Coral communities in a heavily impacted environment: historical changes, current risks and restoration potentials of Tolo Habour and Channel in Hong Kong

M.Phil.Coral communities in Tolo Harbour and Channel (THAC), northeastern Hong Kong, were severely degraded due to uncontrolled sewage pollutions in 1980s. This study followed the 30 years changes in these coral community structures, including conditions before their massive decline, during the peak of pollution, after introduction of pollution abatement scheme based on old records. Signs of coral degradations that lasted from 1980 to 1998 finally stopped in the inner habour in 2012. Some species of coral started to reappear. This could be a result of an overall improvement in water quality for the last 15 years, especially in the reduction of nutrient level, since full sewage diversion was completed in 1998. Yet, the coral cover remained very low ( 90%), suggesting coral transplantation can be a potential tool to accelerate coral recovery in Hong Kong. The four coral species examined, Acropora pruinosa, Pavona decussata, Platygyra acuta and Porites spp., showed differences in their survivorship and growth rate depending on their responses to different stresses, but were all considered suitable for use in transplantation. Degree of human activities, site specific mortality and abundance of potential corallivores were more important site factors than distance from pollution source in affecting transplantation success. Also, autumn may potentially be a better time to carry out transplant deployment than spring in Hong Kong.The sea urchin Diadema setosum appears to be an important destructive force to coral communities in northeastern Hong Kong. A series of laboratory experiments was therefore carried out to evaluate the potential damages, causes and interactions of D. setosum corallivory. The results showed serious tissue damages could be inflicted on Platygyra acuta by D. setosum at high density (17 ind. m-2). D. setosum predation on corals became non-significant when its major algal food was available. Heavily damaged corals due to D. setosum corallivory were also more susceptible to subsequent Drupella attack, another common corallivores in Hong Kong.A mass coral mortality event, of unprecedented scale and rate, occurred in THAC in the spring of 2015. Five out of six coral communities in outer Tolo Channel and in Hoi Ha Wan Marine Park (HHWMP) were heavily impacted. Moon Island in HHWMP lost about half of its live coral cover, and the other four sites suffered a loss of about 20% all within two months. Platygyra spp. suffered the highest mortality among other common corals. A shift in coral community structure in Moon Island and Coral Beach in HHWMP to one now even more highly dominated by