究是指將捕食者和室內(nèi)培養(yǎng)的纖毛蟲共同培養(yǎng),大多數(shù)研究采用一種捕食者攝食一種纖毛蟲,根據(jù)培養(yǎng)前后纖毛蟲豐度的變化估計(jì)纖毛蟲被攝食的情況。目前已開展的實(shí)驗(yàn)室內(nèi)用橈足類進(jìn)行培養(yǎng)的研究較少(表1),采用的橈足類種類有6種,其中研究最多的是Acartia tonsa;采用的纖毛蟲共計(jì)13種,其中7種砂殼類和6種無殼類,使用最多的是具溝急游蟲(Strombidium sulcatum)。這些研究表明橈足類對纖毛蟲的清濾率為0.1—58.1mL/(ind.·h)。

實(shí)驗(yàn)室內(nèi)的研究很少能估計(jì)出橈足類對纖毛蟲的攝食率隨纖毛蟲豐度的變化(即功能反應(yīng)functional response)情況。Frost(1972,1975)根據(jù)功能反應(yīng)曲線提出攝食閾值濃度(threshold concentration),即浮游橈足類的攝食率為零時(停止攝食)的餌料濃度。隨餌料濃度升高,攝食率增加,而清濾率保持不變,餌料濃度升高到一定程度后,攝食率不再增加,而清濾率降低(圖1),這時的濃度稱為攝食飽和濃度(saturation concentration)。為數(shù)不多的研究表明橈足類攝食纖毛蟲時,飽和濃度很高。例如,橈足類A.tonsa在纖毛蟲Tintinnopsis tubulosa的豐度達(dá) 2×103ind./L(Robertson,1983)、Favellasp.的豐度達(dá)到 3.4×103ind./L(Stoeckeret al,1985)時,其攝食率一直線性增加,說明橈足類的清濾率沒有變化。在纖毛蟲Eurytemora pectinis濃度達(dá)(6—9)×104ind./L時,橈足類A.hudsonica清濾率一直在增加(Turneret al,1983)。Saiz等(1995)得出了A.tonsa攝食S.sulcatum的功能反應(yīng)曲線,纖毛蟲的豐度達(dá)到 1×104ind./L后,橈足類的攝食率才不再增加。Kiorboe等(1996)發(fā)現(xiàn)S.sulcatum豐度在(0—2.5)×104ind./L的范圍內(nèi),A.tonsa的清濾率都沒有減少的趨勢。

同樣地,對橈足類攝食纖毛蟲的攝食閾值濃度的研究更少。在橈足類A.tonsa攝食纖毛蟲S.reticulatum的試驗(yàn)中,當(dāng)S.reticulatum的豐度大于900ind./L(生物量 1μg C/L)時,橈足類的清濾率保持較高,當(dāng)?shù)陀谶@個豐度數(shù)值,清濾率就開始逐漸降低;S.reticulatum豐度低于100ind./L(生物量約0.1μg C/L)時,橈足類完全停止攝食纖毛蟲(Jonssonet al,1990)。

2.2 浮游植物對橈足類攝食纖毛蟲的影響

A.tonsa攝食纖毛蟲時,水中的藻類會影響其對纖毛蟲的清濾率,隨著藻類豐度增加,橈足類對藻類的清濾率沒有顯著增加,但是對纖毛蟲的清濾率降低(Stoeckeret al,1985,1987)。Stoecker等(1987)推測藻類濃度過高可能會影響橈足類對纖毛蟲的探測。

與浮游植物相比,橈足類傾向于攝食纖毛蟲(Stoeckeret al,1985)。Jonsson等(1990)認(rèn)為纖毛蟲的粒級并不能解釋這種傾向性,因?yàn)榇蠖鄶?shù)纖毛蟲都比較大,橈足類能達(dá)到 100%過濾截留效率。橈足類對纖毛蟲有機(jī)械感知(Wiadnyanaet al,1989)和化學(xué)感知(Stoeckeret al,1987),可以用來解釋這種傾向性。

表1 實(shí)驗(yàn)室內(nèi)橈足類攝食纖毛蟲的清濾率Tab.1 Clearance rate of copepods on ciliates in laboratory

圖1 浮游橈足類的攝食率(a)和清濾率(b)隨纖毛蟲濃度變化趨勢(Saiz et al,1995)Fig.1 The response of planktonic copepods grazing rate and clearance rate along with ciliates concentration(adapted from Saiz et al,1995)

