環境資訊中心外電；姜唯 翻譯；林大利 審校；稿源：ENS
一篇發表在發現，氣候暖化對大堡礁造成的破壞已經損害了珊瑚自我恢復的能力。
這項獨特的研究沿著大堡礁全長測量存活的成年珊瑚數量，以及2018年回補的新珊瑚數量。與大規模珊瑚白化前的水準相比，成年珊瑚減少導致整體珊瑚數量下降。
2016-2017年大規模白化事件後，科學家在大堡礁放置15處陶磚，研究珊瑚復原狀況。圖片來源：ARC CoE for Coral Reef Stdies/ Gergely Torda
「繼2016年和2017年全球暖化造成前所未有的成年珊瑚減損後，大堡礁的新珊瑚數量下降了89％。」研究主要作者、詹姆斯庫克大學ARC珊瑚礁研究卓越中心主任休斯（Terry Hughes）教授說。
大堡礁在1998年、2002年以及2016年和2017年經歷了四次大規模白化。科學家們預測，隨著全球暖化加劇，珊瑚白化事件之間的間距將繼續縮小。
「每年產出的珊瑚幼蟲數量，以及牠們定居珊瑚礁之前活動的地點，都跟大堡礁恢復力息息相關。我們的研究顯示，珊瑚礁的恢復能力現在嚴重受到全球暖化影響。」共同作者貝爾德（Andrew Baird）教授說。
「回補量降幅最大，與前幾年相比下降了93％，發生在優勢種鹿角珊瑚（Acropora）。成年珊瑚是數千種物種的棲息地。」他說。
「幼年珊瑚的比例已經發生變化，這將影響未來成年珊瑚的比例，因為在接下來的十年或更長時間內，恢復速度將變慢。」貝爾德說。
「珊瑚添入量的減少與大堡礁不同地區珊瑚成體的死亡率相關。失去珊瑚最多的地方，新增個體量的下降幅度最大。」
「假設未來十年內不發生另一次大規模白化，珊瑚的添入會逐漸恢復，因為存活的珊瑚會增長，會有更多的珊瑚達到性成熟。」
「但未來十年肯定會發生第五或第六次大規模白化的，」共同作者普拉切特（ Morgan Pratchett）教授說，「我們曾經以為大堡礁這麼大，韌性很夠，現在不這麼想了。」
「例如，當大堡礁有部分被颶風破壞時，周圍的珊瑚礁可幫助回補幼蟲。但是現在，2016年和2017年極端高溫造成的破壞範圍接近1500公里，遠遠大於颶風路徑。」
普拉切特教授說，逃過白化的南部珊瑚礁狀況仍然很好，但距離太遠，無法幫忙北部珊瑚礁。
「只有一種方法可以解決這個問題，就是盡快將淨溫室氣體排放減少到零，解決全球暖化的根本原因。」
有些陶磚上出現珊瑚藻覆蓋，狀況良好。圖片來源：ARC CoE for Coral Reef Stdies/ Gergely Torda
大堡礁面對的問題還不只這樣。最近昆士蘭州北部的洪水以及大堡礁上大量污水外排使人們開始關注水質對珊瑚礁健康的影響。
但ARC卓越中心的新研究顯示，光是水質得到改善這一點還不足以拯救大堡礁。
發生洪水的昆士蘭河流已讓數百萬公升污水流入珊瑚礁，但直到現在，這起事件對珊瑚礁和海洋生物的影響仍難以評估。
ARC卓越中心、阿德萊德大學和加拿大達爾豪斯大學的研究人員利用先進的衛星成像結合20多年來珊瑚礁監測的資料發現，長期暴露於較差水質限制了大堡礁的回復率。
「我們發現影響大堡礁最主要的污染形式是徑流污染，這大大降低了珊瑚對多種干擾的抵禦能力，特別是在近海地區。」主要作者達爾豪斯大學的麥克尼爾（Aaron MacNeil）博士說。
麥克尼爾表示，水質差導致珊瑚從干擾中恢復的速度降低了25％，他認為改善水質可以提高珊瑚礁的恢復率。
然而水質的影響有限。透過一系列模擬未來氣候變遷和珊瑚白化的可能性，研究小組發現，不管水質變得多好，都無法維持當前最具旅遊觀賞價值的外層珊瑚礁覆蓋水準。
「研究結果顯示，解決大堡礁面臨的威脅沒有靈丹妙藥，」麥克尼爾博士說。
「雖然減少河川徑流污染對珊瑚礁有廣泛的正面影響，應繼續獲得支持。但對於不受水質影響的珊瑚礁區域，我們的重點仍需放在減少碳排放，減緩氣候變遷，」他說。
「我們必須給珊瑚礁恢復的時間和條件，」麥克尼爾說，「否則珊瑚礁美景很快就會消失，從目前的形態變得難以辨識。」
若碳排放沒有減少，溫室氣體珊瑚礁美景很快就會消失。圖片來源：Paul Asman and Jill Lenoble（CC BY 2.0）
Climate Change Disrupts Recovery of Great Barrier Reef TOWNSVILLE, Queensland, Australia, April 13, 2019 (ENS)
The damage caused to the Great Barrier Reef by the warming climate has compromised the capacity of its corals to recover, finds new research published earlier this month in the journal “Nature.”
