主動與被動大陸邊緣的河流－陸棚大型底棲無脊椎動物群聚結構與影響環境因子：珠江與高屏溪之個案研究

Abstract

大陸棚是最具生產力的海洋環境之一，尤其是河流主導的海洋邊緣(River-influenced ocean margins；RiOMar)大陸棚，接收來自鄰近河流大量的淡水與沉積物，通常具有特別高的初級生產力、有機碳再礦化能力、與碳埋藏的速率。儘管RiOMar具有重要的生態意義，針對該區域的底棲生物研究仍然十分缺乏，且大多數研究僅限於溫帶地區的河海系統。然而這些少量的研究仍指出RiOMar的底棲生物群聚同時受到（一）沉積物沉降的物理擾動以及（二）河水營養鹽促進食物供給的調控。即使不同區域的生態研究已確認河流對於底棲生物的影響，不同類型的大陸邊緣（主動與被動大陸邊緣）是否會影響陸棚底棲生物仍屬未知。過去的地質學研究指出主動與被動大陸邊緣的RiOMar透過迥異的沉積物傳輸機制與陸源有機碳組成的差異，在地球碳循環上扮演著不同的角色。例如，位處被動大陸邊緣的大型河川的輸沙與沖淡水較為穩定，而坐落於主動大陸邊緣的山溪型河川則多仰賴洪泛等偶發事件來輸送沉積物。另一方面，大型河川的有機碳會經歷數次的有機碳降解循環，不分活性的將有機碳從沉積物中釋放到水層與大氣之中。相較之下，狹窄且陡峭的主動大陸邊緣能迅速的將有機碳從河流送到沿岸地區，讓陸棚的沉積物維持較高的化石有機碳與現代陸源有機碳訊號。若將這些地質學的發現轉換到生態學的研究上，沉積物傳輸的差異可能會影響沿岸地區的沉積物沉降機制，進而帶給底棲生物不同類型的物理干擾；河流有機碳組成的差異則是可能影響底棲生物食物供給。為了探討大陸邊緣類型對於陸棚底棲生物的影響，本研究調查南中國海北緣同為於亞熱帶的高屏溪與珠江陸棚，比較主動與被動大陸邊緣之生地化過程對大型底棲動物群聚的影響。其中，高屏溪－陸棚系統位於歐亞大陸板塊與菲律賓海板塊匯聚邊界上，具有典型的主動大陸邊緣特徵，如其狹窄的陸棚與陡峭的陸坡，且連接著短促、坡陡流急及高輸砂量的山溪型河川—高屏溪。珠江－陸棚系統則位於歐亞大陸板塊的被動大陸邊緣上，有寬廣的南海大陸棚連接中國的第二大河—珠江。本研究利用海研一號與勵進號共三個研究航次(2019-2020)收集兩個河海系統之水文，沉積物化學、與大型底棲生物資料。分析結果發現，高屏溪-陸棚系統的大型底棲生物群聚主要受到洪水沉積物沉降的影響，而珠江-陸棚系統的大型底棲生物則受多個環境因子共同影響。兩個河川-陸棚系統內的大型底棲生物的生物密度、生物量、次級生產、呼吸率、生物體型頻譜的斜率，與物種組成皆有顯著不同。生物密度的差異與總有機碳與葉綠素a濃度相關，而生物量、次級生產、呼吸率，與生物體形頻譜的斜率的差異則能以兩個區域水層初級生產力的差異來解釋。區域間的物種組成差異更是受到碳同位素、碳氮比、葉綠素a濃度等有機碳品質指標的影響。儘管大型底棲生物群聚結構的各個面向都顯示了高屏溪-陸棚系統與珠江-陸棚系統的顯著差異，採樣區的空間尺度差異及採樣過程造成的誤差皆有可能影響本研究的結果，因此雖然本研究結果顯示兩個河川－陸棚系統間的大型底棲生物群聚有顯著差異，本研究所提出的物理擾動假說與食物供給假說仍無法完全被驗證。後續研究建議以更高解析度的生物多樣性資料與生地化指標，以及更適當的採樣計畫與與工具來檢驗河川類型與地形地貌對於陸棚底棲動物群聚的影響。

The continental shelf is one of the most productive regions in the marine environment. In the river-dominant ocean margins (RiOMar), the continental shelves receive significant inputs of freshwater and sediments from adjacent rivers, facilitating primary production, carbon remineralization, and carbon burial at exceptional rates. Despite their ecological significance, studies investigating the RiOMar benthos remained scarce, scattered, and mainly focused on temperate regions. These limited studies nevertheless suggested that the RiOMar benthic community may be controlled by the interplays between (1) physical disturbance induced by sedimentation and (2) increased food inputs with nutrients from the river discharges. Although these studies spanning different regions confirmed the impacts of river discharges on the shelf benthos, whether the different continental margin types (i.e., active vs. passive) could also play some roles was left unchecked. In a geological context, RiOMars on the active and passive margins play different roles in the global sediment organic carbon cycling via variations in river discharge regime and organic carbon age composition. Large rivers (LR) on passive margins discharge freshwater and sediments more consistently, while small mountainous rivers (SMR) on active margins are more episodic. With large drainage basins, LRs allow multiple organic carbon degradation cycles, removing organic carbon with various reactivities from the sediment into the water column and atmosphere. On the other hand, the narrow and steep active margins immediately deliver the organic carbon from the river into the coastal system, enriching the shelf seabed with fossilized and modern terrestrial organic carbon. Translating geological evidence to ecological context, the difference in sediment discharge regimes could affect the sedimentation regime, inducing various types of disturbances to the benthos. The age difference in riverine organic carbon might also alter the available food source for the shelf benthos. To understand the effects of margin types on the river-shelf seabed, this study compared macrobenthic communities between the active and passive margins in the northern South China Sea in a subtropical setting. The Gaoping river-shelf (GRS) situates on a complex convergent boundary between the Eurasian and Philippine plates, representing a typical active margin. It also connects to an SMR with a narrow shelf receiving large fluvial discharges characterized by high sediment loading. The Pearl river-shelf (PRS) connects to China's second-largest river, the Pearl River, with a broad continental shelf on a classic passive margin setting. This study conducted three research cruises (2019-2020) to compare the hydrography, sediment geochemistry, and macrobenthos community structure between the GRS and PRS. In the GRS, flood sedimentation was the major factor controlling the macrofauna community, while a suite of factors collectively explained the macrofauna community in the PRS. The macrofauna density, biomass, production, respiration, NBSS slopes, and taxonomic composition differed significantly between the GRS and PRS. The macrofauna density corresponded with organic carbon quantity (TOC) and quality (sediment chlorophyll a) indicators, while the macrofaunal biomass, production, and respiration reflected regional primary productivity differences. The organic carbon quality indicators such as δ13C, C/N ratio, and sediment chlorophyll a concentration correlated with the regional taxonomic compositional differences. Although various aspects of the macrofauna communities differed between the two river shelves, the differences in sampling scale, sieving efficiency, and scale-dependent processes might confound the observed patterns. Hence, this study cannot confirm the hypothesized influence of riverine sedimentation disturbance and food supply. Future studies with better taxonomic resolution, enhanced geochemical proxies, relevant sampling designs, and revised sieving protocol are required to verify the geomorphological effect on the RiOMar benthos.