Calanus finmarchicus. Foto Sigrun Jonasdottir.

Copepod migrations are important for the ocean’s uptake of CO2

Tuesday 08 Sep 15
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by Line Reeh

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Sigrun Jonasdottir
Senior Researcher
DTU Aqua
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About Calanus finmarchicus

Copepods are tiny crustaceans, which dominate the zooplankton in most ecosystems.  They form a very important connection between primary production in water column and higher levels in the food chain, i.e. between phytoplankton and, for example, fish larvae. There are thousands of different species of copepods. Calanus finmarchicus is one of the most abundant copepod species in the North Atlantic and is an extremely important source of food for many commercial fish species such as cod larvae, herring and capelin. Icelandic, Faroese and Norwegian fishermen call Calanus 'red-feed', since its pigments can colour the stomach contents of, for example, herring completely orange. C. finmarchicus is 2.5-3 mm long.

Zooplankton no bigger than grains of rice play a much larger role in the transport and storage of  CO2 in the ocean than previously thought

In a scientific article recently published in Proceedings of the Academy of Sciences (PNAS), researchers from DTU Aqua, the University of Copenhagen and the University of Strathclyde, Scotland, have shown that the ocean's tiny copepods actively transport carbon down to the deep water in the North Atlantic during their winter hibernation. The discovery means that our understanding of the planet's carbon cycle, and the ocean's ability to absorb carbon needs to be revised. Changes in the carbon cycle are the cause of climate change.

 

"The active transportation of carbon from the atmosphere into the ocean has never been quantified at this scale before, but our calculations indicate that we may be able to double the previous estimate for the North Atlantic carbon capture," said DTU Aqua’s Senior Researcher Sigrun Jonasdottir, the lead contributor to the article.

"Therefore, we might be running the risk that climate change will weaken the process and as a result reduce the ocean's ability to absorb CO2"
Sigrun Jonasdottir

 

Wintering in deep water
The copepod, Calanus finmarchicus, is a small crustacean that lives in the North Atlantic, where it is an important food source for whales, birds and fish alike. In the summer, when food is plentiful, it manages to reproduce faster than it is eaten, but in winter this is not the case. This possibly explains why the animal developed a life cycle whereby it builds up carbon rich lipids during late summer. It then is carried by currents to the middle of the North Atlantic, where it swims down to a depth of about a kilometre. Here the copepod goes into hibernation and lives off its lipid reserves until spring.

 

The animal’s life cycle has has been known for a long time. But what has not been calculated before is the impact that the copepod’s long journey and  hibernation at depth has on the ability of the ocean to store  carbon dioxide removed from the atmosphere.

"The trick is that the copepod has to swim down so deep to hibernate that it comes down into water which is not in contact with the atmosphere. This means that the CO2 released at these depths by the copepods burning their carbon-containing lipids  into the water will not be exchanged in the atmosphere. In this way, the copepods indirectly remove CO2 from the atmosphere, where it can affect the climate, and deposit it deep down in the ocean, where it can remain for thousands of years," says Sigrun Jonasdottir.

 

Removing 1-3 million tons of carbon
There are billions of Calanus finmarchicus in the North Atlantic, and the research group's calculations show that this species of copepod alone actively moves 1-3 million tons of carbon into the North Atlantic every year. And C. finmarchicus is far from being the only animal in the ocean which spends part of its life cycle in deep water. 

"Once again we can see here a fantastic example of how important biology – and biological diversity – is for the chemical and physical processes on Earth. The ocean's carbon cycle is a vital component of climate models. At the moment, only passive biological processes are calculated into these models, for example when dead material sinks down through the water. But our study shows that we also have to include the active biological processes, such as animal migrations, to predict and calculate the ocean's ability to absorb anthropogenic emissions of CO2," says Professor Katherine Richardson of the University of Copenhagen, who is also one of the authors behind the study.


Copepods are themselves threatened by climate change
This does not mean, however, that we can just rely on water copepods to soak up the increased man-made emissions of greenhouse gases by dragging additional carbon down into the depths of the ocean. On the contrary, a warmer sea can lead to a reduction in the specie’s ability to go into hibernation and thus lessen the effect, according to Sigrun Jonasdottir from DTU Aqua.

"This process has been going on for thousands of years, so it's not a new mechanism by any means. But changes in the ocean, such as the water getting warmer and ocean currents changing, may have consequences for the copepods and their biology. Therefore, we might be running the risk that climate change will weaken the process and as a result reduce the ocean's ability to absorb CO2."

The research group's discovery is based on unique data collected by the article's authors on, for example, winter expeditions in the North Atlantic on DTU's marine research vessel Dana. The research was funded by NAACOS and EURO-Basin.

 

The article in PNAS:
Sigrun Jonasdottir, Andy Visser, Katherine Richardson, Michael R. Heath: A seasonal copepod lipid pump’ promotes carbon sequestration in the deep North Atlantic, PNAS.

The carbon cycle

In the global carbon cycle, carbon (C) is exchanged between the atmosphere, the land and the ocean. This occurs when chemical compounds that contain carbon, for example CO2, are transformed via biological and chemical processes into new carbon-containing substances, for example glucose (C6H12O6) in plants or hydrogen carbonate (HCO3) in the sea.

Carbon dioxide is (aside from water) the greenhouse gas that contributes most to the greenhouse effect, because it has the highest concentration and increases most quickly in the atmosphere. 

The ocean’s pool of carbon is about 50 times that of the atmosphere’s carbon pool . This is due partly to the sheer volume of the sea, and partly due to the fact that water can bind far more CO2 by volumetric unit than atmospheric air.

 

When CO2 is transported deep down into the ocean where it can be stored, it is often popularly described in terms of arriving their either by means of a biological pump or a physical pump. The biological pump involves the living organisms, while the physical pump involves a series of physical processes. In the ocean, the two pumps exchange carbon through a series of chemical processes. It is believed that the biological pump is responsible for approximately 20% of the transportation of CO2 to the ocean, while the physical pump accounts for some 80% of the transportation.  According to the study, a lipid pump must, in future, also be added to the models, which means that the biological contribution to carbon storage in the  sea might be twice that we now assume.