環境資訊中心外電；姜唯 翻譯；林大利 審校；稿源：ENS
南佛羅里達大學聖彼得堡海洋科學院科學家透過美國太空總署（NASA）衛星觀測技術，發現褐藻異常增生現象，範圍從西非延伸到墨西哥灣，規模之大前所未見，而且很可能不是短期現象。
因為由馬尾藻（Sargassum）組成，科學家將其命名為大西洋馬尾藻帶（Great Atlantic Sargassum Belt）。馬尾藻是一種大型褐藻，會大面積增生，就像一座浮島。
2011年，馬尾藻開始在過去不曾出現的地方大量生長，如大西洋中部，接著大規模抵達沿岸，影響當地環境和經濟。
墨西哥坎昆海岸邊的馬尾藻。2015年資料照片，NASA提供。
華盛頓NASA總部生態預測專案經理特納（Woody Turner）說，「這幾波藻華規模大到讓全球衛星圖成為偵測和追蹤其動態的絕佳工具。」
研究主持人、南佛羅里達大學海洋科學院學者胡傳民將這個發現發表在。他從2006年就開始使用衛星研究馬尾藻。
胡傳民說，「海洋的化學反應一定發生了什麼變化，才導致藻類增生失控」。馬尾藻透過親代個體的片段繁殖，在大西洋周圍可能有多個初始生長區域。當養分條件有利，生理時鐘也適合繁殖，便加速生長。
根據電腦模擬，研究人員證實，大西洋馬尾藻帶的形狀順著洋流成形。其規模之大，覆蓋了從非洲西海岸到墨西哥灣的熱帶大西洋表面。
左圖：佛羅里達州大鬆礁島沿岸，馬尾藻異常增生的現象。NASA/佛羅里達大西洋大學提供，Brian Lapointe拍攝。右圖：海豚在藻類增生的海域中生活，NASA提供，王孟秋拍攝。
2018年，超過2000萬噸馬尾藻漂浮在海上（這重量比200艘滿載航空母艦還重），影響熱帶大西洋、加勒比海、墨西哥灣和佛羅里達東海岸線上的休閒活動。
胡傳民與南佛羅里達大學光學海洋學實驗室的博士後學者王孟秋博士合作完成這項研究。研究團隊集結來自南佛羅里達大學、佛羅里達大西洋大學和喬治亞理工學院的學者。
馬尾藻的生態功能是提供海洋生物棲息地。王孟秋去年在墨西哥灣實地考察時，看到海豚在馬尾藻中游泳。
科學家分析NASA 2000至2018年間的中分辨率成像光譜儀（MODIS）資料發現，2011年起，馬尾藻增生時可能引起生態系變遷（regime shift），即導致生態系快速重組，變成新的穩定狀態。
王孟秋說，在衛星圖像中，除了2013年之外，2011年至2018年期間每年都大量繁殖。加上實地測量結果顯示2013年沒有發生增生，因為2012年冬季馬尾藻的種子數量異常低。
在2011年之前，海洋中自由移動的馬尾藻多是在墨西哥灣和馬尾藻海周圍發現。馬尾藻海位於大西洋中部西邊，因馬尾藻多而得名。在開闊的海洋中，一片片的馬尾藻群提供海龜、螃蟹、魚類和鳥類棲息地，並像其他植物一樣，透過光合作用產生氧氣，維持海洋健康。但是一旦過多便會排擠海洋生物，特別是在海岸附近。
科學家分析NASA 2000至2018年間的中分辨率成像光譜儀（MODIS）資料，發現大西洋馬尾藻帶的存在。本圖由NASA/Earth Observatory利用王孟秋、胡傳民資料製圖而成。
該團隊確立了馬尾藻過度增生的關鍵因素：冬季因為前一波增生留下大量種子，加上冬季西非湧升流的養分，以及亞馬遜河春季或夏季帶來的養分。
科學家認為近年來森林砍伐和肥料使用增加，排放的養分有可能增加，不過胡傳民強調養分過多的證據仍在初始階段，而且現有資料有限，團隊需要更多的研究來確認這個假說是否為真。此外，馬尾藻只有在鹽度正常且表面溫度正常或較冷時才會長得好。
NASA地球科學部代理副主任、負責管理NASA海洋生物和生物地球化學計畫的龐德比（Paula Bontempi）博士說：「海洋的生物地球化學正在因自然和人類的力量改變。大西洋馬尾藻帶顯示我們可能正在經歷海洋的生態系的變化，可能對人類賴以生存的海洋生物和生態系服務產生重要影響」
「這最終都與氣候變遷有關，因為氣候影響降水和洋流甚至人類活動（可能導致馬尾藻增生），但我們已經證實，馬尾藻增生不是水溫升高所造成，而是某種長期變化，它們可能會一直待在那裏。」
World’s Biggest Algae Bloom ‘Here to Stay’ ST. PETERSBURG, Florida, July 9, 2019 (ENS)
The largest bloom of macroalgae in the world, an unprecedented belt of brown algae, now stretches from West Africa to the Gulf of Mexico – and it’s likely here to stay warn scientists at the University of South Florida in St. Petersburg’s College of Marine Science. They used NASA satellite observations to discover and document the bloom.