2.3 橈足類攝食纖毛蟲的方式和纖毛蟲的應(yīng)對策略

不同橈足類對纖毛蟲的攝食方式不同。A.tonsa采取伏擊攝食(ambush feeding)(Jonssonet al,1990;Kiorboeet al,1996;Caparroyet al,1998;Broglioet al,2001)的策略。Jonsson等(1990)首先發(fā)現(xiàn)了A.tonsa的伏擊攝食模式,在伏擊攝食時,橈足類感知和捕獲單個纖毛蟲,A.tonsa自由下降,纖毛蟲在距離A.tonsa大觸角0.1—0.7mm的時候就能被A.tonsa感知到,而不用真正觸碰到大觸角,然后A.tonsa開始伏擊攝食,運(yùn)用第二觸角等器官截獲纖毛蟲。A.tonsa完成攝食動作需時僅0.1s。Caparroy等(1998)發(fā)現(xiàn)橈足類Centropages typicus通過游泳來過濾海水中纖毛蟲,C.typicus的游泳速度(3.5mm/s)比纖毛蟲S.sulcatum的游泳速度(0.2mm/s)大得多,S.sulcatum不能逃離C.typicus的攝食流場。A.tonsa體型較小,攝食流場速度低,所以采取伏擊攝食的方式。

有的纖毛蟲(Mesodinium rubrum,Strobilidium velox,Halteria grandinella)有彈跳行為,即快速游泳(Jonssonet al,1990;Gilbert,1994)。在自然情況下,S.velox在一分鐘內(nèi)彈跳1.7—3.6次,在游泳時間中占的比例為0.8%,H.grandinella在一分鐘內(nèi)彈跳8次,在游泳時間中占比為1.0%。在與橈足類接觸之前,這些纖毛蟲能感受到流體力學(xué)的變化,引發(fā)彈跳行為,逃避攝食(Jakobsen,2001;Wuet al,2010)。逃避引發(fā)的彈跳行為和自然狀況下彈跳行為的速度和距離沒有明顯變化(Gilbert,1994)。對于伏擊攝食纖毛蟲的橈足類A.tonsa,自由下降時的流場信號很弱,不足以引發(fā)纖毛蟲的彈跳行為(Kiorboeet al,1999)。所以用彈跳行為逃避攝食者也不是對所有捕食者都有效。彈跳行為本身可以幫助纖毛蟲逃避攝食者,但是由于彈跳對水流造成很大的擾動,也增加了被攝食者發(fā)現(xiàn)的可能性。

由于橈足類和纖毛蟲行為模式的不同,不同種類的橈足類和纖毛蟲的攝食關(guān)系也不同(Gismervik,2006)。例如Tortanus setacaudatus能有效攝食砂殼纖毛蟲Favella panamensis,但是卻不攝食T.tubulosa。這種差異并不能完全用大顆粒可以被優(yōu)先攝食來解釋。Robertson(1983)認(rèn)為這是由于T.setacaudatus是伏擊攝食者,砂殼纖毛蟲T.tubulosa比F.panamensis游泳速度慢,因此T.tubulosa對水流的擾動小,與橈足類的相遇幾率也小。

因?yàn)槔w毛蟲是運(yùn)動的,橈足類與纖毛蟲的接觸幾率(contactment rate)也要大于橈足類與浮游植物的接觸幾率。在這種情況下,纖毛蟲被攝食的情況還與擾動有關(guān)(Saizet al,1995;Kiorboeet al,1996;Caparroyet al,1998)。小尺度的擾動對橈足類攝食纖毛蟲有影響。在一定的擾動強(qiáng)度下(能量耗散率ε=10–3—10–2cm2/s3),A.tonsa對纖毛蟲S.sulcatum的清濾率可以達(dá)到平靜水體的4倍,但是擾動強(qiáng)度增大(ε=10–1—10cm2/s3)時,清濾率減少,不過仍比平靜水體中的清濾率高(Saizet al,1995)。橈足類Centropages typicus在ε=2.9×10–2—3×10–1cm2/s3時,對纖毛蟲S.sulcatum的清濾率可以達(dá)到平靜水體的4倍,但是擾動強(qiáng)度增大(ε=4.4cm2/s3)時,清濾率減少到平靜水體中的水平(Caparroyet al,1998)。

