張武昌 ,李海波,豐美萍,陳 雪,于 瑩,趙 苑,肖 天
(1.中國(guó)科學(xué)院海洋研究所,海洋生態(tài)與環(huán)境科學(xué)重點(diǎn)實(shí)驗(yàn)室,青島 266071;2.中國(guó)科學(xué)院大學(xué),北京 100049)
海洋浮游纖毛蟲(chóng)是一類(lèi)具纖毛并營(yíng)浮游生活的單細(xì)胞原生動(dòng)物,屬于纖毛門(mén)(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(chóng)的粒級(jí)在10—200 μm之間,根據(jù)殼的有無(wú)分為具殼的砂殼類(lèi)(900余種)和無(wú)殼類(lèi)(約150種),它們?cè)谧匀凰w中的豐度約為1000個(gè)/L,在大多數(shù)情況下,無(wú)殼纖毛蟲(chóng)的豐度較高。
浮游纖毛蟲(chóng)與異養(yǎng)腰鞭毛蟲(chóng)(dinoflagellates)和鞭毛蟲(chóng)(flagellates)同屬于海洋浮游生態(tài)系統(tǒng)粒級(jí)劃分的微型浮游動(dòng)物(microzooplankton),是海洋浮游食物網(wǎng)的重要組成部分[1],它們攝食pico和nano級(jí)浮游生物,同時(shí)被中型浮游動(dòng)物(mesozooplankton)攝食,所以浮游纖毛蟲(chóng)是聯(lián)系微食物網(wǎng)和經(jīng)典食物鏈的重要環(huán)節(jié)。同浮游植物相比,纖毛蟲(chóng)具有較高的營(yíng)養(yǎng)價(jià)值,顆粒粒徑較大,在橈足類(lèi)餌料中占的比例較大。
浮游纖毛蟲(chóng)生長(zhǎng)率是估計(jì)纖毛蟲(chóng)生產(chǎn)力的重要參數(shù)。同浮游植物初級(jí)生產(chǎn)和中型浮游動(dòng)物次級(jí)生產(chǎn)是浮游生態(tài)學(xué)研究的重要內(nèi)容一樣,浮游纖毛蟲(chóng)的次級(jí)生產(chǎn)也是海洋浮游生態(tài)學(xué)研究的重要內(nèi)容。浮游纖毛蟲(chóng)生長(zhǎng)率是理解其種群動(dòng)力學(xué)的重要參數(shù)。浮游纖毛蟲(chóng)的豐度和生物量是纖毛蟲(chóng)生長(zhǎng)和死亡共同作用的結(jié)果,無(wú)法反映出種群動(dòng)態(tài)變化的機(jī)制。生長(zhǎng)率是評(píng)價(jià)纖毛蟲(chóng)種群受上行控制(餌料不足)和下行控制(攝食壓力過(guò)大)影響的重要參考指標(biāo)。
目前國(guó)外在實(shí)驗(yàn)室內(nèi)(自 Heinbokel[2]開(kāi)始)和海上現(xiàn)場(chǎng)(自Capriulo[3]開(kāi)始)都對(duì)浮游纖毛蟲(chóng)的生長(zhǎng)率進(jìn)行了研究,國(guó)內(nèi)在這方面研究還比較少,實(shí)驗(yàn)室內(nèi)對(duì)浮游纖毛蟲(chóng)生長(zhǎng)率的研究只有類(lèi)彥立等[4],海上現(xiàn)場(chǎng)生長(zhǎng)率的研究還未涉及。目前已經(jīng)在我國(guó)海區(qū)積累了一些浮游纖毛蟲(chóng)豐度和生物量的資料,纖毛蟲(chóng)現(xiàn)場(chǎng)生長(zhǎng)率的研究是我國(guó)微型浮游動(dòng)物生態(tài)學(xué)亟需進(jìn)行的內(nèi)容之一,本文綜述了海洋浮游纖毛蟲(chóng)生長(zhǎng)率和生產(chǎn)力的研究歷史和現(xiàn)狀,為我國(guó)浮游纖毛蟲(chóng)生長(zhǎng)的研究提供借鑒。
測(cè)定纖毛蟲(chóng)生長(zhǎng)率的方法,都是將一定豐度的纖毛蟲(chóng)放入含有餌料(不同種類(lèi)、種類(lèi)組合或自然餌料)的培養(yǎng)液中培養(yǎng)一段時(shí)間t(d),估計(jì)培養(yǎng)前(C0)和培養(yǎng)后(Ct)培養(yǎng)液中纖毛蟲(chóng)的豐度,假設(shè)纖毛蟲(chóng)的生長(zhǎng)為指數(shù)生長(zhǎng),生長(zhǎng)率(μ,d-1或 h-1)按照公式μ=ln(Ct/C0)/t計(jì)算得出。
