東海地區微米級浮游植物體型大小對其生長率與被取食率之影響-體型大較頻譜影響因子初探

Abstract

本研究將流式浮游生物掃描器 (FlowCAM, Flow Cytometry and Microscopy) 應用於稀釋培養實驗 (Dilution Experiment) 的測量中。此項創新的結合可獲得高解析度的生物體型大小資訊，提供分層過濾葉綠素方法所無法達到的詳細資訊。本研究在東海地區的不同季節進行稀釋培養實驗同時測量自然條件下微米級浮游植物 (Microphytoplankon) 生長率和被取食率。本研究第一個假說為檢驗自然條件下的微米浮游植物群集之單位體積生長率 (size-specific growth rate) 在經過溫度校正後，是否如同代謝理論 (MTE, Metabolic Theory in Ecology) 所預測，與體型大小成一斜率為-1/4之負相關。將單位體積生長率與體型大小進行廣義線性迴歸 (GLMM, Generalized Linear Mixed effect Model) 後的結果指出單位體積生長率與體型大小成一斜率為0.1之正相關。此一結果顯示代謝理論並不為自然條件下的浮游植物單位體積生長率所支持。另一方面，單位體積被取食率 (size-specific grazing mortality) 則與體型大小無顯著相關，但與單位體積生長率成正相關係。此一結果與路徑分析 (Path Analysis) 之結果共同指出體型大小將正向影響生長率，而生長率將再正向影響被取食率。本研究第二項假說為檢驗在不同環境下不同的體型大小頻譜 (NB-SS, Normalized-Biomass Size Spectra) 斜率是為 (1) 不同體型大小之不同生長率所決定，或是 (2) 不同體型大小之不同被取食率所決定，亦或是 (3) 兩者共同作用之結果。單變數分析結果顯示，小體型個體項較於大體型個體所遭受的被取食率將會決定體型大小頻譜斜率。綜合而言，本研究結果顯示，小體型個體的高生長率將會為小體型個體帶來較高的被取食率，而大體型個體被取食率相對降低，最終則可觀察到一較平緩的體型大小頻譜斜率。本實驗有助了解微米級浮游植物體型大小頻譜所受到由下而上 (bottom up control) 與由上而下 (top down control)影響的相對重要性。

Allometric scaling of body size versus growth rate and mortality has been suggested to be a universal macroecological pattern, as described by the metabolic theory of ecology (MTE). However, whether such scaling generally holds in natural assemblages remains debated. Here, we test the hypothesis that the size-specific growth rate and grazing mortality scale with the body size with an exponent of -1/4 after temperature correction, as MTE predicts. To do so, we couple a dilution experiment with the FlowCAM imaging system to obtain size-specific growth rates and grazing mortality of natural microphytoplankton assemblages in the East China Sea. This novel approach allows us to achieve highly resolved size-specific measurements that would be very difficult to obtain in traditional size-fractionated measurements using filters. Our results do not support the MTE prediction. On average, the size-specific growth rates and grazing mortality scale almost isometrically with body size (with scaling exponent ~0.1). However, this finding contains high uncertainty, as the size-scaling exponent varies substantially among assemblages. The fact that size-scaling exponent varies among assemblages prompts us to further investigate how the variation of size-specific growth rate and grazing mortality can interact to determine the microphytoplankton size structure, described by normalized biomass size spectrum (NBSS), among assemblages. We test whether the variation of microphytoplankton NBSS slopes is determined by 1) differential grazing mortality of small versus large individuals, 2) differential growth rate of small versus large individuals, or 3) combinations of these scenarios. Our results indicate that the ratio of the grazing mortality of the large size category to that of the small size category best explains the variation of NBSS slopes across environments, suggesting that higher grazing mortality of large microphytoplankton may release the small phytoplankton from grazing, which in turn leads to a steeper NBSS slope. This study contributes to understanding the relative importance of bottom-up versus top-down control in shaping microphytoplankton size structure.