海洋プランクトンと気候変動　食物連鎖への多大なる影響

【プリマスＡＦＰ＝時事】海から船の甲板に引き揚げられた一風変わった金属の箱。まるで子どもの想像の世界で捕らえた宇宙船のようだ──。（写真英プリマスで調査船に引き上げられた「連続プランクトン採集器」を開ける科学者のクレア・オストル氏）
　箱を開けて、中からシルク製のフィルターを引き出すのは、科学者のクレア・オストル氏。地球上で最も重要な生き物の一つ、植物プランクトンの鮮やかな緑色を探す。
　この魚雷形の箱は「連続プランクトン採集器（ＣＰＲ）」と呼ばれるもので、過去９０年にわたっていくつもの採集器が商船や漁船に引かれ、世界中の航路を回ってきた。
　極小の海洋プランクトンの採集は、海に関する科学的理解の促進に役立つ。
　これまでの調査により、気候変動で海水温が上昇するとプランクトンの移動が起き、海洋生物と人類の双方にとって深刻な結果をもたらす可能性があることが分かっている。
　水中に浮遊し、波まかせで動くプランクトンは、海洋の食物連鎖の基盤をなす。同時に私たち人類の生存を支えている、複雑な均衡の上に成り立つ生態系の一部でもある。人間が呼吸する酸素の多くを生み出し、また地球全体の炭素循環で重要な役割も果たしている。
　ＣＰＲを使い海洋生物調査を実施する国際ＮＰＯ「ＣＰＲサーベイ」でコーディネーターを務めるオストル氏は、英南西部プリマス沖の船上でＡＦＰに「私たちは温暖化という一大事を目にしています」と語った。
　ＣＰＲサーベイの記録を見ると、海流が変化する中、ここ数十年で海洋プランクトンが両極方向へと移動していること、また多くの海洋哺乳類も温度の低い地域へと移っていることがはっきりと示されている。
　こうした移動によって、栄養価の高い冷水性プランクトンは、暖水性の小さなプランクトンに置き換わっている。そのため、プランクトンを餌にしている生物は環境に適応するか、もしくはプランクトンに合わせ移動を余儀なくされる。これら冷水性と暖水性のプランクトンは、季節サイクルが違う場合も多い。
　「変化が急すぎると生態系が回復できません。それが大きな懸念です」とオストル氏は述べ、劇的な気温上昇が「水産業全体を崩壊させる」こともあり得ると警告する。
　人類の約半数は動物性たんぱく質の約２０％を魚に依存しているため、壊滅的な事態となりかねない。

■生物ポンプ
　プランクトンには植物プランクトン（いわゆる藻類）と動物プランクトン（極小動物や魚やカニなどの幼生）がいる。このうち植物プランクトンは光合成を行い、二酸化炭素（ＣＯ２）をエネルギーと酸素に変える。
　地球上の酸素の約半分は海洋で生成されていると科学者らは推定しているが、そのほとんどを担っているのは植物プランクトンだ。
　さらにプランクトンは海の「生物ポンプ」にとっても不可欠だ。生物ポンプとは大気中のＣＯ２を海が吸収するメカニズムのことで、化石燃料の燃焼によって生じたＣＯ２のうち少なくとも４分の１は生物ポンプによって海に取り込まれる。
　樹木は木の幹や葉に炭素を蓄えるが、植物プランクトンは体内に炭素を蓄える。蓄積された炭素は食物連鎖を通過する。植物プランクトンを食べた動物プランクトンは、鳥類からクジラまでさまざまな生き物の餌となる。死んだプランクトンや、その捕食生物が排出した炭素を含んだ有機物は海底に沈む。
　だが、科学者らは気候変動がこのシステムに負荷をかけていると警告する。海温が上昇し、海の深部から上層に養分が移動しにくくなり、海水を酸化させるＣＯ２の量も増加しているという。