砂殼纖毛蟲被攝食的方式目前沒有定論。Robertson(1983)在培養(yǎng)中沒有發(fā)現(xiàn)破損的殼,加上在橈足類糞便中發(fā)現(xiàn)完整的殼,所以認(rèn)為橈足類吞食整個的砂殼纖毛蟲。Turner等(1983)在加入橈足類的培養(yǎng)瓶(實(shí)驗(yàn)瓶)中發(fā)現(xiàn)空殼的比例高于對照瓶(沒有加橈足類),而且對照瓶中的空殼沒有變形,而實(shí)驗(yàn)瓶中的空殼很多都被弄皺變形,所以認(rèn)為不能排除有些砂殼纖毛蟲的肉體被攝食,而殼沒有被攝食的可能性,也有可能當(dāng)橈足類觸碰到砂殼纖毛蟲時,砂殼纖毛蟲肉體從殼中逃出。

3 海上原位培養(yǎng)研究橈足類對纖毛蟲的清濾率

3.1 原位培養(yǎng)研究橈足類攝食纖毛蟲的方法

海上原位培養(yǎng)研究纖毛蟲被攝食的情況是指自然海區(qū)的纖毛蟲群落被攝食的情況。研究橈足類攝食的方法大多是進(jìn)行橈足類培養(yǎng)實(shí)驗(yàn),將自然海水加入培養(yǎng)瓶中,沒有添加橈足類的培養(yǎng)瓶作為對照瓶,添加橈足類的培養(yǎng)瓶為實(shí)驗(yàn)瓶。將培養(yǎng)瓶放在模擬自然環(huán)境(主要是溫度、擾動和光照)中培養(yǎng)一段時間,和對照瓶纖毛蟲豐度相比較,實(shí)驗(yàn)瓶的減少量即被視為橈足類的攝食量。研究橈足類群體在自然狀況下對餌料中纖毛蟲的攝食開始于 Gifford等(1988),其后有約30個相似的研究開展起來(表2)。這些研究的海區(qū)主要是離岸很近的港灣或海邊實(shí)驗(yàn)室,在開闊水域的研究資料很少(表2,圖2)。

大多數(shù)培養(yǎng)的培養(yǎng)體積都沒有超過 3L,一般在2L左右,個體小的橈足類使用的培養(yǎng)瓶小些。培養(yǎng)時間大多為24h,最長為78h。大多數(shù)培養(yǎng)中,用來培養(yǎng)的海水用過濾的方法排除其他攝食者,用來過濾的篩絹孔徑最小為 20μm,最大為 400μm。有的培養(yǎng)中為了消除橈足類代謝營養(yǎng)鹽造成的影響,在培養(yǎng)瓶中加富了營養(yǎng)鹽(表2)。溫度的模擬主要有恒溫室和流水控溫培養(yǎng),擾動的模擬為浮游動物輪。光照方面要考慮的問題主要是模擬自然的光照周期和避免強(qiáng)光照射。

圖2 自然海區(qū)現(xiàn)場的浮游纖毛蟲被橈足類攝食的研究地點(diǎn)分布圖Fig.2 Locations of in situ incubation stations for planktonic ciliates ingested by copepods

Roman等(1980)研究了用培養(yǎng)瓶進(jìn)行現(xiàn)場橈足類攝食實(shí)驗(yàn)的牢籠效應(yīng)(containment effect)或稱瓶子效應(yīng)(bottle effect),那時的橈足類攝食實(shí)驗(yàn)主要是研究對浮游植物的攝食,因此Roman等(1980)的結(jié)果顯示培養(yǎng)過程中橈足類會釋放營養(yǎng)鹽,從而導(dǎo)致培養(yǎng)瓶中浮游植物生長快一些,導(dǎo)致低估對浮游植物的攝食。在橈足類攝食纖毛蟲的培養(yǎng)實(shí)驗(yàn)中,牢籠效應(yīng)也可能導(dǎo)致培養(yǎng)瓶中纖毛蟲的生長也快一些,但是沒有研究對此進(jìn)行評估,所有的研究中都沒有考慮牢籠效應(yīng)。

現(xiàn)場培養(yǎng)的方法也存在一些問題。這個方法的前提是假設(shè)纖毛蟲的減少是由橈足類造成的,但是這個假設(shè)是有問題的,一些其他因素也可能導(dǎo)致纖毛蟲的減少,如遇到氣泡或過度擾動(Gifford,1985,1993)。Gifford等(2007)的培養(yǎng)中,海水沒有過濾的主要目的就是盡量減少對纖毛蟲的傷害。培養(yǎng)過程中無殼纖毛蟲的死亡也是個問題,例如沒有添加橈足類的對照瓶中無殼纖毛蟲的死亡率達(dá)80%(Tiselius,1989)。

