探討淺水域至中光層珊瑚礁過渡帶之底棲組成的區域變異

Abstract

深度及隨深度變化的光強度（light intensity）與光質（light quality）影響底棲生物分布與組成甚鉅，但目前珊瑚礁研究僅著重於淺水域的調查，忽視珊瑚生態系的垂直變異。本研究探討了台灣底棲生物群集（benthic community）與珊瑚群聚（coral assemblage）橫跨緯度與深度的轉變（transition）。實驗區域選定北台灣、綠島以及小琉球，各區域中分別設立三個樣點，並在每個樣點的三個深度（5公尺、15公尺及30公尺深）設置三至五條20公尺長的穿越線，每條穿越線各拍攝21張照片，同時於拍攝期間測量光合作用有效輻射（photosynthetically active radiation; PAR）。最終手動標記來自117條穿越線，共2,457張底棲生物影像，以紀錄生物組成。結果顯示，在較高的PAR條件下，石珊瑚及毛狀藻類生長良好，而在低PAR、深度大的環境中，軟珊瑚、黑珊瑚、柳珊瑚以及殼狀珊瑚藻的覆蓋率增加。底棲生物群集與珊瑚群聚的β多樣性（β-diversity）主要由物種替換（turnover）所驅動，且在光衰減（light attenuation）較快的副熱帶區域尤其明顯。此高物種替換驅使了底棲生物群集從淺水域至中光層的轉變，也使副熱帶地區的動植物群間斷（faunal/floral break）發生在更淺的深度。本研究顯示，相較於深度，光的可用性更適合定義中光層區域的邊界。此外，不同區域的中光層珊瑚擁有獨特的群聚組成，因此儘管中光深度的珊瑚覆蓋率較低，這些群集在生態上仍然與淺水域群集扮演著一樣重要的角色。本研究強調，台灣深層珊瑚生態系的多樣性及生態功能未受到足夠重視，管理者應重新考慮當前珊瑚礁保育所囊括的深度範圍，並將深層珊瑚生態系納入保育規劃中。

Depth influences the composition of the benthic community, as light intensity and quality affect the distribution of benthic organisms. However, changes are often overlooked in coral reef research which focuses primarily on shallow-water reefs. This study investigated transitions in benthic communities across depths (5 m, 15 m, and 30 m) in each of the three sites within three regions of Taiwan (North Taiwan, Green Island, and Xiaoliuqiu). At each location (sites × depths), three to five 20-meter-long transects were established, and 21 photographs were taken along each transect, with photosynthetically active radiation (PAR) measured during the benthic survey. A total of 2,457 benthic images from 117 benthic transects were manually annotated to document organism composition. Stony corals and turf algae thrive under higher PAR, while the cover of soft corals, black corals and gorgonian corals, and crustose coralline algae increase at greater depths where light is limited. β-diversity among benthic communities and coral assemblages is primarily driven by turnover and is especially pronounced in subtropical areas where light attenuation is higher. This high turnover drives shallow-to-mesophotic transitions and results in faunal/floral break occurring at shallower depths in subtropical region compared to tropical ones. Our results support that light availability, rather than depth alone, defines the boundary of the mesophotic zone. Additionally, mesophotic coral assemblages are highly site-specific and often host unique assemblages. Despite lower coral cover at mesophotic depths, these communities may hold ecological value comparable to their shallow-water counterpart. This study underscores the need for managers to think "deeper" about the current extent of coral reefs. The diversity and ecological functions of deeper reef ecosystems in Taiwan remain underappreciated and merit greater attention to ensure their protection in future conservation strategies.