■わずかな減少でも影響は増幅
　植物プランクトンは比較的回復力が強く、海が温暖化すれば生息域を移動し続けるだろう。だが、国連（ＵＮ）の気候変動に関する政府間パネル（ＩＰＣＣ）は来年公表予定の報告で、海洋の状態がさらに悪化すれば、今世紀中に植物プランクトン全体の減少につながると予想している。
　植物プランクトンの生物量（バイオマス）の世界平均は、今後、温室効果ガス排出量のレベルに応じて１．８～６％減少すると予測されている。
　だが、プランクトンの重要度は非常に高く、わずかな減少でもその影響は「海洋食物網の中で増幅」し、最終的には約５～１７％の海洋生物が減少するとＩＰＣＣは見ている。
　英国で始まった国連気候変動枠組み条約第２６回締約国会議（ＣＯＰ２６）に世界の指導者が出席する中、海洋プランクトンの問題は、加速する人間活動の影響で複雑な生命維持システムが揺らいでいる事実を明白に指し示すものとなっている。【翻訳編集ＡＦＰＢＢＮｅｗｓ】

〔ＡＦＰ＝時事〕（2021/11/05-12:45）

The strange metal box hauled from the waves and onto the ship's deck looks like a spaceship fished from a child's imagination.
But when scientist Clare Ostle opens it up and draws out the silk scrolls inside, she is looking for the telltale green glow from some of the most important creatures on Earth: plankton.
This is a Continuous Plankton Recorder, torpedo-like devices that for 90 years have been towed by merchant vessels and fishing boats on a vast network of routes.
They help researchers better understand the ocean by collecting some of its smallest inhabitants.
What they have seen is that as climate change heats the seas, plankton are on the move -- with potentially profound consequences for both ocean life and humans.
Plankton -- organisms carried on the tides -- are the foundation of the marine food web.
But they are also part of an intricately balanced system that helps keep us all alive.
As well as helping produce much of the oxygen we breathe, they are a crucial part of the global carbon cycle.
The big thing that we're seeing is warming, Ostle, coordinator of the Pacific CPR Survey, tells AFP as she demonstrates the plankton recorder off the coast of Plymouth in Britain.
The CPR Survey has documented a decisive shift of plankton towards both the poles in recent decades, as ocean currents change and many marine animals head for cooler areas.
Smaller warm water plankton are also replacing more nutritious cold water ones, often also with differing seasonal cycles, meaning the species that feed on them need to adapt or move too.
The big worry is when change happens so quickly that the ecosystem can't recover, says Ostle, adding that dramatic temperature spikes can lead whole fisheries to collapse.
With nearly half of humanity reliant on fish for some 20 percent of their animal protein, this could be devastating.
- Biological pump -
Plankton is a catch-all term from the Greek for drifting and encompasses everything from photosynthesising bacteria many times smaller than the width of a human hair, to jellyfish with long trailing tendrils.
There are two main types: phytoplankton, diverse plant-like cells commonly called algae; and zooplankton, animals like krill and the larvae of fish, crabs and other marine creatures.
Phytoplankton photosynthesise using the sun's rays to turn C02 into energy and oxygen.
In fact, scientists estimate the seas produce around half the oxygen on Earth, and that is mostly thanks to phytoplankton.
They are also crucial to the ocean's biological carbon pump, which helps the sea lock away at least a quarter of C02 emitted by burning fossil fuels.
While trees store carbon in wood and leaves, phytoplankton store it in their bodies.
It passes through the food web, with phytoplankton consumed by zooplankton which, in turn, are eaten by creatures from birds to whales.
Pretty much everything you can think of in the sea at some stage of its life cycle will eat plankton, says CPR Survey head David Johns.
When organic matter from dead plankton or their predators sinks to the ocean floor it takes carbon with it.
- 'Escalating impacts' -
But scientists warn that climate change has stressed the system, with ocean temperatures rising, fewer nutrients reaching the upper part of the ocean from the deep and increased levels of C02 acidifying seawater.
Climate change has exposed ocean and coastal ecosystems to conditions that are unprecedented over centuries to millennia with consequences for ocean-dwelling plants and animals around the world, says the UN's Intergovernmental Panel on Climate Change (IPCC) in a leaked draft report on climate impacts, due to be published next year, which predicts escalating impacts on marine life.
While phytoplankton are relatively resilient and will likely continue to shift territory as the seas warm, the IPCC expects that deteriorating conditions in the oceans will ultimately lead to an overall decline this century.
Average global phytoplankton biomass -- a measure of total weight or quantity -- is predicted to fall by around 1.8 to six percent, depending on the level of emissions.
But because of its outsized importance, even modest reductions can amplify up the marine food web, eventually leading to reductions in marine life by roughly five to 17 percent.
There could also be changes in carbon cycling and carbon sequestration, as our plankton community changes with smaller plankton potentially drawing down less C02, says plankton ecologist Abigail McQuatters-Gollop of Plymouth University.
