'Marine Snow' Trilogy: Episode 2

Dr Frédéric Le Moigne (CNRS, Marseille, France) and Dr Katsia Pabortsava (NOC, Southamtpon, UK)

20/12/2019

EPISODE 2: What is the ocean’s biological carbon pump?

Currents and biological activity play a critical role in controlling the uptake of atmospheric carbon dioxide by the oceans. This affects marine life from microbes to large fishes everywhere in the ocean, including the remote waters around Antarctica. And this is where we currently are, in the Southern Ocean, aboard the RRS DISCOVERY trying to understand how and why this process works.

‘Marine Snow’ courtesy of Dr N Briggs, NOC

Dr Katsia Pabortsava (NOC, Southamtpon, UK) and I, Dr Frédéric Le Moigne (CNRS, Marseille, France), are investigating the important process of the oceanic carbon uptake, called the “biological carbon pump”. In essence, it represents the amount of carbon that marine particles transport from the surface ocean to depths greater than 1 km as they sink by gravity. We call these sinking particles “marine snow” because they often resemble flocs of snow, as shown in the picture on the right.

Marine snow forms mainly when phytoplankton and zooplankton die and due to the motion of water collide and stick to each other. Eventually marine snow sinks down into the deep ocean carrying along all the organic carbon that originated from photosynthetic plankton. This mechanism is essential for the ocean because it “pumps” carbon from the surface ocean and transfers it to the deep ocean for long periods of time. The deeper marine snow sinks, the longer carbon remains locked at depth and the longer it takes for it to get back to the atmosphere. The central question of our CUSTARD expedition is how deep does this carbon sink?

SAPS being deployed, photo by S Alexiou, NOC

There are multiple ways of collecting and studying marine snow particles. In ‘Marine Snow Episode 1’, Dr Nathan Briggs described various cameras that he uses to detect and describe marine snow. We, however, directly capture marine snow from the ocean to primarily investigate their chemical composition. We are most interested in how much organic carbon these particles contain as this will tell us how strong the biological carbon pump is. We collect sinking particles using 6 water pumps, called  Stand Alone Pumps or SAPS (as shown on the left). We attach the SAPS to the ship’s wire and send them to different depths to pump seawater at the same time. The SAPS usually filter around 1500 L of seawater during just one hour of pumping.

On this CUSTARD expedition, we are deploying SAPS every other day at the three main research sites in order to collect the crucial information on the amount of carbon sinking at various depths and how it may change as phytoplankton grow in the surface. So far, we have deployed the SAPS ten times and collected particles from approximately 50,000 liters of water! That’s equivalent to 71,428 bottles of mulled wine!

‘Marine Snow’ Trilogy: Episode 2 – What is the Ocean’s Biological Carbon Pump?

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