優養化衝擊對副熱帶河川型紅樹林食物網結構的影響:以淡水河紅樹林為例

Abstract

河川型紅樹林是提供多樣生態功能、具高度生產力的重要生態系統，因為承受低海浪侵蝕的影響並接受大量源自河川的養分，然而紅樹林生態系統主要使用的基礎能量源仍有不一致的學術見解。另外，受優養化影響的河川型紅樹林系統的基礎能量源變化在空間性和季節性環境因子影響下的變化少有探討，且食物網結構對於不同優養化程度的變化也不清楚。因此，本研究會探討 (1) 動物性和植物性碎屑在紅樹林生態系統中的重要性；(2) 基礎能量源使用的空間性及季節性變化；(3) 過去20年來食物網結構如何隨著優養化程度降低而改變。本研究在2015-2016年的夏季和冬季，於北台灣的淡水河主流河川紅樹林生態系統中設立三個樣點。穩定同位素混合模型顯示，河川型紅樹林生態系統的主要基礎能量源是異源性能量源(動物屍體及紅樹林落葉)，說明動物性碎屑在紅樹林食物網的重要性，本研究為首個以野外實驗來證實動物性碎屑的重要性。本研究中植物性碎屑的高貢獻度也與前人結果不一致，而造成此差異的原因可能為前人研究通常採集新鮮紅樹林葉片，且僅以單一生物消費者做研究，導致紅樹林落葉的貢獻度被低估。異源性能量源對生物的貢獻度幾乎無樣點間和季節變化，原因為主流河段受來自人為影響的優養化，及紅樹林為常綠樹的特性所提供的穩定的基礎能量源所影響，不過自源性能量源(絲狀藻)從主流上游到河口，對生物的貢獻度增加，可能為優養化的程度改變基礎消費者(底棲大型無脊椎生物)組成有關。本研究顯示，基礎營養能量源的穩定氮同位素與全球紅樹林生態系統相比之下相對較低，可能是受到含輕穩定氮同位素的人為汙染物的影響。與過去20年前的資料相比，淡水河本流的生物組成呈現隨時間改變的趨勢，且物種豐富度增加，這表示過去水質改善措施對食物網結構有正向效益。

Riverine mangroves are important ecosystems with high productivity and many ecological functions. The major basal resources contributed to riverine mangrove ecosystems was still debatable. Also, seasonal and spatial variation of the resource utilisation pattern of consumers in riverine mangrove ecosystems were not clear. Investigation on the effects of urbanisation and eutrophication on the mangrove food web remained scarce. In this study, we investigated that (1) the importance of animal and plant detritus as basal resources to mangrove ecosystems (2) seasonal and spatial resource utilisation pattern of biological assemblages along longitudinal riverine mangrove ecosystems (3) the response of food web structure in riverine mangrove ecosystems with decreased eutrophication level based on the 20-year river water quality monitoring data. Sampling was conducted at three study sites along the tidal main reach of Danshuei River in Northern Taiwan during summer and winter in 2015 to 2016, with the stable carbon and nitrogen isotope analysis. The results of stable isotope mixing model showed the consumers in riverine mangrove ecosystem had high dependence on allochthonous resources (animal corpse and mangrove leaf litter). For animal corpse, our results showed high importance of animal corpse to consumers in mangrove ecosystems, and this was the first field observation to prove the importance of animal corpse. The results that high contribution of mangrove leaf litter to riverine mangrove ecosystems were inconsistent with previous study. The difference might because that the previous studies usually collected fresh mangrove leaf litter and single species, which would underestimate the contribution of mangrove leaf litter. Allochthonous resources consistently represented the dominant basal resources for biological assemblages between seasons and sites. The consistence of resource utilisation might because that mangrove leaf litter with high availability from constant mangrove defoliation and animal corpse caused from eutrophication-related hypoxia with high nutrition would steadily be utilised to biological assemblages all the time. The results showed that the δ15N values of basal resources were relative low in comparison with other tropical mangrove ecosystem, this might be related to low δ15N values of anthropogenic waste in Danshuei River. Compared to the species composition in Danshuei River during past 20 years, species richness increased with reduced eutrophication level. Our results proved that the most important basal resources in heterotrophic riverine mangrove ecosystems were allochthonous resources (e.g. mangrove leaf litter and animal corpse).