翡翠水庫浮游植物色素及群聚組成之季節性變化

Abstract

本研究以高壓液相層析技術(HPLC)，分析翡翠水庫水體中浮游植物的色素組成，以暸解浮游植物群聚結構之季節性變化，並且探討其可能的控制機制。在為期15個月(2007年10月至2008年12月)的週採樣調查中，浮游植物總生物量(以葉綠素表示)表層(20 m以淺)積分平均值介於0.51~5.55 mg Chl m-3，呈現秋季較高的趨勢。整體而言，隱藻、藍綠藻以及矽藻為三個主要優勢種類，分別貢獻25%、24%及20%的葉綠素濃度。此外，綠藻門生物(包括綠藻及青綠藻)佔12%的貢獻量，渦鞭毛藻則佔10%。藍綠藻雖在冬季光照較低時，數量略低，但整體而言，全年皆有穩定的生物量且季節性變化不明顯。相關分析發現，綠藻、青綠藻及隱藻與混和層深度呈正相關。冷季以隱藻及綠藻門生物為優勢種類，反映其適應低溫、低光照和高營養鹽的生態特性。在暖季時，以喜好高溫、高光照，適應低營養鹽環境的渦鞭毛藻為優勢種類。矽藻除在混合層由淺變深的轉換時期外(07年和08年11月)，皆為優勢種類，其生物量的時序變化與環境因子之間無顯著關係。進一步分析發現矽藻與渦鞭毛藻生物量的高值，在暖季時交替出現，且矽藻傾向分佈於較深之水層，而渦鞭毛藻主要出現於高溫、高光照，低營養鹽之表層區域。添加操控實驗，顯示在中光照控制組中，渦鞭毛藻的生長速率為矽藻的3倍；若在高光照及添加磷酸鹽的條件下，渦鞭毛藻生長速率則是矽藻的8倍。推論渦鞭毛藻與矽藻的消長情形及空間區隔現象，應是在高溫高光的環境下，渦鞭毛藻對營養鹽的競爭能力優於矽藻的緣故。此外，本研究所得之玉米黃素(Zeaxanthin ; 藍綠藻之指紋色素)濃度與流式細胞儀計數的藍綠細菌豐度呈顯著的線性關係，其斜率為1.06 ± 0.04 fg zeaxanthin cell-1 (r2 = 0.68, p<0.01, n = 361)，可作為本研究系統中估算藍綠藻豐度之轉換公式。

To understand the annual variation of phytoplankton pigment and community composition as well as the relative importance of different phyla in oligotrophic freshwater ecosystem, pigment analysis via HPLC method was conducted on a weekly basis in the Fei-Tsui reservoir from Oct 2007 to Dec 2008. Epilimnion (20 m deep) integrated averaged chlorophyll-a concentrations (IChl) ranged 0.51~5.55 mg Chl m-3 with a bloom occurring in autumn. Overall, crytophytes, cyanobateria and diatoms were the three most dominant phyla accounting for 25, 24 and 20% of total IChl, respectively. Green algae (chlorophytes and prasinophytes, 12%) and dinoflagellates (10%) ranked the 4th and 5th in term of relative contribution. The analysis between phyla and environmental factors including temperature, light and mixed layer depth (a proxy of nutrient supply) was performed to understand potential controlling mechanisms. Cyanobacteria biomass did not change much during the studied period. Cryptophytes, prasinophytes and chlorophytes prevailed in the cold, low light level and high nutrient supply seasons. On the other hand, dinoflagellates could be categorized as warm water and low nutrient required species. Diatom biomass showed no significant relationships with environmental factors. It is notable that depletion of diatom bloom in turn was replaced by growing dinoflagellates in warm season. Furthermore, dinoflagellates occupied the light-rich surface waters in comparison with diatoms of similar size. Manipulation experiments suggested that plentiful nutrient stimulated the growth of dinoflagellate more than that of diatoms in high light level and phosphorus-enrichment conditions. Additionally, we found that zeaxanthin pigment concentrations showed a good correlation with cyanobacteria abundance with a slope of 1.06 ± 0.04 fg cell-1, which can be used empirically in converting diagnostic pigment concentrations to cyanobacteria abundance in the study area.