How do tiny animals in the ocean influence atmospheric carbon dioxide?

By Emma Cavan 

The important role  small (< 5 mm) plants and animals play in the ocean is widely unknown to the public, as the media prefers to broadcast ‘cuddly,’ charismatic  animals such as dolphins and whales. However, the plankton are very important. Plankton are defined as organisms (both plants and animals) that cannot swim against the currents and range from microscopic algae to huge jellyfish. 

My research is on the biological carbon pump, described by Yonara Garcia in a previous post ‘Ocean fertilization and climate change’ (May, 2016).  The biological carbon pump describes how phytoplankton (plants) and zooplankton (animals) drawdown carbon dioxide from the atmosphere to the deep oceans.  I am most interested in how this biology transports organic carbon (as particles) through the upper ocean (top 500 m).

Image of crustaceous zooplankton 
about 0.5 mm in length. Photo by Emma Cavan.

Zooplankton range from tiny crustaceans (shrimp-like animals) to much larger salps and jellyfish. Here I am just going to concentrate on the crustaceans. One commonly known crustaceous zooplankton is krill, which are large (2-5 mm) for their group and are often found in abundance in the Southern Ocean. They are the food prey for large baleen whales such as humpback whales. Zooplankton change how much organic carbon (originally photosynthesised by phytoplankton in the surface ocean) reaches the deep sea as they:
1. Respire inorganic carbon; 
2. Ingest the carbon and release some as packaged faecal pellets;
3. Break particles into smaller pieces.

To further complicate the process zooplankton can migrate 100s of metres per day vertically, so they may eat at the surface at night, then at dawn sink deeper in the ocean and release faecal pellets there, increasing the amount of carbon reaching the deep ocean and away from the atmosphere. Hence zooplankton are particularly hard to accurately represent in biogeochemical models! I have been to sea in the Southern Ocean and the Equatorial Pacific to find out how zooplankton affect the transfer of organic carbon to the deep ocean.

Southern Ocean

At sea in the Southern Ocean, Elephant Island where
Ernest Shackleton landed is in the background.

Working in the Southern Ocean is an amazing experience. It has to be one of the most beautiful places on Earth. We were surrounded by so many penguins every day! Back to the science though… As I said, the Southern Ocean has a high number of crustaceous zooplankton such as krill and copepods. They can thrive in the cool waters around Antarctica but are very patchy (not evenly spread throughout).

Faecal pellet, 0.3 mm in length.
Photo by Emma Cavan.

Here I collected sinking organic particles (full of carbon) and they turned out to be mostly zooplankton faecal pellets (as opposed to detrital phytoplankton). This suggests that most of the organic carbon reached the seafloor from zooplankton grazing on the phytoplankton and releasing faecal pellets. The number of zooplankton present was shown to actually affect how many particles sunk out of the surface ocean. Further, whether zooplankton were feeding on fresh phytoplankton (brown faecal pellets) or detritus or their own faeces (white faecal pellets – and yes, they eat their own poo!) affected how efficiently organic carbon reached the deep ocean! So these little critters were playing an important role in transferring organic carbon from the surface to the deep ocean here.

Equatorial Pacific
Working here was very different from the Southern Ocean; it was extremely hot, and I saw barely any clouds the entire cruise. We were working off the Pacific coast of Guatemala, and while there was a lot less sea life here, I did see a lot of turtles and even a Thresher shark! 

                  RRS James Cook at port in Panama before the cruise. Photo by Emma Cavan.

Compared to the Southern Ocean, the Equatorial Pacific is very stable with little change in seasons. Between 100-1000 m in this area of the ocean, oxygen concentrations plummet, so organisms are extremely oxygen starved at these depths. Oxygen minimum zones (OMZs) are common around the globe, particularly near coasts such as off of Peru and the west coast of Africa. Many studies have shown that in OMZs, a much higher proportion of organic carbon reaches the deep ocean compared to rest of the world. But the reason for this is still unknown and so I went to sea to find out.

There are two main reasons why organic carbon doesn’t reach the deep ocean:
    1. It is consumed and respired by zooplankton;
    2. Or it is hydrolysed by bacteria.

Micro-respiration system used to measure
bacterial respiration on particles. An oxygen sensor
(blue) is inserted into the small vials which
contain particles to measure oxygen concentrations
over a few hours. Photo by Emma Cavan.

So I wanted to test if bacterial ‘remineralisation’ (process of converting organic carbon back to inorganic carbon, like carbon dioxide) is reduced in OMZs because bacterial metabolism is limited by the low oxygen concentrations. To do this, I measured the respiration of microbes on particles.What this showed was that actually microbes are very well adapted to live in the low oxygen conditions and were responsible for most of the organic carbon degradation! 


This meant that likely a reduction in zooplankton respiration and processing of particles in the OMZ must be why such a high proportion of the organic carbon reaches the deep ocean. This seems like a reasonable hypothesis as studies have shown zooplankton abundance is low in OMZs and their metabolism is greatly reduced. The life cycle of bacteria is much shorter than zooplankton so they can adapt much faster to challenging conditions.  So in the Equatorial Pacific, the absence of zooplankton means more carbon reaches the deep ocean and cannot be exchanged with the atmosphere.

To summarise, zooplankton have a complicated relationship with carbon in the ocean. Both their presence and absence can increase the amount of carbon in the deep ocean, it just depends on the oceanic ecosystem they are part of. This is why it is complicated to model their effect on the carbon cycle and more work is needed to constrain it better. But we should remember tiny animals do indeed influence how much carbon dioxide is in the atmosphere. Who would have thought it?!

About Emma: 
Emma is a marine biologist turned biological oceanographer (which basically means marine biologist of small organisms!). She grew up on the south coast of England and attended the National Oceanography Centre at the University of Southampton, UK, for both her undergraduate and PhD degrees. She has just finished her PhD and is hoping to stay in academia and continue researching. Emma is also very interested in connecting science and policy and spent 3 months working at the Royal Society in London in their science policy centre. Aside from science Emma likes to travel as much as possible and has been able to do so both for pleasure and with work. She also loves kayaking, camping, reading, napping and socialising.
Follow Emma on twitter @emma_cavan or visit http://emmacavan.wix.com/emmacavan