Copepod migrations are important for the ocean's CO2 uptake

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.

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 human-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.