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Oceans are capturing twice as much carbon as previously thought

A biological ‘pump’ that distributes carbon to the bottom of the ocean via tiny plankton is operating twice as rapidly as previously thought, a new study claims.

US scientists say that the level of carbon that is distributed to the ocean floor depends on how much sunlight a microscopic type of plankton gets.

This phytoplankton, which consumes carbon dioxide and releases oxygen just like plants, is present in the ocean’s sun-lit surface area – known as the ‘euphotic zone’.

Phytoplankton then enters the food chain or falls as organic dead matter, indirectly distributing carbon to the ocean depths.

But variations in the depth at which the euphotic zone ends means differences in how much carbon is being distributed to the bottom.

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Masses of dinoflagellates in the water.  Dinoflagellates are one of the two main types of phytoplankton, which require sunlight in order to live and grow. They therefore reside in the uppermost layer of the ocean that receives sunlight – the euphotic zone

The euphotic zone has generally been said to stretch from the ocean’s surface to 650 feet down, followed by the twilight zone, the midnight zone, the abyssal zone and the hadal zone, which exists at in V-shaped depressions at around 20,000 to 36,000 ft

By measuring the actual depth of the euphotic zone – which is usually said to span from the very surface to 650 feet (200 metres) down – scientists say global carbon estimates may be inaccurate.

This has implications for future climate assessments and could lead to more accurate standards by which global climate policy is set.


Phytoplankton are microscopic marine algae.

They contain chlorophyll and require sunlight in order to live and grow.

Phytoplankton are buoyant and float in the upper part of the ocean, where sunlight penetrates the water – known as the ‘euphotic zone’.

 In a balanced ecosystem, phytoplankton provide food for a range of sea creatures including whales, shrimp, snails, and jellyfish, as well as the slightly larger zooplankton. 

The two main classes of phytoplankton are dinoflagellates and diatoms. 

Dinoflagellates use a whip-like tail, or flagella, to move through the water, while diatoms rely on currents. 

Source: US National Ocean Service  

‘This is a good news story, in that we have been underestimating the oceans ability to take up carbon dioxide,’ Professor Ken Buesseler, a geochemist at the Woods Hole Oceanographic Institution (WHOI) in Massachusetts, told MailOnline.

‘Essentially the discovery is not that there is more carbon getting in to the ocean, but realising that we’ve been measuring this carbon flow too deep, and hence missing the full impact of the sun-lit surface zone.

‘So the ocean is taking up more carbon – which is good – and if we consider this boundary of light penetration more properly in our models, we can do a better job understanding and predicting the oceans role in the global carbon cycle, and hence climate.’

Every spring in the Northern Hemisphere, the ocean erupts in a massive bloom of phytoplankton – tiny microscopic marine algae that forms the basis of many food chains.

Above right is a graph of carbon loss traditional measurement at 150 meters compared to carbon loss measurement where the depth of sunlight penetration actually is. ‘Carbon loss’ refers to the amount of carbon that is sinking out as ‘marine snow particles’ – the downward flow of plankton fecal matter and their decay products. Carbon losses are therefore higher when depths of individual euphotic zones are taken into account

These single-celled floating organisms use photosynthesis to turn light into energy just like plants – meaning they consume carbon dioxide and release oxygen.

When phytoplankton die they form part of what is known is ‘marine snow’ – a shower of organic waste that sinks towards the ocean floor.

Phytoplankton are also eaten by the slightly larger zooplankton, which are in turn eaten by larger creatures and distributed as faecal matter.

This process of carbon-rich fragments sinking deeper into the ocean is key to the world’s global carbon cycle – the cycle of carbon between the atmosphere, the oceans, land, and fossil fuels. 

During expeditions over the last few years, Dr Buesseler and his team used chlorophyll sensors that indicate the presence of phytoplankton in order to assess various depths of the euphotic zone.

Marine chemist Ken Buesseler (right) deploys a sediment trap from the research vessel Roger Revelle during a 2018 expedition in the Gulf of Alaska. Buesseler’s research focuses on how carbon moves through the ocean

In his paper, published in Proceedings of the National Academy of Sciences, Dr Buesseler and his co-authors call on fellow oceanographers to consider the differences in euphotic zone boundaries. 

‘If we’re going to call something a euphotic zone, we need to define that – so we’re insisting on a more formal definition so that we can compare sites,’ said Dr Buesseler.

‘Using the new metrics, we will be able to refine the models to not just tell us how the ocean looks today, but how it will look in the future.

‘Is the amount of carbon sinking in the ocean going up or down? That number affects the climate of the world we live in.’

Dr Buesseler said that the next step is looking at processes below the sun-lit layers, starting with the mysterious ‘twilight zone’.

Officially known as the mesopelagic zone, the twilight zone generally spans from just below 200 metres to 1,000 metres deep.

In a joint article published in Nature, Dr Buesseler warns climate change is altering the ‘poorly understood’ twilight zone, which is changing its temperature, acidification and oxygen levels in ways that are likely to affect marine life.  

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