上述纖毛蟲(chóng)的生長(zhǎng)率是根據(jù)纖毛蟲(chóng)豐度變化來(lái)計(jì)算,這樣做的前提是假設(shè)所有纖毛蟲(chóng)的體積是一樣的。但是纖毛蟲(chóng)的體積在不同的營(yíng)養(yǎng)狀況下會(huì)有變動(dòng)。Montagnes 和 Lessard[5]報(bào) 道 Strombidinopsis multitauris在餌料充足時(shí)的體積是饑餓時(shí)體積的2倍。而 Fenchel和 Jonsson[6]報(bào)道 Strombidium sulcatum的體積變化可達(dá) 8倍。Ohman和 Snyder[7]報(bào)道Strombidium sp.在指數(shù)生長(zhǎng)期的細(xì)胞體積小一些,在穩(wěn)定期細(xì)胞體積增大,變化達(dá)2倍。兼性自養(yǎng)的種類(lèi)Laboea strobila和Strombidium conicum在不同的光照下體積變化可達(dá)2倍[8-9]。溫度也可能影響細(xì)胞的 體 積,例 如 Montagnes 和 Lessard[5]報(bào) 道Strombidinopsis multitauris體積在10℃最大,在5—10℃細(xì)胞體積隨溫度升高而變大,在10—22℃細(xì)胞體積隨溫度升高而變小。因此纖毛蟲(chóng)生長(zhǎng)率的這個(gè)計(jì)算方法也有不足之處。
對(duì)于特定的纖毛蟲(chóng)種類(lèi),在一定的溫度下,其生長(zhǎng)率與餌料濃度的關(guān)系叫數(shù)值反應(yīng)。Heinbokel[2]首先研究浮游寡毛類(lèi)纖毛蟲(chóng)的數(shù)值反應(yīng),發(fā)現(xiàn)纖毛蟲(chóng)的生長(zhǎng)率隨餌料濃度的變化成雙曲線(xiàn)型,在餌料濃度低時(shí)不生長(zhǎng),生長(zhǎng)率隨餌料濃度增加而增大,當(dāng)餌料濃度達(dá)到和超過(guò)一定的數(shù)值時(shí),隨著餌料濃度的增加,生長(zhǎng)率穩(wěn)定在一定的水平或者有所下降,這時(shí)的生長(zhǎng)率為內(nèi)稟生長(zhǎng)率(μmax)(圖1)。
圖1 纖毛蟲(chóng)Strobilidium spiralis生長(zhǎng)率(μ)隨餌料藻豐度的變化[10]Fig.1 Therelationship betweengrowth rate(μ)of Strobilidium spiralis and algal cells[10]
Montagnes[11]把特定溫度下纖毛蟲(chóng)的生長(zhǎng)率與餌料濃度的關(guān)系表示為:
式中,P是餌料的濃度,x'是生長(zhǎng)閾值濃度,即生長(zhǎng)率為零(不生長(zhǎng))時(shí)的餌料濃度,k是常數(shù)。當(dāng)餌料濃度為x'+k時(shí),纖毛蟲(chóng)的生長(zhǎng)率為μmax/2。
不同纖毛蟲(chóng)的生長(zhǎng)閾值濃度不同,從最低6 μg C/L到327 μg C/L(表1)。浮游纖毛蟲(chóng)在餌料濃度低于生長(zhǎng)閾值濃度時(shí)會(huì)死亡,但是死亡的時(shí)間各不相同。Fenchel和 Jonsson[6]發(fā)現(xiàn) Strombidium sulcatum饑餓3.5 d 后有 90%的個(gè)體死亡,Montagnes[11]發(fā)現(xiàn)Strobilidium和Strombidium屬的種類(lèi)饑餓1—2 d內(nèi)就會(huì)死亡。
除Strombidium siculum外,Strombidium屬的內(nèi)稟生長(zhǎng)率范圍較窄(0.8—1.1 d-1)。除了S.capitatum,大多數(shù)種類(lèi)生長(zhǎng)率達(dá)到μmax/2的餌料濃度低于50 μg C/L。Strobilidium屬的種類(lèi)生長(zhǎng)率高一點(diǎn),但是也更分散,為 0.7—1.7 d-1,除了 Strobilidium spiralis外,大多數(shù)種類(lèi)生長(zhǎng)率達(dá)到μmax/2的餌料濃度高于100 μg C/L。Strobilidium spiralis的餌料濃度閾值較低,為 42—61 μg C/L[10,12-13]。Strombidinopsis 屬的數(shù)據(jù)不多,既有高的閾值,也有低的閾值,達(dá)到μmax/2的餌料濃度為34—177 μg C/L。個(gè)體較小的混合營(yíng)養(yǎng)種Strombidium vestitum內(nèi)稟生長(zhǎng)率最高[10]。
表1 實(shí)驗(yàn)室內(nèi)培養(yǎng)得出的無(wú)殼纖毛蟲(chóng)的內(nèi)稟生長(zhǎng)率(μmax)Table 1 Intrinsic growth rates of aloricate ciliates in laboratory(μmax)
續(xù)表
在相同的溫度下,對(duì)于不同的餌料,纖毛蟲(chóng)的內(nèi)稟生長(zhǎng)率也不相同。因此內(nèi)稟生長(zhǎng)率常被用來(lái)指示餌料的營(yíng)養(yǎng)價(jià)值、適口性[14-16]。
