Environmental Controls on Macrofaunal Community in the Gaoping Submarine Canyon off the SW Taiwan

高屏海底峽谷源自高屏溪，為台灣陸源有機物進入南海深海的主要通道，也是海底地質災害頻繁的區域。儘管過去累積了許多峽谷環境及沉積物源匯動力學的研究，海洋學界對峽谷的底棲生態卻仍一無所知。為了探討海底峽谷環境時空變化對大型底棲動物群聚結構之影響，本研究於2015年四月、八月、十一月在高屏海底峽谷上段軸心以及鄰近峽谷的斜坡，各設立四個深度自200至1100公尺的測站，進行三次採樣。除了採集大型底棲動物，也測量海床水文及沉積物粒徑、總有機碳、總氮含量等環境資料，並利用流體靜力模式推估近底的內潮流速。研究結果顯示，峽谷內外大型底棲動物的生物群聚、密度、多樣性皆存在顯著差異，且可能受不同機制影響。靠近高屏海底峽谷頭部，受洪水引發異重流及內潮的擾動較強，海床的平均流速高，使富含有機物的粉砂和黏土沉降不易，因此生物密度及多樣性皆隨離峽谷頭距離漸遠及深度變深而增加。而內潮於峽谷內的作用普遍較峽谷外強，而底流流速則隨深度漸深而減緩，因此斜坡上沉積物的有機碳含量較峽谷為高，峽谷內外的有機碳含量也都隨著深度變深而增加。然而斜坡上大型底棲動物的平均密度卻跟沉積物有機碳含量呈負相關，且密度隨著深度變深而減少，此現象可能反映沉積物有機碳含量受到流體動力之影響，而未必代表底棲生物可使用的食物量，因為穩定環境的生物密度通常與食物供給呈正相關。而斜坡上隨深度下降的生物密度也可能減輕生物間的競爭壓力，進而促進分類群間共存，間接解釋分類群多樣性於斜坡隨深度漸深反而增加的趨勢。在高階的分類群中，多毛綱、線蟲綱、猛水蚤科在峽谷內外各採樣點皆佔有相當高的比例，而偏好穩定環境的軟甲綱小型節肢動物則不適應峽谷的擾動環境，主要出現在高屏斜坡。此外，在物理擾動流速較強的高屏峽谷，其大型底棲動物於沉積物中的垂直分布重心，顯著較環境相對穩定的高屏斜坡還深，此垂直分布重心的變化，可能受到峽谷環境移除底表棲性的軟甲綱節肢動物所致。面對全球氣候變遷，颱風及強降雨的變化勢必影響海底地質災害的頻率及強度，本研究探討大型底棲動物群聚結構隨時空變化之機制，將有助於預測峽谷底棲生態系統受氣候變遷之影響。Gaoping Submarine Canyon （GPSC） is the major pathway of terrestrial organic carbon into the deep South China Sea and also an area prone to frequent submarine geohazards. Despite our understanding on the sediment transports in the GPSC has accumulated over the years, their effects on the benthic ecology has never been studied. In order to understand how the spatial and temporal variations of the GPSC environments may affect the community structure of deep-sea macrobenthos, we repeatly sampled the upper GPSC and the adjacent slope （GPS） in April, August and November 2015. A total of 8 stations were sampled from 200 to 1100-m depths for macrofauna, bottom hydrography and sediment geochemistry. A 3-D, hydrostatic, internal tide model was also used to extract the bottom tidal current velocities. Our results suggest that the macrofaunal density, diversity and taxon composition were significantly different between the canyon and slope. Near the head region of GPSC, physical disturbance by hyperpycnal conditions and strong internal tide energy may depressed macrofaunal density and diversity. As the depth and distance increase away from the canyon head, the physical disturbance may be relieved and thus resulted in increase of macrofaunal density and diversity with depths. The internal tide energy was stronger in the canyon than on the slope but in gerneral the energy decreased with depths; therefore, organic carbon associated with the fine grain sediments was less likely to setttle in the canyon and near the shelf breaks, resulting in higher total organic carbon （TOC） contents in the slope sediments, as well as an increase of sedimentary TOC with depths. The macrofauna density on the slope, howerver, displayed a negative relationship with TOC and declined significantly with depth, suggesting that the TOC concentration may be controlled by hydrodnamic engery and not necessary relfected the food availability to the macrofauna. On the slope, the declined in macrofauna density also liklily relieved the compitions, promote coexistence and thus resulted in the significantly increasing diversity with depth. For the taxon composition, polychaetes, nematodes and harpacticoids thrived in both the canyon and slope, but peracarid crustaceans were either rare or disppeared from the canyon and mostly occurred on the relatively stable slope environments. In addition, the abundance-weighted mean vertical distribution of macrofauna was significantly deeper in the canyon sediments than that on the slope sediments, presumably, due to the removal of the epibenthic peracarid crustaceans in the canyon. In the wake of the global climate changes, the changes of storm and precipitations patterns likely lead to changes in frequency and intensity of submarine geohazards in the canyon. This study investigated the interpalys between submarine canyon environemts and benthic communities stucture. The reuslts will enhence our ability to predict the potential climate changes impacts on the submarine canyon ecosystems