Pavona was evident. This coral mortality event could be associated with two exceptionally intense blooms of Noctiluca scintillans, which occurred in Tolo Channel before the observed mortality. There was a general trend showing the correlation between higher mortality in coral communities with higher proximity to locations of blooms.The long-term study of coral community changes in THAC revealed how the habitat could be so easily destroyed and yet natural recovery could be so difficult, especially with the complete demise of adult corals and the lack of coral recruitment success. This present study suggested that transplantation of coral fragments is a feasible active restoration strategy in Hong Kong. Heavy coral damage due to over-population of D. setosum is identified as one of the major risks to current coral communities in THAC. Platygyra spp., one of the dominant corals in Hong Kong, are most susceptible to this threat and in the mass mortality event that may be associated with harmful algal bloom. Their continual dominance in these coral communities may be lost if the threats to their survival are not removed or neutralized.位於香港東北吐露港及赤門海峽的珊瑚群落，於1980年代因為過多的污水排放污染而遭受嚴重破壞。本研究探討了這些珊瑚群落於三十年期間的結構轉變，包括舊記錄中牠們在污染之前、在污染高峰期，及引入污水處理計劃後的情況。吐露內港珊瑚群落自1980至1998年的退化跡象終於在2012停止。一些珊瑚品種開始於吐露港內重現。這可以是因為通過吐露港污水輸出計劃在1998年起全面實施，吐露港整體水質，特別是富營養化，在過去十五年有了明顯的改善。但是，這些珊瑚群落的複蓋率仍然很低 (< 2%)。依靠珊瑚幼體的自然增添來復原這些珊瑚群落的可能性不大。因為除了捲曲黑星珊瑚 (Oulastrea crispata)，一個細小、對地形複雜性貢獻有限的品種，珊瑚幼新添入量非常低。相比之下，雖然赤門海峽的珊瑚群落在1980年代受到最少破壞的，過去十五年間複蓋率持續下降. 這可能與其他最近發生的生物干擾有關，如刺冠海膽 (Diadema setosum) 對優勢屬扁腦珊瑚 (Platygyra spp.) 的捕食和侵蝕。由於吐露港及赤門海峽的珊瑚群落不太可能自然恢復，本研究對珊瑚移植輔助珊瑚復原的可行性進行了調查。使用水泥移植天然珊瑚碎片的方法顯示出高存活率 (大多數超過90％)，表示珊瑚移植可以是有效加速香港珊瑚群落復原的措施。研究中測試的四個珊瑚品種， 霜鹿角珊瑚 (Acropora pruinosa) 、十字牡丹珊瑚 (Pavona decussata) 、 尖邊扁腦珊瑚 (Platygyra acuta)及濱珊瑚 (Porites spp.) ，對不同的壓力呈現出不同的存活率和生長率，但都適合用於移植。地點因素方面，相對於與污染源頭的距離，人類活動程度、個別地點死亡率與潛在珊瑚捕食者的數量，更影響移植的成敗。此外，在香港，秋季可能是比春季進行珊瑚移植的更好時機。刺冠海膽似乎正在嚴重破壞香港東北部的珊瑚群落。因此，本研究以一系列的實驗室實驗，評估了刺冠海膽的捕食對珊瑚的潛在損害，原因和相互作用。 結果顯示，高密度 (每平方米17隻) 的海膽捕食可對尖邊扁腦珊瑚的組織造成嚴重損傷。當主要的藻類食物可供其用時，刺冠海膽對珊瑚的捕食變得不顯著。因刺冠海膽的捕食而遭受嚴重損傷的珊瑚，隨後也更容易受到另一香港常見珊瑚捕食者核果螺 (Drupella spp.) 的攻擊。2015年的春季，在吐露港及赤門海峽發生了前所未有的大規模和快速的珊瑚死亡事件。赤門海峽和海下灣海岸公園的六個珊瑚群落中，有五個受到嚴重影響。 在兩個月內，海下灣海岸公園內的磨洲損失了大約一半的珊瑚複蓋，其他四個地點亦損失了大約20%的珊瑚複蓋率。扁腦珊瑚的死亡率是常見珊瑚中最高的。海下灣海岸公園中的磨洲和珊瑚灘的珊瑚群落結構，變得比從前更由十字牡丹珊瑚主導。此大規模珊瑚死亡事件可能與此前兩宗在赤門海峽，異常嚴重的夜光藻大量暴發有關。此事件中有一個總體趨勢，就是與夜光藻暴發的地點較接近的珊瑚群落有更高的死亡率。吐露港及赤門海峽珊瑚群落的長期變化研究，揭示了此生境可以如何被輕易地破壞，但自然恢復則如此困難。特別是因成年珊瑚完全滅亡，以及珊瑚幼新添入量非常低。本研究結果顯示，移植珊瑚碎片是復育香港珊瑚一可行策略。 當前這些珊瑚群落的主要風險之一是因刺冠海膽過多而造成的嚴重珊瑚損害。香港主要的珊瑚之一，扁腦珊瑚則最受到這種威脅及與有害藻華可能有關的珊瑚死亡事件所影響。 如果這些生存威脅沒有被消除或被抵銷，扁腦珊瑚可能將會喪失在這些珊瑚群落中的優勢。Wong, Kwan Ting.Thesis M.Phil. Chinese University of Hong Kong 2017.Includes bibliographical references (leaves 195-214).Abstracts also in Chinese.Title from PDF title page (viewed on …)