對(duì)于同一種類(lèi)而言,纖毛蟲(chóng)的內(nèi)稟生長(zhǎng)率與溫度有關(guān)。在餌料適合時(shí),不同溫度下纖毛蟲(chóng)的內(nèi)稟生長(zhǎng)率隨溫度升高而升高,但是溫度過(guò)高或過(guò)低對(duì)纖 毛 蟲(chóng) 也 有 抑 制 作 用 (圖 2)[20]。 例 如Strombidinopsis multiauris在5℃會(huì)死亡,8.5—15℃時(shí)內(nèi)稟生長(zhǎng)率會(huì)從0.14—0.77 d-1線(xiàn)性增長(zhǎng),15—19℃時(shí)內(nèi)稟生長(zhǎng)率(約0.85 d-1)不再增加,22℃時(shí)內(nèi) 稟 生 長(zhǎng) 率 會(huì) 降 低 (0.13 d-1)[5]。Strombidium sulcatum在18℃時(shí)幾乎不生長(zhǎng),在25℃時(shí)世代時(shí)間為 5.7 h,30 ℃時(shí)世代時(shí)間為 2 h[4]。
在實(shí)驗(yàn)室內(nèi)測(cè)定過(guò)生長(zhǎng)率的砂殼纖毛蟲(chóng)有12種(表2),溫度范圍為8—25℃,各種的內(nèi)稟生長(zhǎng)率變化范圍為0.45—2.7 d-1。在實(shí)驗(yàn)室內(nèi)測(cè)定過(guò)生長(zhǎng)率的無(wú)殼纖毛蟲(chóng)有22種(表1),溫度范圍為5.5—30℃,除具溝急游蟲(chóng)的內(nèi)稟生長(zhǎng)率可達(dá)8.3 d-1外,其它各種的內(nèi)稟生長(zhǎng)率變化范圍為0.1—3.5 d-1。
在內(nèi)稟生長(zhǎng)率隨溫度升高而升高的溫度范圍內(nèi),生長(zhǎng)率隨溫度變化的關(guān)系可以用Q10來(lái)表示:
式中,μ2和μ1是溫度T2和T1時(shí)的內(nèi)稟生長(zhǎng)率。
圖2 纖毛蟲(chóng)內(nèi)稟生長(zhǎng)率與溫度的關(guān)系圖[30]Fig.2 Generalized description of the relationship between ciliates intrinsic growth rate and temperature[30]
Montagnes等[30]分析了原生生物的內(nèi)稟生長(zhǎng)率和溫度的關(guān)系,認(rèn)為線(xiàn)性關(guān)系比Q10關(guān)系要好,平均的斜率為0.07。目前根據(jù)5種寡毛類(lèi)纖毛蟲(chóng)的內(nèi)稟生長(zhǎng)率隨溫度升高而變化的資料計(jì)算得出內(nèi)稟生長(zhǎng)率隨溫度變化的斜率為 0.052—0.077(表 3)。Gismervik[10]利用 0.07 d-1℃-1的系數(shù)對(duì)寡毛類(lèi)幾個(gè)種的生長(zhǎng)率進(jìn)行轉(zhuǎn)換成15℃的內(nèi)稟生長(zhǎng)率進(jìn)行比較。
表2 實(shí)驗(yàn)室得出的砂殼纖毛蟲(chóng)的內(nèi)稟生長(zhǎng)率(μmax)Table 2 Intrinsic growth rates of tintinnids in laboratory(μmax)
Pedersen和Hansen[18]使用室內(nèi)實(shí)驗(yàn)檢查pH值升高對(duì)幾種纖毛蟲(chóng)(Favella ehrenbergii,Rimostrombidium caudatum,R.veniliae)生長(zhǎng)的影響,當(dāng)pH值升高到8.8—8.9時(shí),生長(zhǎng)率開(kāi)始降低,pH值上升到8.9—9.0時(shí),纖毛蟲(chóng)停止生長(zhǎng),當(dāng)pH值上升到9.2—9.3時(shí),纖毛蟲(chóng)快速死亡。
對(duì)于不同種類(lèi)的無(wú)殼纖毛蟲(chóng),其內(nèi)稟生長(zhǎng)率與個(gè)體大小有關(guān),一般情況下,體長(zhǎng)大的種類(lèi)其內(nèi)稟生長(zhǎng)率較低。
但是也有報(bào)道體型較大的種類(lèi)內(nèi)稟生長(zhǎng)率可能相當(dāng)于或超過(guò)體型小的種類(lèi)[10-11,42]。Montagnes[11]在實(shí)驗(yàn)室內(nèi)測(cè)定幾種寡毛類(lèi)纖毛蟲(chóng)的內(nèi)稟生長(zhǎng)率,個(gè)體最大的種類(lèi)(Strobilidium neptuni)內(nèi)稟生長(zhǎng)率最高。實(shí)驗(yàn)室內(nèi)測(cè)定幾種寡毛類(lèi)纖毛蟲(chóng)的內(nèi)稟生長(zhǎng)率數(shù)據(jù)表明,個(gè)體最大的異養(yǎng)纖毛蟲(chóng) Strobilidium spiralis的內(nèi)稟生長(zhǎng)率較高[10,12,43]。