The scientists named it the Great Atlantic Sargassum Belt because it is composed of Sargassum, a genus of large brown seaweed, a type of algae, that floats in island-like masses.
In 2011, Sargassum populations started to explode in places it had never been before, like the central Atlantic Ocean, and then it arrived in huge gobs that suffocated shorelines and introduced a new nuisance for local environments and economies.
“The scale of these blooms is truly enormous, making global satellite imagery a good tool for detecting and tracking their dynamics through time,” said Woody Turner, manager of the Ecological Forecasting Program at NASA Headquarters in Washington.
Chuanmin Hu of the USF College of Marine Science, who led the study, published in the journal “Science,” has studied Sargassum using satellites since 2006.
“The ocean’s chemistry must have changed in order for the blooms to get so out of hand,” Hu said. Sargassum reproduces from fragments of the parent plant, and it probably has several initiation zones around the Atlantic Ocean. It grows faster when nutrient conditions are favorable, and when its internal clock ticks in favor of reproduction.
Based on computer simulations, the researchers confirmed that this belt of the brown macroalgae Sargassum forms its shape in response to ocean currents. It can grow so large that it blankets the surface of the tropical Atlantic Ocean from the west coast of Africa to the Gulf of Mexico.
In 2018, more than 20 million tons of it – heavier than 200 fully loaded aircraft carriers – floated in surface waters and became a problem to shorelines lining the tropical Atlantic, Caribbean Sea, Gulf of Mexico, and east coast of Florida, carpeted popular beach destinations and crowded coastal waters.
Hu spearheaded the work with first author Dr. Mengqiu Wang, a postdoctoral scholar in his Optical Oceanography Lab at University of South Florida, USF. The team included others from USF, Florida Atlantic University, and Georgia Institute of Technology.
Sargassum contributes to ocean health by providing habitat for marine life. Dr. Wang was doing fieldwork in the Gulf of Mexico last year when she saw dolphins appearing to enjoy their swim through the Sargassum.
The data they analyzed from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) between 2000-2018 indicates a possible regime shift in Sargassum blooms since 2011.
In the satellite imagery, major blooms occurred in every year between 2011 and 2018 except 2013. This information, coupled with field measurements, suggests that no bloom occurred in 2013 because the seed populations of Sargassum measured during winter of 2012 were unusually low, Wang said.
Before 2011, most of the free-floating Sargassum in the ocean was found in patches around the Gulf of Mexico and Sargasso Sea. The Sargasso Sea is located on the western edge of the central Atlantic Ocean and named after its popular algal resident. In patchy doses in the open ocean, Sargassum contributes to ocean health by providing habitat for turtles, crabs, fish, and birds and, like other plants, producing oxygen via photosynthesis. But too much of this seaweed can crowd out marine species, especially near the coast.
The team identified key factors that are critical to bloom formation: a large seed population in the winter left over from a previous bloom, nutrient input from West Africa upwelling in winter, and nutrient input in the spring or summer from the Amazon River.
They think that discharged nutrients may have increased in recent years due to increased deforestation and fertilizer use, though Hu noted that the evidence for nutrient enrichment is preliminary and based on limited available data, and the team needs more research to confirm this hypothesis. In addition, Sargassum only grows well when salinity is normal and surface temperatures are normal or cooler.
“Earth’s ocean biogeochemistry is changing in response to natural and human forcings. The Great Atlantic Sargassum Belt suggests that we may be witnessing ecosystem shifts in our ocean that could have important implications for marine organisms and ecosystem services, which humans depend on,” said Dr. Paula Bontempi, who manages NASA’s Ocean Biology and Biogeochemistry Program and serves as acting deputy director of NASA’s Earth Science Division at NASA Headquarters.
“This is all ultimately related to climate change, as climate affects precipitation and ocean circulation and even human activities [that can lead to Sargassum blooms], but what we’ve shown is that these blooms do not occur because of increased water temperature,” Hu said. “They are probably here to stay.”
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