利用穩定同位素探討浮游動物之個體大小與營養階層關係在不同水生生態系統下的變動

Abstract

人為干擾及氣候變遷對生態系的協同效應日益受到關注，因此我們急需一套有效率且可信賴的指標來研究食物網的動態，並用來做為生態系統監測及保育的基礎。大型生物捕食小型生物在水生環境系統中是非常普遍的現象，因此利用生物個體大小為理論基礎來探討水生生態系統動態是一個潛在的指標。有許多文獻指出生物個體體型大小以及營養階層之間存在著良好的線性關係，然而支持此現象的研究證據大多數僅侷限於大型生物所形成之食物網，然而此種線性關係是否存在於浮游生物群聚中至今仍然沒有明確的定論。捕食者與獵物之間體型大小的比例是決定食物網結構、穩定性及動力學的重要參數，然而直到現在探討何種機制影響捕食者與獵物體型大小比例的研究非常有限。本研究假設資源可利用性為影響捕食者與獵物體型大小比例變異之重要因子，並且提出利用（1）環境中物理及營養鹽參數、（2）浮游植物生物量，以及（3）顆粒性有機物質之碳氮比，來作為資源可利用性的代用指標。本研究藉由碳氮穩定同位素分析來探討在不同環境下浮游生物各體型組距間的食階關係，以及捕食者和獵物之間的交互作用。本研究結果顯示在大型浮游生物群聚中，體型大小及營養階層之間仍然存在明顯的正相關性；然而在小型浮游生物群聚中，體型大小和營養階層之間正相關性非常薄弱，甚至有反向的關係發生。本研究也指出捕食者與獵物體型大小比例在生態系統中確實會受到某些特定因素的影響，例如: (1)捕食者與獵物體型大小比例有空間上的變異。在相對變動較大的沿岸環境中，捕食者與獵物體型大小比例不會有太大的變動；而捕食者與獵物體型大小比例在外洋環境中卻存在有非常大的變異性。(2)捕食者與獵物體型大小比例會隨著浮游植物總生物量增加而呈現了非線性的下降趨勢。(3)食物來源的品質(元素組成)對於捕食者與獵物體型大小比例會產生影響。最後，在結合體型大小和食物階層關係以及捕食者與獵物體型大小比例之分析當中發現，在捕食者與獵物體型大小比例較小的時候，生態系統中會有較長的食物鏈；而較小的捕食者與獵物體型大小比例通常發生在環境變異較大的沿岸環境中。

Effects of anthropogenic disturbance and global climate changes on ecosystems are pressing concerns. Developing reliable and efficient indicators to study trophodynamics is essential for ecosystem management and conservation. Aquatic food webs are strongly size-based; therefore, one potential indicator to investigating trophodynamics is variation in body size. Substantial evidence suggests a positive linear relationship between log size and trophic level; however, whether such positive relationship holds in plankton community remains elusive. Predator-prey mass ratio (PPMR) is an important parameter to determine the stability, structure and dynamic of foodweb. Nevertheless, only one mechanism (gape limitation) was proposed to explain the observed pattern in predator-prey body mass ratios (PPMR) so far. We hypothesized and tested that PPMR was sensitive to the variation of resource availability and used three factors as proxy for resource availability: (1) concentration of nutrient; (2) phytoplankton biomass; and (3) C:N ratio of particulate organic matter. The present study used stable isotope analysis to assess size-TL relationship and PPMR within plankton communities. Our result suggested that larger size plankton community generally showed a strong linear positive size-TL relationship, but the small size plankton community showed non-significant and sometimes even inverse pattern. The present result also manifested some patterns in the PPMR. First, PPMR was steady in highly changeable costal environments; by contrast, PPMR were relatively variable in stable offshore environments. Second, PPMR showed a nonlinear declining trend with an increase in total phytoplankton biomass. Third, food quality has influence on PPMR. Coupled analyses of size-TL and PPMR may provide a basis for assessing the structure of food webs. Our result indicated that longer food chains were found in the food webs where average PPMR were smaller, and mean PPMR were smaller in highly variable inshore environments.