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23 August 2022 6 min read

From microscopic phytoplankton to massive whales, all life in our oceans becomes the sinking dead at some point. It might be eaten by something higher up the food chain first or its time might just have run out.

When that life dies, it begins a journey that can lead to carbon being locked away in the deep ocean.

This is an important biological process – called the biological carbon pump – that removes carbon from the atmosphere. Our story starts with phytoplankton.

Bongo nets deployed from RV Investigator scoop up a phytoplankton bloom. Image: Max McGuire.

From light comes life

Phytoplankton, also known a microalgae, are the ocean’s primary producers. Like land plants, they use light to grow, creating carbohydrates via photosynthesis and absorbing carbon dioxide from the atmosphere in the process. At the same time, they release oxygen. Lots of oxygen. About half the oxygen produced on the planet.

Phytoplankton play a key role in the global carbon cycle. They do this by locking away atmospheric carbon dioxide into their cells. In fact, phytoplankton are estimated to capture 10-20 billion tons of carbon dioxide each year.

This is about three times more than what is captured by global forests.

In the light filled waters near the ocean surface, phytoplankton thrive and form the basis of the ocean food web. They also become part of the sinking dead.

A hunter from the twilight zone, the dragonfish feasts on zooplankton and small fish. Image: Amy Rose Coghlan.

There’s always a bigger fish

Phytoplankton become food for zooplankton. Zooplankton are the smallest animals in our oceans. They include invertebrates such as copepods and other tiny crustaceans. They also include soft and squishy animals like squid, jellyfish and salps, the jelly beans of the sea.

Zooplankton, and other small marine animals, become food for fish, crustaceans and other bigger species. All the way up to the largest animals ever to live on earth – whales.

Zooplankton are largely understudied, but they play an important role in the movement of carbon into the deep ocean. They do this in two ways. The first is as they move up and down the water column each day to feed on phytoplankton. The second is when they die and drift down into the deep ocean as the sinking dead.

As the bodies of the sinking dead drift deeper into the ocean, they take with them the carbon locked in their cells. This helps create an unusual underwater weather phenomenon in our oceans.

Spot the baby squid! A sample of marine snow collected from 1000 metres below the surface. Image: Cathryn Wynn-Edwards.

Say it ain’t snow

The carcasses of the sinking dead join with other sinking organic matter, including animal poop, to create ‘marine snow’. Marine snow forms a continuous shower of biological debris in our oceans, drifting from waters near the surface down into the deep ocean.

This is a key process in the ocean's biological carbon pump.

Recent research published by the Institute for Marine and Antarctic Studies (IMAS) examined the contribution of zooplankton, specifically a species of copepod, to the sinking dead. This research was carried out with help from our research vessel (RV) Investigator. Researchers found zooplankton likely make a significant contribution to carbon movement into the deep ocean. Their carcasses fall at a rapid rate and stimulate other life in the ocean along the way.

However, we still have much to learn. It's a big ocean and the forecast is for a 100 per cent chance of marine snow for the foreseeable future.

So how do we study marine snow across a vast and remote ocean?

A sediment trap, containing a year's worth of the sinking dead, is recovered on board RV Investigator. Image: Elizabeth Shadwick.

It’s a sediment trap!

One way is by using sediment traps. Sediment traps look like a big yellow funnel attached to a vacuum cleaner filter. The latter is actually a carousel of sample bottles into which the falling marine snow is funnelled. Literally.

The traps are about 1.5 metres tall. They are attached to deep water mooring lines to capture marine snow at various depths in the ocean.

RV Investigator is supporting this research through a long-term partnership with the Integrated Marine Observing System (IMOS). IMOS maintain two deep-water moorings in the Southern Ocean called the Southern Ocean Time Series (SOTS).

One of the SOTS moorings collects atmosphere and ocean data. The other collects marine snow and the sinking dead via a series of sediment traps.

The sediment traps are located at depths of 1000, 2000 and 3800 metres along the mooring line. On each sediment trap there are 21 cups. Each cup rotates under the giant yellow funnel and stays open for about a fortnight to collect the marine snow.

RV Investigator recovers the old moorings and deploys new ones each year. On return, the voyage brings back important data, as well as a year’s worth of marine snow for researchers to sift through.

Samples of marine snow filled with the sinking dead await processing. Image: Cathryn Wynn-Edwards.

Decade of the decayed

Over the past decade, with the help of RV Investigator and its predecessor, RV Southern Surveyor, these moorings have continuously gathered data and collected marine snow. This offers an unbroken record of life and changes in the Southern Ocean.

These data are critical to our understanding of both the interaction of ocean and atmosphere, as well as the changes in them over time. Altogether, the time series has been running since 1997, with continuous data being collected for the past 10 years.

Whether it be salps, copepods or other zooplankton. Bigger fish or whales. The death of ocean life sends millions of tonnes of carbon into the deep ocean every year. Locking it away for thousands of years.

While researchers estimate that less than one per cent of captured atmospheric CO2 ends up in marine sediments, this still equals millions of tons of carbon per year. Without the biological carbon pump and the sinking dead, atmospheric carbon dioxide levels would be much higher.

No small job. A sediment trap is deployed into the Southern Ocean from RV Investigator. Image: Ben Arthur.

Sinking a solution

Researchers are still seeking to understood how the sinking dead and strength of the biological carbon pump will be impacted by global climate change.

The research is ongoing. But one thing is clear. The ocean's biological carbon pump makes the deep ocean a large store of carbon. Furthermore, the research we’re helping to deliver shows the Southern Ocean is a globally significant region for carbon storage.

Without a doubt, these observations are essential to increase our understanding about how climate variability is affecting us now. And how it will likely affect us into the future.

A surface float from an IMOS deep-water mooring maintains its lonely vigil in the Southern Ocean. Image: Andrew Martini.

Research in the spotlight

The SOTS moorings are operated through a partnership between us, IMOS, the Bureau of Meteorology, and the Australian Antarctic Program Partnership, and supported by grants of sea time on RV Investigator from the Marine National Facility.

The Marine National Facility is a national research infrastructure funded by the Australian Government, and operated by Australia’s national science agency, on behalf of the nation.


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