台灣北部鹽寮灣海域珊瑚群聚之研究

Abstract

台灣北部的鹽寮灣海域屬於亞熱帶及邊緣性的環境，珊瑚的生長多以珊瑚群聚的方式呈現，這些群聚的特徵是珊瑚覆蓋率和物種多樣性偏低以及珊瑚礁的發育受到限制。為了更清楚瞭解這些珊瑚群聚的動態變化，從2003年到2009年，在鹽寮灣海域的三處岩礁，以固定樣框的調查方式進行了以每半年為區間的長期監測研究。本研究的目的是：（1）調查鹽寮灣石珊瑚的群聚結構與特徵，（2）研究珊瑚群聚的動態和長期變化，（3）研究石珊瑚的幼生入添模式和入添率，（4）探討珊瑚群聚的變動與環境因子的關係。 以多維尺度分析法（ MDS ）分析12次連續的調查結果發現，在2005年7月與11月的兩次調查之間，珊瑚群聚出現明顯的變化，時間點與2005年夏天經過鹽寮灣的三個強烈颱風一致。由於珊瑚的生長形態隨種類而有所不同，其對颱風擾動的忍受度亦有所不同。比較2005年7月與11月的兩次調查發現，在局部死亡率(partial colony mortality)與整個群體死亡率(whole colony mortality)上，不同型態的珊瑚出現明顯差異。其中葉片型和表覆形的珊瑚例如微孔珊瑚屬(Montipora)、波紋珊瑚屬(Pachyseris)和碓珊瑚屬(Hydnopora)等死亡率較高，而團塊形的珊瑚包括菊珊瑚屬(Favia)、角菊珊瑚屬(Favites)、細菊珊瑚屬(Cyphastrea)及圓菊珊瑚屬(Montastrea)等在颱風擾動後死亡率較低。 從2006年5月開始至2009年9月，利用附著板在本海域進行珊瑚群聚的幼生入添研究，其結果顯示，珊瑚幼生主要出現在夏至秋季，以鹿角珊瑚科(Pocilloporidae，52〜90％）和軸孔珊瑚科（Acroporidae，10〜41％）幼生最為常見。本研究也發現珊瑚幼生具有附著偏好，幼生在附著板上表面和垂直面的數量和水深呈現負相關，顯示在這些亞熱帶珊瑚群聚，光照強度可能是控制亞熱帶及邊緣性珊瑚群聚珊瑚幼生附著和生存的主要因素。此外，珊瑚幼生的入添率(recruits/m2)具有顯著年間變動，從2006年和2007年的結果來看，幼生入添率和一般熱帶珊瑚礁的入添率相當接近，這顯示珊瑚幼生的入添率很可能不是鹽寮灣珊瑚群聚發展和維持的限制因子。 經由分析珊瑚覆蓋率和環境因子之間的關係發現，鹽寮灣的磷酸鹽，總氮和總磷的濃度與珊瑚覆蓋率在時間序列的變化上呈現正相關。此結果顯示，在光強度較低的亞熱帶環境中營養鹽可能對於珊瑚的生長是有益處的。推測可能是豐富的營養鹽提升了共生藻的光合作用因而促進了珊瑚生長。 綜合而論，本研究的結果顯示，在鹽寮灣的亞熱帶珊瑚群聚容易受到颱風的擾動影響而出現明顯的年間變動。颱風擾動引起的珊瑚死亡率會因珊瑚的生長形態不同而有所差異，從而導致珊瑚群聚的變動。由於鹽寮灣的幼生入添率與熱帶珊瑚礁入添率相當，此海域的珊瑚群聚的恢復和維持可能不是受到幼生入添率的限制。此外因為無機營養鹽和珊瑚的覆蓋率之間有相當一致的正相關，營養鹽可能對珊瑚群聚的恢復是有益的。由於邊緣型的環境可能作為熱帶地區的造礁珊瑚面臨氣候變遷威脅時的避難所，本研究的結果可以增加我們對邊緣型環境中的珊瑚群聚動態的了解，有利於面臨日益嚴重環境壓力的珊瑚群聚的管理和保育。

Coral communities in Yenliao Bay were characterized by low coral cover, low species diversity and limited reef-building activities due to its subtropical and marginal environment for reef corals. To better understand the dynamics of these coral communities, a long-term monitoring study by permanent belt quadrats at semi-annual intervals was conducted on three rocky reefs in Yenliao Bay, northern Taiwan, from 2003-2009. The objectives of this study were (1) to examine the general characteristics of the scleractinian coral communities, (2) to examine the dynamics and long-term changes of coral communities, (3) to study the recruitment pattern and recruitment rate of scleractinian corals, (4) to investigate the possible environmental factors related to the changes of coral communities in Yenliao Bay. Multidimensional Scaling (MDS) analysis of coral communities among 12 consecutive surveys showed distinct temporal variations and the major change occurred between July 2005 and November 2005 which was coincident with major typhoon disturbances in summer 2005. Coral species with different growth forms varied greatly in partial and whole colony mortalities due to their susceptibility to typhoon disturbances. Foliaceous and encrusting corals such as Montipora, Pachyseris, and Hydnopora species suffered higher mortality, while massive corals including Favia, Favites, Cyphastrea, and Montastrea species suffered lower mortality after typhoon disturbances. Coral recruitment in these subtropical coral communities was studied by settlement plates from May 2006 to September 2009. The results showed that coral recruits occurred in summer and early autumn only and the most common taxa were Pocilloporidae (52~90%) and Acroporidae (10~41%). The number of coral recruits on top and vertical surfaces was negatively correlated with depths suggesting that light intensity is possibly the primary factor controlling settlement and survival of coral recruits in these subtropical coral communities. In addition, there was a large variation of recruitment rates (recruits/m2) among years. The high recruitment rates in 2006 and 2007 were comparable with those of tropical reefs suggesting that recruitment might not be a limiting factor for the maintenance and development of coral communities in Yenliao Bay. Studies on the relationships between coral coverage and environment factors showed that the concentrations of phosphate, total nitrogen, and total phosphorus were positively correlated with the changes of coral coverage among years in Yenliao Bay. This suggests that nutrients enrichment might be beneficial for coral growth through the enhancement of photosynthesis of zooxanthellae in subtropical environment where light intensity is relatively low. In conclusion, this study demonstrated that the subtropical coral communities in Yenliao Bay were susceptible to typhoon disturbances and large changes might occur among years. Typhoon disturbances induced differential mortality to coral species related to their growth forms, hence resulted in the changes of coral communities. The recovery and maintenance of coral communities was possibly not limited by recruitment since the high recruitment rates were comparable with those of tropical reefs. Besides, nutrients enrichment might be beneficial to the recovery of coral communities since there were consistent positive correlations between dissolved inorganic nutrients and coral coverage among years. Since marginal coral areas might act as refuges for tropical reef corals to face with the threats of climate change, this study can contribute to our knowledge of coral community dynamics in marginal environment and benefit the management and conservation of coral community facing the increasing environmental stresses in the future.