Tiny zooplankton can reduce atmospheric carbon dioxide levels

Zooplankton are tiny microorganisms that are consumers, made up of heterotrophs and detritivores (decomposers). Many of these organisms are found living in colonies and are essential for food webs that make up marine ecosystems.

In a scientific article recently published in the Proceedings of the National Academy of Sciences (PNAS), researchers from the Technical University of Denmark (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 directly linked to climate change.

"Copepods are unique organisms that construct carbon-rich lipids during late summer."

The effect of copepods' lipid pump on carbon dioxide emissions
Copepods make up a group of crustacean zooplankton that are typically 1-2 millimeters long with a thin, transparent exoskeleton. They are unique organisms that construct carbon-rich lipids during the late summer as a food reserve when they are in the surface of ocean waters. The copepods use the lipid (fat) reserves during the winter hibernation months for survival. These organisms usually hibernate approximately one mile down in the ocean, taking the carbon-rich lipids with them out of the atmosphere. During cellular respiration, the copepods release carbon dioxide. However, because they are around a mile deep in the ocean, instead of the carbon dioxide going back into the atmosphere, it travels deep into the ocean where it can remain in the ocean floor for thousands of years without being disturbed or released.

The University of Strathclyde research team has named this process the "copepod lipid pump".

All forms of ocean life depend on copepods and other zooplankton species. All forms of ocean life depend on copepods and other zooplankton species.

The research team focused on one species of copepod, Calanus finmarchicus, which they determined carried between 1-3 million tons of atmospheric carbon dioxide into the deep North Atlantic Ocean each year.

"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," DTU Aqua's senior researcher Sigrun Jonasdottir, the lead contributor to the article, said.

Copepod migration and hibernation
The copepod lipid pump is a unique biological pump that has not been considered by the Intergovernmental Panel on Climate Change (IPCC). The lipid pump does not remove nutrients from the ocean's surface waters when the copepods migrate to hibernate deep in the waters of the North Atlantic. According to the PNAS, these microorganisms are able to decouple carbon sequestration from nutrient replenishment, called a "lipid shunt". This allows phytoplankton and other small organisms to still thrive in the nutrient-rich waters while carbon dioxide is being removed from the atmosphere. Carbon dioxide is removed from the oceans via detritus, decomposing organisms that sink to the floor of the ocean, but that process, called the biological pump, can remove important nutrients found in the surface waters.

"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," Jonasdottir said. "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."

Climate change is creating challenges for copepod productions, which signals rifts throughout the marine food chain. Climate change is creating challenges for copepod productions, which signals rifts throughout the marine food chain.

The correlation between carbon dioxide emissions and climate change is well understood and researched, although now, researchers focus on how to reduce atmospheric carbon dioxide via natural processes.

"We need to look into this further to find out whether the same thing is happening in other oceans of the world and how it can be included in the next generation of IPCC models," University of Strathclyde's Professor Michael Heath stated. "These copepod migrations don't provide a solution to the emissions problem, but our results are certainly part of the process of building up a better understanding of how the planet is responding to increasing CO2 levels."

Climate change threatening copepod populations
Climate change has accelerated at an alarming rate worldwide, and oceans are grappling with this global event in many ways. Warming waters cause many marine species, including zooplankton, to travel in adverse patterns in search of food. As a result, climate change may also affect the copepod populations. The warmer waters are decreasing the hibernation period for these organisms, which in turn decreases the amount of carbon they carry with them into the deep ocean. The copepod lipid pump has been occurring for thousands of years; however, this is just the first time researchers have proven that the carbon cycle involves these microorganisms.

"This process has been going on for thousands of years, so it's not a new mechanism by any means," Jonasdottir explained. "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."