Eyes on Environment: zooplankton migration traps carbon in deep ocean

copepod

Read the original post at Eyes on Environment on Nature’s Scitable Network!

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If you wander off the east coast of Puerto Rico as the sun sets, there’s a good chance you’ll catch a shimmer of light flooding the water’s surface. As the sun falls below the horizon and the darkness deepens, the water will erupt in a bright blue shine contrasting the blackness around you.

The sources of this illuminating display are bioluminescent phytoplankton, which attract thousands of tourists each year to the Puerto Rican shores. These microscopic organisms drift along the surface of the ocean (their name is derived from the Greek word for wanderer), photosynthesizing light for their energy needs. Their unique luminescence has several evolutionary purposes, including keeping predators away that don’t want to eat a bright piece of food that make them perfect targets for even bigger fish. But phytoplankton aren’t just important for Puerto Rico’s tourist industry – all that energy from the Sun stored in the organic carbon of the plankton serves as the foundation for the ocean’s massive food web.

A larger variety of plankton, the zooplankton, often feeds on these little phytoplankton, and the resulting fecal and dead organic matter drifts towards the bottom of the ocean. Scientists have known for a long time that this transport of carbon from the ocean surface to the deep depths, known as the biological pump, plays a huge role in trapping carbon in the ocean and limits the amount of CO2 in the atmosphere that warms the planet. But researchers in Denmark and Scotland have discovered another large source of carbon transport: migration of zooplankton known as copepods to the deep ocean.1 These findings could significantly alter our predictions about how much carbon can be stored in the ocean with implications about the future global warming of the planet.

Understanding why plankton migration can store carbon requires following the life cycle of the ovular little sea creatures. Copepods of the genus Calanus inhabit the upper ocean from the coasts of Maine to Norway. During the spring and summer, these copepods drift happily with the turbulence of the surface waters, living a gluttonous life of sex and phytoplankton feeding. The copepods store all the energy from the phytoplankton in lipids, made of omega-3 fatty acids and long-chain carbons. After the summer of love and food, the copepods prepare for a long sleep through the autumn and winter, descending 600-1400 m to the dark, deep waters of the ocean to hibernate. It’s quite cold at these depths – between -1 to 8 degrees C – so the copepods must metabolize all that energy stored in the lipids to keep them alive until the next summer, when they return to the surface to produce the next generation.

It is this lipid metabolism that stores carbon deep in the ocean. Essentially, the copepods act as a deep water transport vehicle for carbon, eating it up in the form of phytoplankton in the surface waters, and metabolizing it during hibernation, trapping it in deep ocean basins.

Scientists have known about this copepod life cycle for some time, but never before have researchers quantified just how much carbon sequesters in the deep ocean due to this migratory behavior. To do this, the researchers followed groups of the copepod C. finmarchicus throughout the North Atlantic, measuring total masses of hibernating groups and the associated lipid content. The migratory groups are extremely dense – 15,000-40,000 individual plankton in just one square meter! Each copepod only stores about 200 micrograms of lipids, about the mass of a fruit fly, but with such a high density, this adds up to a sizable amount of carbon. To determine how much carbon is actually sequestered due to migration, the authors calculated plankton respiration rates based on the average temperature in the ocean basin, and determined that 50-90% of the total lipid content is respired during hibernation. Some poor zooplankton also don’t survive the hibernation journey, so their entire carbon content will be sequestered as dead organic matter on the ocean floor. Thus, results given here are conservative lower estimates of total carbon sequestration.

Based on these calculations, the authors determined that this lipid storage and respiration of migrating copepods – which the authors call the ‘lipid pump’ – stores the same order of magnitude of carbon as particles of organic carbon drifting down into the deep ocean in the form of fecal or dead matter. Previous estimates have indicated that the North Atlantic biological pump pushes 1-3 gigatons of carbon per year, compared to about 30-40 gigatons of carbon per year we emit through fossil fuel emissions.2 The lipid pump could potentially double the size of this biological pump to 2-6 gigatons of carbon, providing another avenue for carbon sequestration and limiting the pace of global warming.

Such a change will affect how much carbon is left in the atmosphere, and if confirmed by future studies, will likely need to be included in climate models for more accurate predictions of global warming. These are also conservative estimates, as the study only looked at one species – C. finnmarchicus – in the North Atlantic. Future studies will need to assess other oceans to see if a similar migration pump is occurring.

Nevertheless, this is another sign that we are still discovering all the mysteries of the ocean and how they affect our lives above sea level. It’s truly amazing that the annual trek of a trillion miniature sea creatures could affect worldwide carbon levels – a reminder that we are not the only species shaping the planet. Something to think about the next time you blindly plunge a fishing line into these rich, complex waters.

References

  1. Jonasdottir, SH et al. “Seasonal copepod lipid pump promotes carbon sequestration in the deep North Atlantic.” PNAS, September 3, 2015.
  2. How many gigatons of carbon dioxide…?” Information is beautiful. Accessed September 29, 2015.

Photo Credit

Photo of copepod courtesy of Uwe Kils via Wikipedia

 

Jónasdóttir SH, Visser AW, Richardson K, & Heath MR (2015). Seasonal copepod lipid pump promotes carbon sequestration in the deep North Atlantic. Proceedings of the National Academy of Sciences of the United States of America PMID: 26338976

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