大多數(shù)海上添加橈足類培養(yǎng)是對幾個種進(jìn)行培養(yǎng),對橈足類群體進(jìn)行的研究較少。在海上培養(yǎng)的橈足類種類(表2)有:Acartia屬(A.bifilosa,A.clause,A.hongi,A.longiremis,A.tonsa),Calanus屬(C.finmarchicus,C.helgolandicus,C.pacificus,C.sinicus,C.glacialis,C.hyperboreus),Centropages屬(C.abdominalis,C.cherchiae,C.hamatus,C.typicus),Eucalanus屬(E.bungii,E.pileatus),Neocalanus屬(N.cristatus,N.plumchrus,N.flemingeri),Oithona屬(O.davisae,O.similis)和Temora屬(T.longicornis)。橈足類六足幼體的攝食也得到研究(Stoeckeret al,1987;Dolan,1991;Fessendenet al,1994;Merrellet al,1998)。

3.2 原位培養(yǎng)研究橈足類攝食纖毛蟲的結(jié)果

橈足類對纖毛蟲的清濾率最大為C.hyperboreus的 162.5mL/(ind.·h),其次為C.helgolandicus的 98 mL/(ind.·h)和N.cristatus的 70.8mL/(ind.·h),其他大多數(shù)清濾率低于 40mL/(ind.·h)(表2)。橈足類體長(或體重)增加,清濾率增加,但是單位體重的清濾率卻隨體長增加而減小,即個體小的橈足類攝食比較活躍(Levinsenet al,2000)。

有的培養(yǎng)實(shí)驗(yàn)把纖毛蟲分成不同的粒級,例如Levinsen等(2000)、Liu等(2005a)和 Fileman等(2007)把纖毛蟲分為 20μm 以上和 20μm 以下兩個體長組,Vincent(2001)分為40μm以上和以下,Koski等(2002)分為30μm以上和以下。這些培養(yǎng)結(jié)果表明,橈足類對纖毛蟲的清濾率隨纖毛蟲尺寸的增加而增加(Tiselius,1989;Nejstgaardet al,1997)。

目前幾乎所有的實(shí)驗(yàn)結(jié)果(Giffordet al,1988;Atkinson,1996;Castellaniet al,2005)都表明,橈足類對纖毛蟲的清濾率大于對浮游植物的清濾率,說明橈足類傾向于選擇攝食纖毛蟲。這種選擇性攝食的原因可能是纖毛蟲的粒級比較大,懸浮攝食的橈足類對大顆粒的清濾率較大,也可能是伏擊攝食的橈足類的行為具有選擇性。與攝食藻類相比,橈足類攝食纖毛蟲有很多好處,纖毛蟲的粒級較大,大多數(shù)的橈足類等浮游動物傾向于攝食粒級較大的顆粒。纖毛蟲C:N比例比藻類低,蛋白質(zhì)含量較高(Kiorboeet al,1985;Stoeckeret al,1990),纖毛蟲還可以合成橈足類不能合成的不飽和脂肪酸(Sanderset al,1993;Breteleret al,1999,2004),從而有利于橈足類的生長。

Calbet等(2005)對已有的橈足類攝食纖毛蟲資料進(jìn)行了總結(jié),估計(jì)了纖毛蟲在橈足類餌料中的比例,當(dāng)浮游植物的生物量為＜50μg C/L、50—500μg C/L、＞500μg C/L時,纖毛蟲在橈足類餌料中的平均貢獻(xiàn)分別為 49%、25%、22%,即隨著浮游植物生物量的增加,纖毛蟲在橈足類餌料中的貢獻(xiàn)減少。

