西菲律賓海浮游動物類群間元素含量變化

Abstract

浮游動物在海洋中元素生地化循環扮演著舉足輕重的角色，其攝食、同化、代謝和排泄等生理作用及日夜垂直遷移和死亡後殼體沉降等行為影響微量元素在食物鏈中的傳遞與海洋各層水中微量元素的分布。因此，藉由探討各種浮游動物體內微量元素的組成與種間之差異可以更進一步了解微量元素在海洋中的生物地球化學循環。本研究使用超純淨採樣及分離技術(Ultraclean Size Fraction Techniques)，於西菲律賓海採集浮游動物，並配合元素分析儀(Elemental Analysis)、感應耦合電漿質譜儀（Inductively Coupled Plasma Mass Spectrometry)等分析技術進行基本建構元素(C, N, S, P)及微量金屬元素(Fe, Zn, Cu, Ni, Cd)含量分析。 實驗結果顯示，殼體組成成分不同使甲殼類浮游動物與軟體浮游動物體內基本建構元素碳、氮含量存在類群間差異，軟體浮游動物翼足目的碳氮含量明顯低於其他類甲殼浮游動物；由變異數分析與雪費(Scheffé)事後檢定等結果指出橈足亞綱、端足目、介形綱等甲殼類浮游動物對於微量金屬的攝取策略相似，其體內微量元素含量無明顯的差異。翼足目對於微量元素的生理需求與代謝速率不同於甲殼類浮游動物，使得浮游動物體內必需微量元素鋅、銅與鎳含量存在類群間異質性;微量元素在食物鏈中的傳遞，藉由浮游動物自我調控體內過量的微量元素，使得微量元素在食物鏈中隨著食階上升，微量元素莫爾比值下降。浮游動物體內所需微量元素(鐵、鋅、銅與鎳)的生物放大係數(BMF)皆小於1；而對於生物具有毒性的鎘元素，在傳遞至浮游動物時會有累積現象(BMF>1)，但在高營養階層的魚類中卻不見累積現象。 綜合上述實驗結果，體型大小、身體組成結構與生理需求等因子影響浮游動物體內微量元素含量的變化，透過浮游動物主動調控體內元素的多寡，進而影響海洋中微量元素的循環。

Zooplanktons play an important role in biogeochemical cycling of metals in marine systems. Through vertical migrations, as well as sedimentation of pellets and dead individuals, it accelerates the element transfer rate in water columns. Hence,to understand trace elemental content and interspecific heterogeneity in zooplankton can further know trace metals biogeochemical cycle in marine environment. In this study, using Ultraclean Size Fraction Techniques, zooplanktons (size in 363-500 μm, 500-1000 μm, 1000-2000 μm, 2000 μm-2 cm and >2 cm) and fish larvae were collected by vertical plankton tow at oligotrophic Western Philippine Sea. Samples were then separated into several species of zooplankton (i.e. Eucarida, Copepoda, Amphipoda, Ostracoda, and Pteropoda) under microscopes. Each sample fractions were further analyzed for their elemental (C, N, S, P and Ca) and trace metal (Fe, Zn, Cu, Ni and Cd) components. The results show distinct heterogeneity and differences in elemental(C, N) components between crustacean zooplankton and mollusc zooplankton. Due to their differences in body structure, both C and N content of Pteropoda is significantly lower than the other crustacean zooplankton. No significant difference of trace metal content (except Fe) among Copepoda, Amphipoda and Ostracoda were found by ANOVA and Scheff&eacute;'s Statistic method. However, there is significant difference between crustacean and Pterpoda. This difference may due to their physiological needs and metabolic rates, changes in Zn, Cu, and Ni between Pterpoda and crustacean zooplanktonconfirms such interspecific heterogeneity. Since zooplanktons can accumulate essential metals and actively remove excess metal, essential metals (Fe, Zn, Cu and Ni) were found with decreasing molar ratio with higher trophic level through the food web transfer. Moreover, the results indicate that biomagnification factor (BMF) of essential trace elements (Fe, Zn, Cu, and Ni) measured for zooplankton are all less than 1. On the other hand, Cd biomagnifies (BMFs > 1) from small plankton (10-63 μm and 63-153 μm) to zooplankton (153-2000 μm) and potential biodiminishes (BMFs < 1) from zooplankton to fish. In sum, trace metal concentrations in zooplankton are controlled by body size, body structure and physiological characteristics.