Hansen 等[44]總結(jié)體長(zhǎng)范圍為 2—2000 μm 的浮游原生動(dòng)物和浮游后生動(dòng)物的生長(zhǎng)率,發(fā)現(xiàn)隨體長(zhǎng)增加,生長(zhǎng)率下降。但是對(duì)于浮游無(wú)殼纖毛蟲(chóng)這一類(lèi)群而言,這個(gè)規(guī)律并不成立。
內(nèi)稟生長(zhǎng)率與體長(zhǎng)和溫度的關(guān)系可以用函數(shù)來(lái)表達(dá)。利用文獻(xiàn)中實(shí)驗(yàn)室培養(yǎng)的結(jié)果,Montagnes等[45]得出纖毛蟲(chóng)內(nèi)稟生長(zhǎng)率(μmax)與溫度(T)和個(gè)體體積大小(V)的關(guān)系為:
Müller和 Geller[23]則根據(jù)不同類(lèi)群的資料總結(jié)了如下公式:
Perez 等[46]根 據(jù) Nielsen 和 Kiorboe[47]的 公式ln(μmax)=a ln(T)+b ln(V)+c進(jìn)一步給出了混合營(yíng)養(yǎng)和異養(yǎng)纖毛蟲(chóng)內(nèi)稟生長(zhǎng)率與溫度及體長(zhǎng)間的不同系數(shù)。
表3 浮游寡毛類(lèi)纖毛蟲(chóng)生長(zhǎng)率和溫度的關(guān)系Table 3 The relationship between growth rates of planktonic oligotrichous ciliates and temperature
Montagnes[11]認(rèn)為現(xiàn)場(chǎng)的餌料水平無(wú)法判斷是不是內(nèi)稟生長(zhǎng)率,所以只總結(jié)了當(dāng)時(shí)已有實(shí)驗(yàn)室內(nèi)的生長(zhǎng)率資料,認(rèn)為 Müller和 Geller[23]的公式最適合用來(lái)預(yù)測(cè)Strombilidium和Strombidium兩個(gè)屬的內(nèi)稟生長(zhǎng)率。
目前有關(guān)不同營(yíng)養(yǎng)類(lèi)型纖毛蟲(chóng)的內(nèi)稟生長(zhǎng)率有沒(méi)有差異尚無(wú)定論。有的無(wú)殼纖毛蟲(chóng)是專(zhuān)性營(yíng)養(yǎng)的,如Strombidium conicum需要進(jìn)行光合作用,在黑暗中培養(yǎng)4 d以后的生長(zhǎng)率會(huì)下降。所以在進(jìn)行不同種類(lèi)的生長(zhǎng)率實(shí)驗(yàn)時(shí)要注意它們的食性[10]。Perez等[46]根據(jù)已有的文獻(xiàn)中不同種類(lèi)的生長(zhǎng)率,用Montagnes[11]的公式總結(jié)了混合營(yíng)養(yǎng)的種類(lèi)和異養(yǎng)種類(lèi)在內(nèi)稟生長(zhǎng)率上的差異,發(fā)現(xiàn)混合營(yíng)養(yǎng)種類(lèi)的生長(zhǎng)率比相同體積的異養(yǎng)種類(lèi)要低0.5 d-1,而Dolan和Perez[48]在海水的營(yíng)養(yǎng)鹽加富培養(yǎng)中發(fā)現(xiàn)自養(yǎng)和異養(yǎng)的纖毛蟲(chóng)的生長(zhǎng)率都能夠隨餌料濃度的增加迅速增加到 1.2 d-1,Gismervik[10]也沒(méi)有發(fā)現(xiàn)自養(yǎng)和異養(yǎng)纖毛蟲(chóng)在生長(zhǎng)率方面的差異。
自然海區(qū)纖毛蟲(chóng)生長(zhǎng)率是自然界中的纖毛蟲(chóng)種群或群體在自然狀況下(自然餌料、溫度、光照和搖動(dòng))的生長(zhǎng)率。要估計(jì)自然海區(qū)的生長(zhǎng)率,可以用上述實(shí)驗(yàn)室的研究結(jié)果外推到自然界中[49-50],但是室內(nèi)培養(yǎng)的結(jié)果與自然界的情況有很大不同,自然環(huán)境對(duì)生長(zhǎng)率的影響在實(shí)驗(yàn)室內(nèi)是很難模擬的[51],所以這種方法不是理想的方法。
其他方法有通過(guò)檢測(cè)自然海區(qū)纖毛蟲(chóng)的分裂細(xì)胞頻率(FDC,frequency-of-dividing-cells)來(lái)估計(jì)纖毛蟲(chóng)的生長(zhǎng)率[52],也可以根據(jù)攝食率和毛生長(zhǎng)效率(GGE)來(lái)估計(jì)生長(zhǎng)率,例如Sherr等[53]估計(jì)如果西北地中海中粒級(jí)較小的纖毛蟲(chóng)(<15 μm)只吃細(xì)菌的話(huà),生長(zhǎng)率可以達(dá)到 0.5 d-1[47]。
