浮游動物食性在東海中隨體型大小與環境食物組成之變化

Abstract

在海洋食物網中，浮游動物扮演能量傳遞的重要角色。過去研究指出浮游動物的食性會隨著其體型大小以及環境中食物組成而有所變化。然而先前較少研究同時探討體型大小及環境中食物組成對浮游動物食性之影響。在本篇研究中我們利用脂肪酸營養標記 (fatty acid trophic marker, FATM) 來探討浮游動物的食性變化。我們於2016年5月在東海五個不同測站中採集樣本，並從2012~2013年間於基隆嶼周圍海域採集共九個月份的樣本，來分別比較浮游動物食性在空間與時間上的變化。在空間上不同測站的採樣中，我們將浮游動物從1000微米到50微米分成五個大小，而在基隆嶼海域則將浮游動物從 >2000微米到50微米共分成七個大小層級，並利用這些不同大小浮游動物體內的脂肪酸組成推論其食性。此外我們在各採樣點同時也收集水體中的浮游生物，來與浮游動物食性做比較。此研究測試兩個假說：一、浮游動物食性主要受其體型大小決定，亦即較大的浮游動物主要攝食各種大型藻類(例如：雙邊毛藻與矽藻)，相對較小的浮游動物則是只能攝食細菌或是原生動物。二、浮游動物食性主要受環境中食物組成決定。研究結果發現，大於 363 μm 的浮游動物主要食用大型浮游植物，小於 200 μm 的浮游動物則主要食用依靠細菌為食的微浮游動物，而在 363~200 μm 的浮游動物食性则依環境食物組成而有所不同。然而當環境中的食物多樣性變少時，浮游動物的食性便會被環境中的食物所限制。在此研究中可以發現浮游動物的食性主要依照其體型大小不同而有所變化，但是在特別的環境中食物多樣性明顯降低時，環境中的食物組成便會限制所有大小浮游動物的食性。

Zooplankton play an important role in transferring energy from primary producers to higher trophic levels. Their diet has been suggested affected by their body size and food composition in the environment. However, few studies examined simultaneously how zooplankton diet varies with respect to their size and environmental food composition in marine ecosystems. To tackle this issue, I collected zooplankton with 5 size classes in a transect of 5 stations in May 2016 in the southern East China Sea (ECS) for examining spatial variation and 7 size classes in 9 months from July 2012 to August 2013 at Keelung Island for examining temporal variation through analyzing diets of zooplankton using fatty acid trophic markers (FATM). Meanwhile, plankton were collected to represent food composition in the environment. I tested two hypotheses. First, zooplankton diet is determined by their body size; specifically, larger zooplankton ingest more larger phytoplankton such as dinoflagellates or diatoms, whereas smaller zooplankton prefer bacteria or protists. Second, zooplankton diet is affected by the environmental food composition. My results show that generally zooplankton of body size larger than 363 μm mainly feed on large phytoplankton and those of body size smaller than 200 μm mainly forage on bacteria-derived food sources, while the diets of zooplankton of body size between 363~200 μm vary depending on food composition in the environment. However, when food diversity in the environment is low, for example, when phytoplankton are over-abundant or nutrient condition is oligotrophic, the diet of zooplankton in all size classes is restricted by the environmental food composition. These results indicated that both hypotheses are supported to some extent: the diet of zooplankton is significantly influenced by their body size; however, in some specific conditions of low food diversity, zooplankton would change their diet in accordance of food composition in the environment.