Copepod. Photo: Erik Selander

Climate change is impacting the ocean’s ability to fix carbon dioxide

Thursday 14 Feb 19

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Thomas Kiørboe
Professor
DTU Aqua
+4535 88 34 01

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Andre Visser
Professor
DTU Aqua
+4535 88 34 25

The scientific article

'Climate change has altered zooplankton-fuelled carbon export in the North Atlantic'

Philipp Brun, Karen Stamieszkin, Andre W. Visser, Priscilla Licandro, Mark R. Payne, and Thomas Kiørboe
Nature Ecology & Evolution, 2019

Read the article at nature.com 
Climate change is influencing the distribution of zooplankton in the sea, thereby affecting the contribution made by plankton to removing carbon dioxide from the atmosphere.

Researchers from the Danish Centre for Ocean Life at DTU have been the first to document that the important contribution made by zooplankton to the carbon cycle is changing, and that zooplankton in the temperate regions of the North Atlantic are today removing significantly less carbon dioxide from the atmosphere than they did 55 years ago.

However, in the colder regions of the North Atlantic from Iceland and down to the northern USA, the situation is the opposite: Here, the zooplankton are transporting more carbon from the ocean surface towards the seabed. The findings were published on 11 February 2019 in an article in the prestigious scientific journal Nature Ecology & Evolution.

The sea plays a major role in the Earth’s carbon cycle, because atmospheric carbon dioxide is fixed as organic carbon in the surface and subsequently transported by zooplankton to the ocean interior. Thereby carbon dioxide is removed from the atmosphere for many years, where it would otherwise contribute to climate change

Storage in the sea is carried out by different processes, collectively known as the biological pump, and which is estimated to remove as much carbon dioxide from the atmosphere as that emitted by human activities.

Plankton affected by climate change
The article in Nature Ecology & Evolution deals with marine copepods, mm-sized crustaceans that are the dominant group of zooplankton and which are found everywhere.

“The changes we see in the North Atlantic are largely due to the fact that climate change is affecting marine copepods. In the central and eastern part of the North Atlantic, there are areas with fewer or smaller copepods than previously, and these changes in the biomass are significant for how much carbon can be transported from the atmosphere and down into the ocean,” explains Professor Thomas Kiørboe from the Centre for Ocean Life, who has lead the study.

In the north-western parts of the North Atlantic near Iceland and the USA on the other hand, the study shows that the copepod population has increased since 1960, so more carbon can be transported here from the atmosphere and down into the ocean. However, these are complex biological, chemical, and physical processes that are not yet sufficiently well understood to allow us to predict what the changes will mean for the climate in future.

“Our study is the first attempt to describe the global significance of the contribution made by copepods to the carbon cycle. We know that the changes in copepod populations affect the global carbon cycle directly through the changes in the transport of carbon from the ocean surface to the seabed, but as yet we are unable to quantify the consequences,” says Thomas Kiørboe.

The study is based on a unique data set which extends from 1960 to 2014, and which contains 200,000 observations of the occurrence of different copepod species. The researchers have combined this data with information on copepod body size, together with their food, migration behaviour, etc., and processed the data in complex models.

Copepods transport carbon downwards from the ocean surface

Phytoplankton is at the base of the food webs in the ocean. Phytoplankton grow—like plants on land—in the sunshine and by absorbing carbon dioxide from the air, which they convert to organic carbon. Phytoplankton is then eaten by copepods and other zooplankton.

Fecal pellets from the copepods and dead plankton organisms sink to the bottom, and t carbon may that way be buried in the seabed. In this way, the copepods passively transport carbon down into the sea.

Copepods also transport carbon actively from the ocean surface and downwards. This happens because each day the copepods migrate between the surface, where they eat, and deeper layers, where they hide from predators. In addition, in the winter they hibernate in deep water. When they are here, their metabolism converts organic carbon to carbon dioxide, which is thus released at depth where it remains for up to 1,000 years.

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