估計(jì)自然海區(qū)纖毛蟲(chóng)生長(zhǎng)率最好的方法是現(xiàn)場(chǎng)培養(yǎng),首先使用這一方法的是Stoecker等[35],他們?cè)?982年春天在Massachusetts的海邊測(cè)量了幾種纖毛蟲(chóng)的生長(zhǎng)率。此后這種方法逐漸推廣開(kāi)來(lái)。
現(xiàn)場(chǎng)培養(yǎng)纖毛蟲(chóng)估計(jì)其生長(zhǎng)率的方法主要是海水分粒級(jí)培養(yǎng)法[51,54],即將海水用一定孔徑的篩絹過(guò)濾,假設(shè)過(guò)濾去除了纖毛蟲(chóng)所有的捕食者,將海水裝入一定體積的培養(yǎng)瓶中在模擬的現(xiàn)場(chǎng)條件(水溫、光照、搖動(dòng))下培養(yǎng)一段時(shí)間,根據(jù)培養(yǎng)前后纖毛蟲(chóng)豐度的變化計(jì)算生長(zhǎng)率。
有關(guān)野外現(xiàn)場(chǎng)培養(yǎng)容器的選擇上,人們經(jīng)過(guò)了很長(zhǎng)時(shí)間的探索。為了避免培養(yǎng)過(guò)程中的牢籠效應(yīng)[55],人們使用了很多方法。為了使培養(yǎng)水體與外界進(jìn)行交換,Stoecker等[35]使用了微型生物籠(體積400 mL),籠的兩邊有多孔(孔徑12 μm)的膜,允許籠內(nèi)的水與外面交換,Verity[54]和 Dolan[56]分別使用了2 L和0.6 L的透析袋。有的研究使用較大的培養(yǎng)體積,如 Gilron和 Lynn[51]使用了20 L的培養(yǎng)體積,Landry等[57]使用60 L的培養(yǎng)體積。但是后來(lái)大多使用2 L的培養(yǎng)瓶。
纖毛蟲(chóng)通過(guò)分裂方式進(jìn)行無(wú)性繁殖,這種繁殖有沒(méi)有晝夜節(jié)律還沒(méi)有統(tǒng)一的結(jié)論,Heinbokel[52,58]認(rèn)為有節(jié)律,而Verity[38]則認(rèn)為沒(méi)有節(jié)律。用培養(yǎng)的方法得出的是纖毛蟲(chóng)在一段時(shí)間內(nèi)分裂的結(jié)果,所以培養(yǎng)實(shí)驗(yàn)最好超過(guò)1 d,實(shí)際上大多數(shù)培養(yǎng)的時(shí)間為1 d。
但根據(jù)現(xiàn)場(chǎng)培養(yǎng)實(shí)驗(yàn)來(lái)估算生長(zhǎng)率的一個(gè)問(wèn)題是經(jīng)常得出負(fù)值,例如Verity等[59]稀釋培養(yǎng)稀釋度為零的一組中纖毛蟲(chóng)的生長(zhǎng)率,3個(gè)實(shí)驗(yàn)中得出2個(gè)負(fù)值。Levinsen等[60]發(fā)現(xiàn)培養(yǎng)1 d后纖毛蟲(chóng)死亡很多。Fileman等[61]的實(shí)驗(yàn)中有一半的結(jié)果是負(fù)值。
由于纖毛蟲(chóng)非常脆弱,培養(yǎng)過(guò)程中對(duì)海水的轉(zhuǎn)移、過(guò)濾等操作都可能造成纖毛蟲(chóng)死亡,所以對(duì)于出現(xiàn)負(fù)值的原因,很多作者很難確定是自然界的實(shí)際情況,還是操作造成的。
為減少實(shí)驗(yàn)操作對(duì)纖毛蟲(chóng)的影響,現(xiàn)在大多數(shù)培養(yǎng)中使用逆向過(guò)濾的方法,即將篩絹裝在兩端開(kāi)口的圓筒的一端,將篩絹的一端浸入水中,開(kāi)口的一端向上露出液面,將圓筒中的海水取出(虹吸或用容器如燒杯舀出)。但是這種操作本身對(duì)這些纖毛蟲(chóng)的影響也沒(méi)有評(píng)估。
海水分粒級(jí)培養(yǎng)法假設(shè)過(guò)濾去除了纖毛蟲(chóng)的所有捕食者,但是,有些纖毛蟲(chóng)的捕食者(例如Didinium spp.和砂殼纖毛蟲(chóng)Favella sp.等其他捕食性的纖毛蟲(chóng)、異養(yǎng)甲藻和寄生甲藻)與纖毛蟲(chóng)處于同一粒級(jí),甚至小于后者,所以不可避免的部分或全部通過(guò)了篩絹[47]。另外,實(shí)驗(yàn)室培養(yǎng)的纖毛蟲(chóng)在餌料不足時(shí)還出現(xiàn)自食現(xiàn)象[62],一些甲藻的分泌物能降低纖毛蟲(chóng)的生長(zhǎng)率[15,33],這些現(xiàn)象在自然界也可能發(fā)生。因此海水分粒級(jí)培養(yǎng)法得出的生長(zhǎng)率數(shù)據(jù)是保守估計(jì)。