4 自然海區(qū)橈足類對纖毛蟲的攝食壓力

根據(jù)自然海區(qū)橈足類的豐度和橈足類對纖毛蟲的清濾率可以估計(jì)橈足類群體對纖毛蟲的攝食壓力。根據(jù)現(xiàn)場測定的清濾率和橈足類豐度進(jìn)行估計(jì)的研究不多(Dolan,1991;Fessendenet al,1994;Atkinson,1996;Atkinsonet al,1996;Lonsdaleet al,2000;Batten,2001;Calbetet al,2002;Zeldis,2002;Broglioet al,2004),分布的海區(qū)也主要是海灣和離岸很近的海區(qū)(圖2),除 Dolan(1991)和 Fessenden 等(1994)報(bào)導(dǎo)橈足類群體攝食纖毛蟲生物量的攝食壓力為 45%—200%外,其他研究結(jié)果均比較一致:橈足類群體對纖毛蟲生物量的攝食壓力大約為每天5%(表3)。Dolan(1991)研究發(fā)現(xiàn),在切薩皮克灣的表層橈足類(主要是幼體)的豐度高達(dá)80ind./L,這是近岸富營養(yǎng)水體中的情況,因此橈足類對纖毛蟲的攝食壓力較高。

表3 不同海區(qū)現(xiàn)場培養(yǎng)得出的纖毛蟲每天被中型浮游動物(主要是橈足類)攝食的壓力Tab.3 Grazing pressure of mesozooplankton(mainly copepods)on ciliates by in situ incubation

此外,Fileman等(2007)的研究結(jié)果表明,英國Celtic海的橈足類群體對原生動物 PZP(protozooplankton,主要為纖毛蟲和異養(yǎng)甲藻)的攝食壓力與水華暴發(fā)有關(guān),水華期和非水華期的攝食壓力分別為 2%—10%和12%—17%。Fileman等(2010)研究發(fā)現(xiàn),英吉利海峽橈足類Calanus helgolandicus和Acartia clausi的種群對 PZP的攝食壓力在 2002年為 3%—32%(平均17%),2003年為 0.1%—15%(平均 3%),其研究結(jié)果表明橈足類對PZP的攝食壓力與調(diào)查年份也有關(guān)。

根據(jù)橈足類的攝食能力在一定程度上可以估計(jì)攝食壓力與橈足類豐度的關(guān)系。Stoecker等(1985)計(jì)算Acartiaspp.的豐度為3—4ind./L時,就可以控制砂殼纖毛蟲Favellasp.的生長。Calbet等(2005)總結(jié)了橈足類攝食纖毛蟲的攝食率為 0.016—0.029μg C纖毛蟲/(μg C橈足類·d),要每天攝食纖毛蟲生物量的 5%,需要橈足類(每只體重為10μg C)的豐度為0.7ind./L。

Nielsen等(1994)使用文獻(xiàn)中的清濾率和橈足類的豐度估計(jì)橈足類對＞50μm ESD(equivalent spherical diameter,等效球體直徑)纖毛蟲的攝食壓力與纖毛蟲的生長率相抵,但是小個體纖毛蟲的生長率要大于橈足類群體對其攝食壓力。

5 結(jié)語

自20世紀(jì)80年代以來,人們開始在實(shí)驗(yàn)室內(nèi)和海上現(xiàn)場研究浮游橈足類對纖毛蟲的攝食。實(shí)驗(yàn)室內(nèi)有限的研究發(fā)現(xiàn):橈足類對纖毛蟲的攝食存在功能反應(yīng);相比于浮游植物,橈足類傾向于攝食纖毛蟲;不同橈足類對纖毛蟲有不同的攝食方式,有的纖毛蟲有逃避橈足類攝食的行為。自然海區(qū)橈足類對纖毛蟲的攝食研究主要在近岸海區(qū),所用的方法較一致,多是在自然海區(qū)采集纖毛蟲群體的樣品,添加幾個種類的橈足類,模擬現(xiàn)場環(huán)境進(jìn)行對照培養(yǎng)實(shí)驗(yàn)。目前的研究方法還存在一些問題,例如沒有考慮牢籠效應(yīng)、氣泡或過度擾動對纖毛蟲的影響以及培養(yǎng)過程中纖毛蟲的自然死亡等,這些都是在今后的研究中亟待解決的。原位培養(yǎng)研究橈足類攝食纖毛蟲,得出了橈足類對纖毛蟲的清濾率與橈足類和纖毛蟲的粒級有關(guān);橈足類對纖毛蟲的清濾率大于對浮游植物的清濾率,這個結(jié)果與室內(nèi)研究一致。根據(jù)自然海區(qū)橈足類的豐度和橈足類對纖毛蟲的清濾率可以估計(jì)橈足類群體對纖毛蟲的攝食壓力,得出的結(jié)果較一致,大約為每天 5%。目前已經(jīng)開展研究的海區(qū)多是近岸海區(qū),對大洋的研究較少,需要加強(qiáng)對大洋中橈足類攝食纖毛蟲的研究。

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