亞熱帶高山湖泊食物網結構的季節變化

Abstract

高山湖泊是位在高海拔的自然的靜止水體，具有獨特的生態系結構，亦是對環境變化相當敏感的生態系。前人有關於高山湖泊生態系的研究主要集中在溫帶地區，在亞熱帶亞洲地區的高山湖泊生態系卻很少被探討，由於亞熱帶地區的高山湖泊生態系統常受到季節性的環境擾動影響，如夏季的強降雨事件而造成的水文擾動或是夏季強烈的太陽輻射所導致的能量源的變化，可能會影響其中食物網能量源變化。 本研究探討位在亞熱帶台灣北部的翠峰湖及鴛鴦湖，這兩個研究樣點海拔高度及氣候條件相近，然而，這兩個研究樣點的沿岸植被有明顯差異，翠峰湖是相對較為開闊的自源生態系統且只有稀疏的沿岸植被；但是鴛鴦湖則是部分水面受到陸域植物遮蔽且較為封閉的異源生態系統。在本研究期間，翠峰湖食物網中生物的穩定碳同位素數值整體來看是高於鴛鴦湖，同時，我們的結果也發現雖然鴛鴦湖周圍植生茂密，然而落葉並非最重要提供鴛鴦湖整個生態系的能量源，穩定同位素混合模型（SIAR）的結果證實，在異源的鴛鴦湖，細菌是食物網中最主要的能量源。然而，翠峰湖是一個典型的自源生態系統，其主要提供整個食物網的能量源是自源性的浮游藻類以及附生藻類。另外，我們的研究結果顯示鴛鴦湖食物網結構其複雜度高於翠峰湖，主要原因是鴛鴦湖周圍植生茂盛而湖中有繁茂的水生植物床與豐富的藻類物種，大大提升鴛鴦湖的棲地複雜度，為浮游動物及水生無脊椎動物等營造合適躲藏的棲地。至於在兩個高山湖泊食物網結構之季節變異，主因為隨著夏季到來而提高的水溫以及太陽輻射所導致自源能量源的光合作用率提升，進而增加整個生態系統的初級生產力。我們的研究結果顯示，異源性鴛鴦湖的水生生物主要的基礎能量源是細菌，而自源性的能量源(如: 浮游藻類及附生藻類)是自源性翠峰湖中水生生物的主要基礎能量源。

Mountain lakes are natural and remoted lentic water bodies with unique ecosystem structure housing high level of biological endemism. They are sensitive to environmental changes though most studies of the mountain lake ecosystems have been focused on the temperate region. Yet, only limited investigation has been undertaken in such ecologically important habitats in tropical/subtropical Asia. As the subtropical mountain lake ecosystems are strongly influenced by seasonal environmental perturbations, such as periodic and stochastic hydrological disturbances due to heavy rainfall events, and the intense solar radiation during summer. It could be the major factor characterizing the energy basis for the lake food webs. In this study, the trophic structure and food utilization patterns of primary consumers, secondary consumers, and top predators in two subtropical mountain lakes in Taiwan, including Lake Tsuifeng (TFL) and Lake Yuanyang (YYL) were investigated using stable carbon and nitrogen isotope analyses. The two study lakes exhibited marked difference in riparian vegetation pattern, TFL was relatively unshaded with only sparsely distributed riparian vegetation whereas YYL was a densely shaded system with continuously distributed riparian forest. This resulted in distinct difference in the availability of different food sources to the food webs. During the study period, the variation of water temperature and precipitation pattern was similar between the two study lakes with most heavy rainfall events in summer. Our results showed that the food web components were more δ13C-enriched in TFL than in YYL. Despite the high availability of the two most dominant allochthonous food sources including fine particulate organic matter (FPOM) and leaf litter in the densely shaded YYL, they were least utilized by aquatic consumers. Instead, results of the stable isotope mixing model (SIAR) indicated that the δ13C-depleted bacteria represented the major energy basis in the allochthonous YYL. In contrast, TFL was a typical autochthonous ecosystem dependent on autochthonous food sources (periphyton and phytoplankton) which accounted for the major primary production supporting the food web. In addition, the increased complexity of food web structures in YYL as compared to TFL could be primarily due to higher habitat complexity in YYL where dense macrophyte bed and accumulation of leaf litter mass on the lake bottom. Moreover, the observed seasonal shift of food web structures in the two study lakes could be due to the enhanced primary production due to higher light intensity and water temperature in summer. Our study confirmed that the most important basal food sources for aquatic organisms in allochthonous YYL was bacteria, whereas autochthonous basal food sources (e.g. phytoplankton and periphyton) had the higher contribution to aquatic organisms in autochthonous TFL during the study periods.