自 Stoecker 等[35]后,Tumantseva 和 Kopylov[26],Verity[54],Gilron 和 Lynn[51]及 Dolan[56]研究了幾種纖毛蟲(chóng)(多為砂殼纖毛蟲(chóng))的生長(zhǎng)率(表4),因?yàn)檫@些研究不是針對(duì)纖毛蟲(chóng)群體的,所以 Sherr和Sherr[63]認(rèn)為還沒(méi)有有效的現(xiàn)場(chǎng)纖毛蟲(chóng)生長(zhǎng)率的資料。1993年以后,纖毛蟲(chóng)群體的現(xiàn)場(chǎng)生長(zhǎng)率測(cè)定得到重視,但是目前的文獻(xiàn)較少26,43,47,50-70(表 5),研究海區(qū)主要在溫帶近岸區(qū)域(圖3)。這些資料顯示現(xiàn)場(chǎng)纖毛蟲(chóng)的生長(zhǎng)率的最大值為3.3 d-1。
表4 不同海區(qū)現(xiàn)場(chǎng)培養(yǎng)得出的砂殼纖毛蟲(chóng)生長(zhǎng)率Table 4 Growth rates of tintinnids by in situ incubations
續(xù)表
表5 不同海區(qū)現(xiàn)場(chǎng)培養(yǎng)得出的纖毛蟲(chóng)群落的生長(zhǎng)率Table 5 Growth rates of ciliate community by in situ incubations
在現(xiàn)場(chǎng)的生長(zhǎng)率同樣得出生長(zhǎng)率隨溫度升高而增加的趨勢(shì)。Verity[54]在Narragansett Bay測(cè)定了20多種砂殼纖毛蟲(chóng)周年溫度為0—25℃的生長(zhǎng)率和溫度的關(guān)系為 Q10=1.8。Dolan[56]在 Chesapeake Bay 4月、6月和8月得出以pico級(jí)浮游生物為食的纖毛蟲(chóng)群體的生長(zhǎng)率在7.4—26.8℃生長(zhǎng)率和溫度的關(guān)系為 Q10=2。Nielsen 和 Kiorboe[47]在丹麥 Kattegat Bay將纖毛蟲(chóng)分為 3 個(gè)粒級(jí)(10—15 μm、30 μm、40—60 μm),進(jìn)行了一周年的生長(zhǎng)率調(diào)查,3個(gè)粒級(jí)的 Q10為 2.2—3.0,平均為 2.6。Hansen 等[44]總結(jié)了體長(zhǎng)為2—2000 μm的浮游動(dòng)物的攝食率和生長(zhǎng)率的 Q10的平均值為 2.8,此后,Levinsen 等[60]和Hansen 等[43]都用 Q10=2.8。
圖3 自然海區(qū)現(xiàn)場(chǎng)的浮游纖毛蟲(chóng)生長(zhǎng)率的測(cè)定地點(diǎn)(·)分布圖Fig.3 Locations of in situ incubation stations for planktonic ciliate growth rates
Nielsen 和 Kiorboe[47]用 Q10=2.6 測(cè)量了不同粒級(jí)纖毛蟲(chóng)生長(zhǎng)率的季節(jié)變化,發(fā)現(xiàn)3個(gè)粒級(jí)異養(yǎng)纖毛蟲(chóng)(10—15 μm、30 μm、40—60 μm)和自養(yǎng)纖毛蟲(chóng)Laboea strobila(55 μm)的生長(zhǎng)率有共同的季節(jié)變化:春季生長(zhǎng)率開(kāi)始增加,7月和8月生長(zhǎng)率最高,秋季降低。
Nielsen 和 Kiorboe[47]利用現(xiàn)場(chǎng)實(shí)驗(yàn)的數(shù)據(jù),得出纖毛蟲(chóng)生長(zhǎng)率和溫度(T)、纖毛蟲(chóng)體積(V)的關(guān)系為:
Levinsen等[60]在格陵蘭一個(gè)海灣在1.4℃下進(jìn)行培養(yǎng)得出的纖毛蟲(chóng)生長(zhǎng)率高于用 Müller和Geller[23]的公式計(jì)算出的數(shù)值,Levinsen 等[60]認(rèn)為Müller和 Geller[23]的公式是根據(jù)較高溫度(17 ±6)℃下得出的數(shù)值回歸的,不太適用于低溫條件下的情況,極地生物可能具有適應(yīng)極端環(huán)境的能力,需要更多的研究完善這個(gè)公式。
