Atmospheric carbon dioxide (CO2) has steadily increased since the industrial revolution. The levels would be even higher without the ocean, which is an overall carbon ‘sink’ to about a quarter of global emissions. Increased uptake by the ocean contributes to Ocean Acidification, which is a decrease in pH that can impact biological populations in the ocean.

The cold, productive North Atlantic around the PAP-SO is a strong CO2 ‘sink’. Although seawater levels have not increased here in recent years there is an increase in seasonal (winter to summer) changes and a decline in pH. Most of the seasonal variability at PAP is due to changes in biological production and mixing.

CO<sub>2</sub> concentration measured at PAP from 2002 to 2018.

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Upper ocean (epipelagic zone)

In spring, the top 50m of the water column becomes warmer and more stable. Phytoplankton, small photosynthetic organisms, swiftly increase in numbers, using the energy from sunlight combined with carbon dioxide absorbed from the atmosphere. This burst of growth creates the spring bloom that forms the base of the food web, supporting increasing numbers of zooplankton, small fish and then on through predation up the food web, ultimately supporting marine mammals, tuna and sharks.

As well as drawing down carbon dioxide, phytoplankton also use nitrate, silicate, phosphate and micronutrients such as iron and release oxygen changing the qualities of the water column itself.

These phytoplankton not only support the food web in the sunlight upper oceans, but also aggregate with faecal pellets and other small particles and sink out of the epipelagic zone, into the deep ocean where they support the food web all the way to the seabed.

Food production rate between 2000 and 2020.

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Deep ocean – the deep water column (meso-, bathy-, and abyssopelagic zones)

In the deep ocean, below 1,000m are the bathypelagic and abyssopelagic zones. There is no light so no production; the food source is the sinking particles from the epipelagic zone and the other animals that feed on them.

As the particles sink and are consumed, whether by animals or bacteria, the carbon that had been fixed by the phytoplankton is respired (used for energy) so is released back into the water column. This is the basis of what scientists call the Biological Carbon Pump (BCP), the transfer of carbon from the atmosphere to the deep ocean where it is not accessible for thousands of years.

The Biological Carbon Pump along with other ocean chemical and physical processes have absorbed 2–30% of all human carbon dioxide emissions (since the 1980s). Much of our work is focused on the BCP.

Rates of particles falling in the water column 1994–2018. (Black is observed, shaded area is predicted.)

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Seabed (the abyss)

The animal communities of the abyssal seabed have been studied since 1985. In 1996, and again in 2002, there was a dramatic rise in the abundance of one species of sea cucumber Amperima rosea. The abundance of this small animal jumped from three to 4610 individuals in an area the size of a football pitch in the first of these population “bloom” events.

A number of other small organisms also had populations blooms at the same time, including the shelled protozoan Quinqueloculina, an opheliid polychaete, the brittle star Ophiocten hastatum, and a second small sea cucumber Ellipinion molle. The species that have increased most in abundance may be specialist feeders on “phytodetritus”, the seasonally deposition of particulate matter that rains from the ocean above.

It seems that changes in the amount and type of material that falls to the seabed may cause major changes in the animal communities that live there. Climate variability in the surface ocean may have an immediate effect on deep-sea communities, even three miles deep!

Relative abundance and biomass of animal communities on the seabed.