高屏峽谷之底棲生物食物網中的碳循環

Abstract

雖然台灣西南海域高屏海底峽谷的地質和生物群落結構已被廣泛研究，然而我們對沉積物的碳循環和棲地對底棲生態系統功能性的影響仍不清楚。本研究透過量化底棲食物網的碳循環以了解與山溪型河川對接的海底峽谷生態系統功能。本研究使用2014年至2020年共12個航次的沉積物樣本來估算高屏峽谷頭（簡稱GC1）和相鄰斜坡（簡稱GS1）中的生物和非生物的碳存量，並檢驗其中是否存在季節性差異。接著，我結合不同食階的生地化數據來構建線性逆推模型（Linear Inverse Model; LIM）並比較兩地食物網中的碳流量。最後，我使用網路指數檢驗兩棲地食物網的生態系統功能。 本研究結果顯示生物以及沉積物的碳存量並無顯著季節性差異，然而不同棲地之間生物以及沉積物的碳存量則顯著不同。除了細菌的碳存量在峽谷頭較高之外，相鄰斜坡的生物以及非生物(沉積物)的碳存量都顯著高於峽谷頭。峽谷中相對較低的生物多樣性和生物碳存量顯示峽谷頭生態系受到劇烈物理擾動，其中峽谷的大型（Macrofauna）與中型（Meiofauna）底棲無脊椎生物的碳存量遠低於斜坡，然而沉積物群落耗氧量在兩棲地之間並沒有顯著差異，顯示了峽谷頭中細菌可能貢獻了絕大部分的耗氧量。相比之下，相鄰斜坡上生物對於碳循環的貢獻較高，為相對成熟的生態系統。LIM結果顯示兩棲地之間的碳流量大小和分佈有巨大差異。峽谷頭和相鄰斜坡分別需要131.08 mg C/ m2/ d 和 78.95 mg C/ m2/ d 的有機碳通量來支持其生物系統。峽谷頭較高的碳埋藏率顯示高屏峽谷不僅將沉積物輸送到南海深處，也在碳封存中扮演重要角色。網絡指數分析顯示，峽谷頭的總系統通量（Total system throughput ; T..）和總系統穿流量（Total system through flow ; TST）相當高，代表流經峽谷系統的碳流量較大。而峽谷的芬恩循環指數（Finn Cycling Index; FCI）邊緣顯著低於相鄰斜坡，意味著高比例的碳未被利用而直接被埋藏，因此在系統中的分佈效率低下。 本研究是第一個量化高屏峽谷及相鄰斜坡底棲食物網碳流量的研究。透過LIM與網路指數分析，我們更理解海底峽谷的生態系統功能。未來山溪型河川的洪水強度跟頻度可能受氣候變遷的影響而增加，並且更頻繁地引發海底峽谷的地質災害。研究海洋環境中的物質和能量轉移將有助於我們了解生態系統功能以及其儲存碳的能力，而透過模擬高屏峽谷底棲生態系統的碳循環，我們或許能夠將本研究建構的模式運用在預測氣候變遷或人類活動對深海生態系統的影響。

The Gaoping Submarine Canyon (GPSC) off Southwest Taiwan has been extensively studied for its geology and biological community structure. However, the carbon cycle across the sediment-water interface and the environmental control on benthic ecosystem functioning remained unclear. This study attempts to contribute knowledge gap in the benthic food web by quantifying the carbon cycling in this small mountain river (SMR)-fed submarine canyon ecosystem. First, biotic and abiotic carbon stocks in the upper GPSC (GC1) and adjacent slope (GS1) were estimated and examined for the seasonal difference. Then, I combined biological and geochemical data from 2014 to 2020 with literature data to construct linear inverse models (LIM) and compared the carbon food webs between GC1 and GS1. Finally, I used selected network indices to examine the ecosystem function and food web characteristics between the canyon and slope habitats. The analyses did not find seasonal differences in organic carbon stocks. However, except for the bacteria stocks, the biotic and abiotic carbon stocks in the GS1 were significantly higher than in GC1. The relatively lower biodiversity and faunal carbon stocks in the canyon show that the GC1 was a fragile ecosystem under severe physical perturbation. Nevertheless, despite the drastic difference in fauna stocks, the sediment community oxygen consumption (SCOC) rates were similar between habitats, indicating the relatively higher microbial carbon remineralization in the GC1 than in GS1. By contrast, the higher fauna contribution to carbon processing in the GS1 suggests that the slope may be a relatively more mature ecosystem than the canyon. The LIM food web results showed that the magnitude and distribution of the carbon flows differed between GC1 and GS1. The GC1 required 131.08 mg C/ m2/ d, and GS1 needed 78.95 mg C/ m2/ d of total organic carbon (TOC) flux to support the biological systems. The higher carbon burial rate in GC1 indicates that the GPSC not only transports sediment to the deep South China Sea but contributes considerably to carbon sequestration. Moreover, two network indices, the total system throughput (T..) and total system through flow (TST), were markedly higher in GC1, indicating greater energy flowing through the system. The Finn Cycling Index (FCI) was marginally lower in the canyon, revealing that a large fraction of carbon is buried, and the remaining carbon were distributed inefficiently in the system. This study presents the first food web model to examine carbon cycling in the GPSC and on the adjacent slope. Moreover, it is the first study that applied the LIM technique in Taiwan. The LIM model results provided a rare opportunity to study how the canyon affects food web structure compared to the slope habitat. Moreover, the LIM model offered an insight into the ecosystem functioning of the submarine canyon from the aspect of energy flows and food web characteristics such as total system throughput, energy recycling, and food web maturity. Due to the ongoing climate change, the geohazards in the submarine canyons might become more frequent owing to new weather systems with a higher intensity of flooding in SMRs. Studying matter and energy transfer in the submarine canyon will help us determine the capacity of deep ecosystems to capture and store carbon. By better understanding carbon cycling in GPSC, we may be able to predict the impact of climate change or human influence on deep-sea ecosystems.