比較微型浮游動物於沿岸海域及河口區時空分布之研究

Abstract

沿岸區及河口區都是營養鹽充足的海域；而河口區有河流持續帶入營養鹽及有機顆粒，因此河口區的浮游植物及其他有機物顆粒，都較沿岸地區為多。對橈足類而言，此兩海域意味了兩種不同食物補充率的狀態。因此本研究要探討的主軸為:在亞熱帶海域，溫度都較高的情形下，橈足類在兩種不同食物量的環境中，種類及豐度變化是否也受到食物供給量的影響。沿岸及河口區的捕食者數量也很多，包括水母及仔稚魚等。因此除了食物供應外，也將針對捕食者對橈足類的影響進行討論。 橈足類族群中，小體型橈足類佔了將近90%的數量，但常因採樣以較大網目的拖網(200~330 um)而忽略。臺灣附近海域小體型橈足類的種類及數量的研究是很缺乏的。因此本研究將以較細的網目(100 um)捕捉小體型的橈足類，不漏過橈足類族群的數量。除了研究橈足類種類組成外，探討橈足類族群在此兩海域的種類及豐度變化與攝食的偏好，並探討影響豐度變化的因子。本研究以核二廠入水口為沿岸海域，淡水河口為河口區。本研究提出幾點假說: 1. 河口區橈足類的豐度及種類較沿岸區多。 2. 河口區橈足類對浮游植物的攝食率及攝食衝擊較沿岸海域高。 3. 河口區橈足類不受食物供應的限制，而沿岸區則可能因食物短缺而受限。 如何評估豐度有無受到食物影響，筆者以豐度與溫度相關做為基準，若橈足類食物充足，個體有足夠的能量反應在成長及生殖，而溫度的增加會加速其成長及繁殖速率，因此族群數量的累計會與反應在溫度的變化上。因此當兩者呈正相關，表示橈足類食物並不缺乏。橈足類與食物間的關係，筆者以浮游植物為目標，以14C標定浮游植物，藉由橈足類攝食的量，推算橈足類的攝食率及族群攝食衝擊。而在食物不缺乏的狀況下，才能討論溫度及捕食者的影響，筆者以捕食者與橈足類族群的相關性做為判定，當橈足類豐度與捕食者豐度呈負相關，表示捕食者效應造成橈足類豐度的改變。 核二廠結果顯示，2002年11月至2004年5月期間，溫度(平均值23.7 ± 3.6 ^oC, 17.8~29.9 ^oC)有明顯的季節性變化，營養鹽濃度(3.8 ± 3.6 uM, >0.01~19.0 uM)與溫度趨勢相反，葉綠素濃度(0.55 ± 0.40 mg Chl-a m^-3, 0.03~2.02 mg Chl-a m^-3)在2002年僅有三次高峰值，而2003年4~6月則是緩緩增加的。橈足類種類發現11種，多以小體型的沿岸種類為主，Paracalanus parvus、Acrocalanus gibber及Temora turbinata是主要的優勢種類。橈足類豐度變化(5103 ± 6304 ind. m^-3, 42~29902 ind. m-3)，2002年的5月至10月都維持在高值(>10,000 ind. m^-3)，而2003年5至10月則是在6月有高值後(29902 ind. m^-3)，就急速減少(4254 ind. ^m-3)。橈足類豐度與溫度呈正相關，表示橈足類的成長不受食物的限制。個體攝食率(0.02~0.60 ugC ind. d^-1)及攝食衝擊(2.68 ± 11.0% Chl-a d^-1)都偏低的，表示其他食物來源是會影響橈足類成長的。在核二廠發現高濃度的溶解性有機碳(DOC)，此經由微生物環提供的碳，應是讓2002年橈足類可以維持高豐度的原因。而捕食者數量與橈足類豐度呈正相關，橈足類豐度仍隨著溫度累積，因此對於橈足類豐度並無顯著影響。 淡水河溫度及營養鹽濃度變化與核二廠類似，但河口持續帶入營養鹽，因此葉綠素濃度(1.6±2.0 mg Chl-a m^-3, 0.3~8.5 mg Chl-a m^-3)較核二廠為高。橈足類豐度(1665 ± 1754 ind. m^-3, 275~7088 ind. m^-3)有明顯的季節性變化，且與溫度呈正相關，表示沒有受到食物的限制。淡水河發現的種類也多是沿岸種類，優勢種類包括Parvocalanus crassirostris、Acrocalanus indicus、Oithona attenuata及O. nana，種類組成有明顯的季節性不同。個體攝食率(0.12 ugC ind. d^-1)及攝食衝擊(~2% Chl-a d^-1)都是偏低的，說明橈足類不完全以浮游植物為食，其他食物來源，包括纖毛蟲等都可以供給橈足類食物。而水母平均值在春夏季間大量增加(9->4525 ind. 1000m^-3)，使得大體型橈足類平均豐度(254 ->47 ind. m^-3)減少，另外大體型橈足類與水母空間分布呈顯著負相關(r= -0.63, p<0.01)，但小體型橈足類並未因此受到影響。推測是因為小體型橈足類成長率大過水母的捕食率所致。 比較兩地之橈足類豐度顯示，核二廠的豐度明顯較淡水河為高(ANOVA, p<0.05)，兩地都不受到食物的影響下，水母在淡水河的捕食效應是造成兩地豐度差異的原因。而淡水河小體型橈足類豐度，在水母的攝食效應強烈下，支持橈足類數量及族群的重要角色。而核二廠2003年DOC濃度銳減後，是否影響來年的橈足類豐度，及是否橈足類對浮游植物的攝食因食物缺乏而增加，則有待進一步的研究。