As global leaders prepare to meet at a crucial UN summit on climate change, the issue is a stark example of how accelerating human impacts are destabilising intricate life-sustaining systems.
- Thinking small -
Tackling this is not as simple as planting trees, McQuatters-Gollop notes.
But fishing sustainably, reducing pollutants and curbing C02 emissions can all help improve ocean health.
In the past, she says conservation has focused on the big things, the cute things, or the things that are directly worth money -- like whales, turtles and cod.
But all rely on plankton.
While this blindness could be because they are microscopic, people can see plankton traces at the beach -- in foam on waves, or the nighttime twinkle of bioluminescence.
Or on the children's television show SpongeBob SquarePants, whose character Plankton is the most famous plankton out there, says McQuatters-Gollop.
And when they bloom in vast numbers, plankton are visible from space, turning the water a startling emerald, or creating Van Gogh swirls of milky blue, in seasonal displays critical for ocean life.
Like land plants, phytoplankton need nutrients like nitrates, phosphates and iron to grow.
But they can have too much of a good thing: The runoff of nitrogen-rich fertilisers is blamed for creating harmful algae blooms, like the glutinous sea snot off Turkey's coast this year.
These can poison marine life or choke oxygen out of the water and may be exacerbated by warming, warns the IPCC.
Meanwhile, research published in Nature last month found that iron carried in smoke from huge 2019 and 2020 wildfires in Australia sparked a giant swell of phytoplankton thousands of miles away, which could have sucked up substantial amounts of C02.
Blooms can be seeded by nutrients from sand storms or volcanic eruptions and it is these ?natural processes? that have inspired David King, founder of the Centre for Climate Repair at Cambridge.
King supports a hotly-debated idea to fertilise plankton blooms by sprinkling iron on the surface.
The theory is that this would not only help suck up more C02, but lead to a surge of ocean life, including eventually helping to increase whale populations that have been devastated by hunting.
More whales equals more whale poo, which is full of the nutrients plankton need to bloom, and King hopes could restore a wonderful circular economy in the seas.
A pilot project will try the technique in an area of the Arabian Sea carefully sealed off in a vast plastic bag, but King acknowledges that the idea raises fears of unintended consequences: We certainly don't want to de-oxygenate the oceans and I'm pretty confident we won't.
- Sea mysteries -
Ocean organisms have been photosynthesising for billions of years -- long before land plants. But we still have much to learn about them.
It was only in the 1980s that scientists named the planktonic bacteria prochlorococcus, now thought to be the most abundant photosynthesiser on the planet.
Some drifters it turns out can swim, while others are masters of communal living.
Take the partnership between corals and plankton -- it is so important that when it breaks due to warming the corals bleach.
Or Acantharea, a single cell shaped like a snowflake that can gather photosynthesising algae and manipulate them into an energy-generating battery pack, says Johan Decelle, of the French research institute CNRS and the University of Grenoble Alpes.
They have been overlooked because they dissolve in the chemicals used by scientists to preserve samples.
To study plankton under a high-resolution electron microscope, Decelle used to collect samples at the French coast and drive for hours back to Grenoble with them in a special cool box.
But this year he worked with the European Molecular Biology Laboratory on a pioneering project bringing high-tech freezing virtually onto the beach.
This enables the study of these delicate organisms as close as possible to their natural environment.
By contrast, Continuous Plankton Recorders end up mashing their samples into roadkill, says Ostle.
But the value of the survey, which began in 1931 to understand how plankton affected herring stocks, comes from decades of data.
Scientists have used it to look back to track climate changes and it played an important role in the recognition of microplastics.
Ostle used CPR ships' logs to show that macroplastics like shopping bags were already in the seas in the 1960s.
By the time it was awarded a Guinness World Record last year for the greatest distance sampled by a marine survey, it had studied the equivalent of 326 circumnavigations of the planet.
From the boat in Plymouth, the water appears calm as sunlight slides across its surface. But every drop is teeming with life.
There's just a whole galaxy of things going on under there, Ostle says.