台灣與東沙環礁淺海的八放珊瑚群聚

Abstract

八放珊瑚為珊瑚礁生態系中重要的底棲生物，牠們常與石珊瑚相互競爭生存空間。為了瞭解八放珊瑚在台灣與東沙環礁淺海礁區的分布特性，本研究共選擇4個區域，包含台灣東北角、綠島、墾丁及東沙外環礁北區，每區3至6個測站，共調查了16個測站。每個測站分別做水深5及10公尺兩條穿越線的方框取樣調查，共調查了26條穿越線。方框影像資料利用CPCe (Coral Point Count with Excel extensions)來量化八放珊瑚與其他底棲生物的覆蓋率，進行定量分析。群集分析與MDS (nonmetric multidimensional scaling)分析的結果顯示，26條穿越線可分成5群 (A~E)，分別代表不同的八放珊瑚群聚。其中，A與B皆為綠島海域的八放珊瑚群聚，主要由異軟珊瑚科(Xeniidae)組成。C與D群包括台灣東北角、墾丁與東沙的八放珊瑚群聚，主要由軟珊瑚科(Alcyoniidae)組成。E群以恆春半島西岸的萬里桐測站為代表，屬於八放珊瑚覆蓋率極低的底棲群聚。綠島八放珊瑚群聚以Xenia、Heteroxenia、Cespitularia、與Clavularia等為主要組成屬，這些屬的物種大多生長在水質清澈的環境，此環境條件可能是促成異軟珊瑚科成為綠島海域八放珊瑚群聚主要組成的重要因子。除此之外，回歸分類樹 (classification and regression tree)的結果顯示，在藻類與石珊瑚覆蓋率較低或懸浮顆粒濃度低的環境中，八放珊瑚的覆蓋率較高，本研究顯示八放珊瑚的分布受到藻類、石珊瑚與懸浮顆粒濃度等環境因子的影響。

Octocorals are one of the major benthic organisms on coral reefs and they often compete for space with scleractinian corals. To reveal the distribution of octocoral communities in Taiwan, reef areas across a latitudinal gradient from northeastern Taiwan, Lyudao off southeastern Taiwan, Kenting in southern Taiwan, and Dongsha Atoll in the South China Sea were selected for field surveys. From 3 to 6 sites in each reef area and a total of 16 sites were surveyed. At each site, benthic communities at depths of 5 and 10 m were surveyed by the photoquadrat method and a total of 26 transects were surveyed. The covers of benthic groups on each transect were estimated with CPCe (Coral Point Count with Excel extensions). The results of cluster analysis and nonmetric multidimensional scaling (MDS) showed that the octocoral communities at 26 transects could be classified into 5 clusters (A~E). Clusters A and B including reef sites at Lyudao represent the octocoral communities dominated by species of Xeniidae. Clusters C and D including most reef sites at northeastern Taiwan, Kenting and Dongsha represent the octocoral communities dominated by species of Alcyoniidae. Cluster E including the sites with very low abundance of octocorals. The octocoral communities on Lyudao reefs are characterized by the dominance of Xenia, Heteroxenia, Cespitularia and Clavularia. Since these Xeniidae species prefer clear-water habitats, it is suggested that lower turbidity and sedimentation may play an important role in the formation and maintenance of octocoral communities on Lyudao reefs. Furthermore, the results of classification and regression tree (CART) showed that octocoral cover was negatively associated with the cover of space competitors and sedimentation. These results suggest that macroalgae, scleractinian corals, and environmental factors might be the main factors responsible for the distribution pattern of octocoral communities.