在某一時(shí)刻,比較現(xiàn)場(chǎng)培養(yǎng)得出的纖毛蟲(chóng)生長(zhǎng)率和用細(xì)胞大小及溫度模型得出的纖毛蟲(chóng)內(nèi)稟生長(zhǎng)率,可以用來(lái)估計(jì)這個(gè)時(shí)刻纖毛蟲(chóng)在現(xiàn)場(chǎng)的餌料供應(yīng)情況。Gilron和Lynn[51]在寡營(yíng)養(yǎng)的熱帶發(fā)現(xiàn)實(shí)際測(cè)得的生長(zhǎng)率比模型得出的生長(zhǎng)率低一個(gè)量級(jí),所以這個(gè)海區(qū)纖毛蟲(chóng)餌料不足。Leakey等[64]發(fā)現(xiàn)在Sutton Harbor這兩個(gè)數(shù)值相差不大,因此纖毛蟲(chóng)的餌料供應(yīng)是充足的。Nielsen和Kiorboe[47]發(fā)現(xiàn)除了小型的纖毛蟲(chóng)外,優(yōu)勢(shì)纖毛蟲(chóng)類(lèi)群的生長(zhǎng)率與餌料濃度關(guān)系不大且都接近內(nèi)稟生長(zhǎng)率,因此,這些優(yōu)勢(shì)類(lèi)群的餌料是充足的,而小型纖毛蟲(chóng)的餌料是細(xì)菌,它們的餌料濃度不足,因此生長(zhǎng)率不高。而那些不占優(yōu)勢(shì)的類(lèi)群因餌料不足,生長(zhǎng)率較低。
纖毛蟲(chóng)的生長(zhǎng)率受溫度和餌料的共同影響,對(duì)于纖毛蟲(chóng)周年的生長(zhǎng)率變化,Nielsen和Kiorboe[47]認(rèn)為溫度變化對(duì)生長(zhǎng)率變化的貢獻(xiàn)為75%—97%,纖毛蟲(chóng)很少受到餌料不足的影響。
實(shí)驗(yàn)室中得出的纖毛蟲(chóng)的生長(zhǎng)率接近甚至超過(guò)許多浮游植物在最佳培養(yǎng)條件下的生長(zhǎng)率(0.5—2 d-1)[63],也超過(guò)一些異養(yǎng)甲藻的生長(zhǎng)率[22],所以纖毛蟲(chóng)被認(rèn)為對(duì)浮游植物豐度的變化反應(yīng)迅速,和異養(yǎng)甲藻一起,可以有效抑制初起水華[22],這也被認(rèn)為是高營(yíng)養(yǎng)鹽低葉綠素(HNLC,High Nutrient Low Chlorophyll a)海區(qū)浮游植物葉綠素濃度低的原因(例 如 Miller 等[71],Sherr 和 Sherr[63],Sherr 和Sherr[72])。
但是自然條件下觀察到纖毛蟲(chóng)在水華前期的生長(zhǎng)是很少的,在一些赤潮研究中,在赤潮的中后期發(fā)現(xiàn)纖毛蟲(chóng)的豐度很高,例如Admiraal和Venekamp[73]發(fā)現(xiàn)在北海的Phaeocystis pouchetii發(fā)生赤潮時(shí)有兩種砂殼纖毛蟲(chóng)的豐度很高。但是也有一些調(diào)查發(fā)現(xiàn)在一些水華期間,微型浮游動(dòng)物(包括纖毛蟲(chóng))豐度很低,如 Bjornsen 和 Nielsen[74]及 Nielsen 等[75]。在自然界會(huì)遇到單種或幾種纖毛蟲(chóng)豐度很高的情況,由于當(dāng)時(shí)的餌料并不是很高,所以物理的匯聚作用也是纖毛蟲(chóng)濃度高的原因之一,但也不排除此前可能發(fā)生過(guò)餌料濃度高的情況,只是沒(méi)有被監(jiān)測(cè)到。因?yàn)槔w毛蟲(chóng)迅速生長(zhǎng)的空間小,時(shí)間短,所以現(xiàn)場(chǎng)觀測(cè)到纖毛蟲(chóng)豐度升高和降低的資料幾乎沒(méi)有。在這些纖毛蟲(chóng)豐度升高的事件中,每次事件只有一種或幾種纖毛蟲(chóng)豐度升高,是優(yōu)勢(shì)種,所以特定餌料種類(lèi)對(duì)應(yīng)特定纖毛蟲(chóng)種類(lèi)的營(yíng)養(yǎng)關(guān)系很重要[5,76]。
在人為制造的水華事件中可以監(jiān)測(cè)纖毛蟲(chóng)的變化,其中最著名的是在HNLC海區(qū)進(jìn)行鐵加富實(shí)驗(yàn)制造的水華,但是在這些實(shí)驗(yàn)中,纖毛蟲(chóng)的動(dòng)態(tài)不同。例如,在亞北極太平洋,在Landry等[57]用圍隔制造的水華中,纖毛蟲(chóng)生長(zhǎng)率并沒(méi)有隨之增加,作者懷疑實(shí)驗(yàn)水體中的小型橈足類(lèi)、捕食性的纖毛蟲(chóng)和異養(yǎng)甲藻限制了植食性纖毛蟲(chóng)的豐度。在東赤道太平洋進(jìn)行的現(xiàn)場(chǎng)鐵加富實(shí)驗(yàn)表明,在添加鐵后,浮游植物生長(zhǎng),>20 μm 的纖毛蟲(chóng)的生物量也增加[77],在亞北極太平洋進(jìn)行的現(xiàn)場(chǎng)鐵加富實(shí)驗(yàn)SEEDS計(jì)劃中,在加富水體中從加富的第2天到第13天纖毛蟲(chóng)豐度減少[78]。在南大洋進(jìn)行的的現(xiàn)場(chǎng)鐵加富實(shí)驗(yàn)EisenEx計(jì)劃中,纖毛蟲(chóng)在加富水體中并沒(méi)有明顯升高的趨勢(shì)[79]。
