探討東海橈足類營養級聯效應之影響因子

Investigating factors influencing copepod-induced trophic cascades in the East China Sea

陳子謙(Tz-chian Chen)

指導教授 ： 謝志豪

國立臺灣大學/理學院/海洋研究所/碩士(2019年)

https://doi.org/10.6342/NTU201901843

摘要

在浮游生物食物網中，橈足類捕食原生生物時會使原生生物對浮游植物的捕食壓力降低，並引發營養級聯(trophic cascade)效應。然而，該營養級聯在自然環境下的強度變化仍鮮少被探究。本研究提出四項因子會增加橈足類對浮游植物的攝食強度，進而降低營養級聯的強度：大型浮游植物的相對濃度、浮游植物化學計量品質、溫度及橈足類捕食壓力。較高濃度的大型浮游植物使橈足類能直接攝取大型浮游植物。較高的浮游植物品質(亦即化學計量；例如，較低的C：N比率）會增加橈足類對浮游植物的偏好。較高的溫度增強了橈足類的代謝及碳營養的需求，因此使橈足類傾向攝取浮游植物碳營養。較高的攝食者和獵物之密度比增加了橈足類對浮游植物的捕食壓力。本研究採甲板培養橈足類攝食實驗，比較沒添加及有添加橈足類的組別間浮游植物濃度減少之差異來量化營養級聯的強度，並研究該強度與浮游植物群聚的體型大小頻譜之斜率、溫度、攝食者和獵物之密度比和顆粒有機物 C：N之關係。我們發現營養級聯的強度與浮游植物群聚的體型大小頻譜之斜率、溫度或顆粒有機物的C：N比率沒有任何顯著關係。營養級聯的強度與攝食者和獵物之密度比呈負相關，但該關係在統計上並不顯著。本研究結果並不支持大型浮游植物的相對濃度、浮游植物品質、溫度和橈足類捕食壓力會影響營養級聯的強度。我們認為其他關鍵因子，像是原生生物的豐度，可能於影響東海浮游生物食物網中營養級聯效應發揮更重要的作用。

關鍵字

下行控制 ； 營養級聯 ； 浮游生物食物網 ； 橈足類攝食效應 ； 東海

並列摘要

Copepods induce trophic cascades when they consume protists, which cause a reduction in protists’ grazing pressure on phytoplankton. However, how the intensity of copepod-induced trophic cascades varies under different resource conditions remains elusive. We hypothesize that the proportion of large phytoplankton, phytoplankton stoichiometric quality, temperature and copepod grazing pressure (quantified as predator-prey density ratio) reduces the intensity of trophic cascade by increasing copepod grazing impact on phytoplankton. Specifically, higher proportion of large phytoplankton attracts copepods to ingest large phytoplankton in the community. Higher phytoplankton stoichiometric quality (e.g. lower C: N ratio) shifts copepod dietary preference toward phytoplankton. Higher temperature enhances the demand of copepods for metabolic carbon and shifts copepod dietary preference toward phytoplankton. Higher predator-prey density ratio increases copepod grazing pressure on phytoplankton. To explore this issue, we quantified the intensity of trophic cascade by estimating the difference in phytoplankton concentration reduction using in situ incubations with and without copepods in the East China Sea, and investigated the relationship between the intensity of trophic cascade versus normalized biomass size spectrum (NBSS) of phytoplankton community, C: N ratio of particulate organic matter (POM), temperature, and prey-predator density ratio. We found that the intensity of trophic cascade does not exhibit any relationship with phytoplankton NBSS slopes, C:N ratio of POM or temperature. The trophic cascades exhibit a negative relationship with the predator-prey density ratio; however, the relationship is not statistically significant. Our results do not support the hypotheses that proportion of large phytoplankton, phytoplankton stoichiometric quality, temperature and copepod grazing pressure affects trophic cascades. We suggest that other critical factors, such as protist abundance, are likely to play a more important role in affecting trophic cascades in the plankton food web in the East China Sea.

並列關鍵字

top-down control ； trophic cascade ； plankton food web ； copepod grazing impact ； East China Sea

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