The unique study measured how many adult corals survived along the length of the world’s largest reef system following extreme heat stress, and how many new corals they produced to replenish the Great Barrier Reef in 2018.
The loss of adults resulted in a crash in coral replenishment compared to levels measured in the years before mass coral bleaching.
“The number of new corals settling on the Great Barrier Reef declined by 89 percent following the unprecedented loss of adult corals from global warming in 2016 and 2017.” said lead author Professor Terry Hughes, director of the ARC Centre of Excellence for Coral Reef Studies at James Cook University.
To date, the Great Barrier Reef has experienced four mass bleaching events due to global warming, in 1998, 2002, and back-to-back in 2016 and 2017. Scientists predict that the gap between pairs of coral bleaching events will continue to shrink as global warming intensifies.
“The number of coral larvae that are produced each year, and where they travel to before settling on a reef, are vital components of the resilience of the Great Barrier Reef. Our study shows that reef resilience is now severely compromised by global warming,” said co-author Professor Andrew Baird.
“The biggest decline in replenishment, a 93 percent drop compared to previous years, occurred in the dominant branching and table coral, Acropora. As adults these corals provide most of the three-dimensional coral habitat that support thousands of other species,” he said.
“The mix of baby coral species has shifted, and that in turn will affect the future mix of adults, as a slower than normal recovery unfolds over the next decade or longer,” said Baird.
“The decline in coral recruitment matches the extent of mortality of the adult broodstock in different parts of the Reef,” said Hughes. “Areas that lost the most corals had the greatest declines in replenishment.”
“We expect coral recruitment will gradually recover over the next five to 10 years, as surviving corals grow and more of them reach sexual maturity, assuming of course that we don’t see another mass bleaching event in the coming decade,” he said.
“It’s highly unlikely that we could escape a fifth or sixth event in the coming decade,” said co-author Professor Morgan Pratchett. “We used to think that the Great Barrier Reef was too big to fail – until now.”
“For example, when one part was damaged by a cyclone, the surrounding reefs provided the larvae for recovery. But now, the scale of severe damage from heat extremes in 2016 and 2017 was nearly 1,500 kilometers, vastly larger than a cyclone track.”
Professor Pratchett said the southern reefs that escaped the bleaching are still in very good condition, but they are too far away to replenish reefs further north.
“There’s only one way to fix this problem,” says Hughes, “and that’s to tackle the root cause of global heating by reducing net greenhouse gas emissions to zero as quickly as possible.”
Flooding Dumps Contaminated Water on the Reef
The world’s largest reef faces other issues, too. Recent north Queensland flooding and the mass outflows of polluted water onto the Great Barrier Reef have focused attention on the impact of water quality on the reef’s health.
But new research from the ARC Centre of Excellence reveals that even if water quality is improved, that alone won’t be enough to save the Great Barrier Reef.
Flooded Queensland rivers have dumped millions of liters of polluted water onto the reef, but until now the impact of these events on reef corals and marine life has been tough to assess.
Using a combination of advanced satellite imaging and over 20 years of coral monitoring across the reef, a team of researchers from ARC Centre of Excellence, the University of Adelaide, and Canada’s Dalhousie University has found that chronic exposure to poor water quality is limiting the recovery rates of corals across wide swathes of the Great Barrier Reef.
“We found the Great Barrier Reef is an ecosystem dominated by runoff pollution, which has greatly reduced the resilience of corals to multiple disturbances, particularly among inshore areas,” said lead author Dr. Aaron MacNeil of Dalhousie University.
Poor water quality reduced the rates at which corals recover after disturbances by up to 25 percent, said MacNeil, who believes that by improving water quality, the rates of reef recovery can be enhanced.
Yet the effects of water quality only go so far. Using a series of scenarios modeling future changes in climate and the likelihood of coral bleaching, the team found that no level of water quality improvement was able to maintain current levels of coral cover among the most scenic and valuable outer-shelf reefs that sustain much of the reef tourism industry.
“What these results emphasize is that there is no silver bullet for addressing the threats facing the Great Barrier Reef,” said Dr. MacNeil.
“Clearly reducing pollution in river runoff can have widespread, beneficial effects on reef corals and should continue to be supported. But for areas of the reef not impacted by water quality, our emphasis must be on mitigating carbon emissions to slow down climate change,” he said.
“We must give our reefs the time and conditions to recover,” MacNeil said. “Without that, the most stunning and iconic parts of the reef will soon decline and be unrecognizable from their current form.”
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