缺氧對(duì)纖毛蟲(chóng)的生長(zhǎng)有不同的影響。在Chesapeake灣,6月下層水體發(fā)生缺氧,在缺氧區(qū)有與上層不同的纖毛蟲(chóng)群落,生長(zhǎng)率是上層纖毛蟲(chóng)的兩倍,達(dá)1.18—2.36 d-1。在8月的缺氧區(qū),上層和下層缺氧區(qū)的纖毛蟲(chóng)群落是相同的,但是缺氧區(qū)的纖毛蟲(chóng)幾乎不生長(zhǎng)[56]。
根據(jù)纖毛蟲(chóng)生長(zhǎng)率和生物量就可以估計(jì)纖毛蟲(chóng)的生產(chǎn)力,Lynn 和 Montagnes[80]根據(jù)當(dāng)時(shí)已經(jīng)報(bào)導(dǎo)的纖毛蟲(chóng)生物量數(shù)據(jù),估計(jì)全球不同海區(qū)纖毛蟲(chóng)的生產(chǎn)力,由于當(dāng)時(shí)缺乏生長(zhǎng)率的資料,他們的估計(jì)使用了一個(gè)數(shù)值:0.69 d-1(即加倍時(shí)間為1d),所以是很簡(jiǎn)單的估計(jì)。
大多數(shù)對(duì)纖毛蟲(chóng)生產(chǎn)力的估計(jì)是根據(jù)纖毛蟲(chóng)生長(zhǎng)率和生物量,與橈足類(lèi)的生產(chǎn)力相比,纖毛蟲(chóng)的生產(chǎn)力較大。在Solent Estuary,砂殼纖毛蟲(chóng)的年生產(chǎn)力為0.6 mg C/L[49];在 Kattegat Bay,纖毛蟲(chóng)群落(包括紅色中縊蟲(chóng))的年生產(chǎn)力為57 g C/m2或2 g C/m3,橈足類(lèi)群體的生產(chǎn)力為 12 g C m-2a-1[47];在Narragansett Bay,從3月到10月,砂殼纖毛蟲(chóng)群體的生產(chǎn)力日平均為14 mg C/m3,年生產(chǎn)力為3.3 g C/m3,橈 足 類(lèi) 的 生 產(chǎn) 力 為 13 mg C m-3d-1[81];在Southampton Water,浮游纖毛蟲(chóng)的日生產(chǎn)力為<1—141 mg C/m3,年生產(chǎn)力為 2.2—9.2 g C/m3,而橈足類(lèi)生產(chǎn)力為 0.6—1.6 g C m-3a-1[50];在 Gulf of Maine 纖毛蟲(chóng)的生產(chǎn)力為 0.16 g C m-3a-1[45];在Georgia estuary,夏季無(wú)殼纖毛蟲(chóng)群體(<20 μm)的生產(chǎn)力在潮溪內(nèi)為 1.2 mg C m-3h-1,在河口為 0.03 mg C m-3h-1,冬季則分別為0.05 mg C m-3h-1和0.07 mg C m-3h-1[82]
Zervoudaki等[83]利用纖毛蟲(chóng)最大清濾率、攝食率和毛生長(zhǎng)率0.33 d-1來(lái)估計(jì)纖毛蟲(chóng)的生產(chǎn)力,在土耳其海峽纖毛蟲(chóng)生產(chǎn)力為0.2—41 mg C m-2d-1。
綜上所述,國(guó)外在實(shí)驗(yàn)室內(nèi)和海上現(xiàn)場(chǎng)對(duì)浮游纖毛蟲(chóng)的生長(zhǎng)率已進(jìn)行了比較廣泛的研究。在實(shí)驗(yàn)室內(nèi)12種砂殼纖毛蟲(chóng)的內(nèi)稟生長(zhǎng)率變化范圍為0.45—2.7 d-1;22種無(wú)殼纖毛蟲(chóng)的內(nèi)稟生長(zhǎng)率(除Strombidium sulcatum的內(nèi)稟生長(zhǎng)率可達(dá)8.3 d-1外)變化范圍為0.1—3.05 d-1。纖毛蟲(chóng)內(nèi)稟生長(zhǎng)率與餌料種類(lèi)、溫度等有密切關(guān)系。雖然纖毛蟲(chóng)內(nèi)稟生長(zhǎng)率高于浮游植物內(nèi)稟生長(zhǎng)率,但是在自然海區(qū)纖毛蟲(chóng)隨水華發(fā)生而迅速增長(zhǎng)的情況并不常見(jiàn)。現(xiàn)場(chǎng)培養(yǎng)(海水分粒級(jí)培養(yǎng)法)是估計(jì)自然海區(qū)纖毛蟲(chóng)生長(zhǎng)率的常用方法,目前研究的海區(qū)主要局限在溫帶近岸海區(qū),熱帶和寒帶大洋海區(qū)的資料較少。對(duì)纖毛蟲(chóng)次級(jí)生產(chǎn)的研究表明纖毛蟲(chóng)次級(jí)生產(chǎn)大于橈足類(lèi)次級(jí)生產(chǎn)。
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