浮游動物及浮游植物間元素比不平衡影響海洋浮游動物群聚內之競爭優勢

Abstract

消費者與食物間的元素比不平衡 (stoichiometric imbalance) 是影響消費者生長速率的重要因子。因為作為食物的浮游植物其碳、氮、磷比 (C:N:P) 變化較大，具有相對穩定元素比的浮游動物經常遇到元素比不平衡的情形。食物中元素含量過低或過高皆屬於元素比不平衡的範疇，而兩者皆會限制浮游動物的生長及生殖能力，進而降低其適存度與競爭力。浮游動物類群間對元素需求的差異會導致各類群與浮游植物間產生不同程度的元素比不平衡。理論上，受到較高程度不平衡的類群競爭力較差，因此不平衡程度較低的類群會成為優勢。過去研究關於元素比不平衡對浮游動物群聚的影響，多半為控制實驗或理論模型，且聚焦在淡水浮游動物，而針對海洋浮游動物的研究仍相當缺乏。本研究於東海南部採集植食性浮游動物及顆粒有機物 (POM) ，並測量其碳、氮、磷比，以探討元素比不平衡如何影響海洋浮游動物在科層級的類群組成。本研究旨為驗證三項假說：(1) 浮游動物的元素比在科內恆定；(2) 浮游動物的元素比有科間差異；(3) 浮游動物類群受到的計量不平衡越高，其相對豐度越低。結果證實浮游動物元素比在科內恆定，但科間存在顯著差異(尤其是C:P 與 N:P)。此外，磷的元素比不平衡對浮游動物群聚組成影響最大。其中缺磷會減少浮游動物相對豐度，而過量的磷則有正向影響，顯示浮游動物有儲存而利用過量磷的可能。本研究首次於野外研究中證實元素比不平衡為影響海洋浮游動物群聚組成的重要因子之一，並提出海洋浮游動物具有對過量磷適應能力之可能。

Stoichiometric imbalance between consumers and their food sources plays a crucial role in determining growth efficiency. Zooplankton often experience such imbalance due to their relatively stable internal C:N:P ratios compared to the more variable stoichiometry of their phytoplankton prey. The imbalance is bi-directional: both nutrient deficiency and excess can impair zooplankton growth and reproduction, thereby reducing fitness and competitive performance. Because zooplankton taxa differ in their elemental requirements, they experience differential degrees of stoichiometric mismatch with phytoplankton. Taxa experiencing stronger imbalance are likely to be less competitive, leading to community shifts favoring taxa with lower imbalance. Most research on the effects of stoichiometric imbalance on zooplankton competition has been conducted under laboratory conditions or using theoretical models, and primarily focuses on freshwater systems. In contrast, the role of stoichiometric imbalance in shaping marine zooplankton communities remains largely understudied. In this study, I conducted field sampling in the southern East China Sea and analyzed the carbon, nitrogen, and phosphorus ratios (C:N:P) of herbivorous zooplankton and particulate organic matter (POM). The objective was to examine how stoichiometric imbalance influences the family-level taxonomic composition of marine zooplankton. I tested the following hypotheses: (1) Zooplankton stoichiometry is homeostatic within families; (2) Zooplankton stoichiometry varies among families; and (3) A higher degree of stoichiometric imbalance reduces the relative abundance of a zooplankton family. My analyses revealed that while zooplankton exhibit stoichiometric homeostasis within families, significant inter-family differences were observed, especially in C:P and N:P ratios. Phosphorus imbalance, in particular, was found to influence zooplankton community composition: P deficiency negatively affected zooplankton relative abundance. Unexpectedly, excess P had a positive effect, suggesting the potential for luxury phosphorus uptake. This study provides novel field-based evidence that taxon-specific stoichiometric imbalance shapes marine zooplankton community composition. Furthermore, zooplankton might have the capacity to store and utilize excess phosphorus.