浮游食物網中跨營養階層攝食行為對於體型大小頻譜之影響

Abstract

如何有效率地取得良好的指標用以監測生態系統功能為當今環境保護及生態系統管理的重大議題。在生態系統中，經常以個體的體型大小頻譜為監測水域生態系統的重要指標。然而，體型大小頻譜形成的機制至今仍未有定論，過去用以解釋體型大小頻譜的理論模式，皆假設經對數轉換的體型大小以及營養階層之間存在著一良好的線性關係，然而支持此假設的證據多侷限於由大型生物所形成之食物網，未考慮存在於微生物食物網中的各樣複雜交互作用可能會破壞此線性關係，因此本研究以碳氮穩定同位素分析來檢驗此基本假設，並調查當有違反此假設的情況時，體型大小頻譜結構會受到怎樣的影響。欲達此目的，本研究中收集了在翡翠水庫中浮游生物的體型大小頻譜的時間序列資料，自2008年2月至2009年2月。結果發現此線性關係的假設並非恆成立，且體型大小與營養階層的關係主要受下列二因子影響 (1)微生物或是大型浮游藻類的能量貢獻差異；(2)浮游動物跨營養階層取食行為，皆使得營養階層與體型大小的關係非預期顯著。而營養階層與體型大小之間的薄弱關係使得體型大小頻譜偏離理論預測的冪次分布，並強化了體型大小頻譜中的次級結構。

One of the most important issues in environmental conservation and ecosystem management is to develop good indices for efficiently monitoring ecosystems. Size spectrum is one of the potential candidate indices and has often been used as an index to detecting functional processes in aquatic ecosystems. However, the mechanism for formation of size spectrum is still under debate. Most theoretical models concerning size spectrum assume a linear relationship between log size and trophic level (size-TL relationship). However, this linear size-TL relationship in microbial food webs has not been examined. In this study, we examined the size-TL relationship and structure of size spectrum under different size-TL relationships. To do so, we sampled time series of plankton size spectra from Feb. 2008 to Feb. 2009 in the Feitsui Reservoir and carried out size fractionated stable isotope analyses. We found that the linear size-TL relationship does not always exist in the microbial ecosystem of the Reservoir, possibly due to the following two factors: (1) the extent of energy contribution from microbes and larger phytoplankton to higher trophic levels, and (2) the omnivorous interactions between zooplankton and microbes. The weak size-TL relationship caused the size spectrum to deviate from power-law distribution and intensify the secondary structure in size spectrum.