The estuary and coastal water have sufficient nutrients that support high phytoplankton biomass and primary production. Higher nutrients and organic particle by river input indicate that the estuaries supply more food to copepods than the coastal water. One would easily predict copepods would not limited by food supply in estuary. On the other hand, verity of copepod abundance would control by temperature and predator in estuary. Fish larvae, Chaetognatha and jellyfish are common predators in estuary and coastal water, and also affect the variety of copepod abundance. Our hypotheses are as followed: first, copepod abundance was higher in estuary than in coastal water, because the differences of food supply conditions. Second, the copepod grazing rate and grazing impact in estuary are higher than in coastal water. Finally, copepod abundance is not controlled by food supply in estuary. Sampling sites are in Tan-Shui River estuarine (as estuary site, TSR) and the inlet of Taiwan Nuclear Power Plant II (as coastal site, TPII). A 100 um mesh size of plankton net were employed to collect the smaller copepods and other zooplankton. A 330 um size net were collected the larger plankton and compare the abundance to smaller copepods. Temperature, salinity, nutrients, Chl-a concentrations and predator abundance were also measured. The linear regression was used to the relation between measurements. Copepod abundance is positive correlation with temperature indicating copepods are not in food limited condition. Under this condition, we could discuss the predation effect by negative correlation between copepods and predators abundance. We sampled weekly or bi-weekly during November 2001 to May 2004 in TPII. Temperature (17.9 to 29.9 ^oC), nitrate concentration (0.01 to 19.0 uM) showed strong seasonal fluctuation. Chl-a concentration (0.03~2.02 mg Chl-a m^-3) were positive correlation with temperature and negative correlation with nitrate. Paracalanus parvus, Acrocalanus gibber and Temora turbinata were most dominant species. Copepod abundance (42~29902 ind. m^-3) were positive correlation with temperature, indicating food is sufficient. Copepod specific grazing rate (SGR, 0.02 to 0.66 ugC Chl-a d^-1) and mean copepod grazing impact (2.68 ±11.0 Chl-a d^-1) were lower than others study, indicating phytoplankton was not the major food to copepods. The higher dissolve organic carbon concentration was obtained in TPII, which may support another carbon source from microbial loop. The jellyfish and Chaetognatha abundance were showed positive correlation with copepods, showed predators were not affected to copepod abundance. The large copepods were collected using 330 um mesh size net in 19 sampling stations, while small copepods were also collected by using 100 um mesh size net in 5 stations. During four seasons in 2006 in TSR, temperature (18.4~29.9 ^oC) varied seasonally with lowest and highest values recorded in winter and summer, respectively. Nitrate concentrations (0.3~8.8 uM) showed a significant seasonal trend with higher values in winter and lower values in summer. Chl-a concentration (0.3~8.5 mg Chl-a m^-3) varied with temperature, and recorded higher values near the river mouth. The neritic species were dominant in TSR, that including Parvocalanus crassirostris, Acrocalanus indicus, Oithona attenuata and O. nana. The species composition showed significant different by Cluster Analysis. Small copepods abundance (275~7088 ind. m^-3) varied with temperature, but large copepods (2~3272 ind. m^-3) were decreased from spring to summer. Medusae abundance (0~32584 ind. 1000m^-3) were significant decreased from winter to spring, and increased almost 444-folds of abundance in summer. The significant negative correlation between large copepod and medusa abundance indicated the strong predation appeared during spring to summer. The significant decouple of spatial pattern also support large copepods were under the strong predation effect by medusae. But small copepod abundance did not affect by medusae. It suggested that small copepods have higher growth rate and reproductive rate. Small copepod abundance in TPII were significant high than in TSR. According to the similar temperature variation and the sufficient foods in both areas, it is suggested that predator effect by medusae played the key factor that explain the difference of copepod abundance. The medusa bloom is occurred in many of estuary, small copepods may play more important role in secondary production when in a medusa bloom. It is also obtained the decreasing DOC in TPII in 2003, indicating it may be in food limited condition in 2004 or following years. The role of small copepods in microbial loop and secondary production may be important in study trophodymanics and carbon cycling.