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Salps play a key role in pumping carbon to the deep ocean

When assessing the efficiency of the ocean’s biological carbon pump (BCP) that carries carbon from the surface layers to the deepest depths, few people consider the role of salps. This is probably because these creatures, also known as sea squirts, are mostly small, gelatinous and transparent, and their patchy distribution in the ocean makes them difficult to study. But they have certain characteristics that potentially make them potent players in the ocean’s carbon cycle. They may even play a significant role in mitigating global warming. 

Salps are reminiscent of small, tubular jellyfish but they are tunicates, more closely related to humans than to jellyfish. Their larvae have a notochord – a tough, flexible rod that protects the central nerve cord – running down the back. Although the adults lose this notochord, its presence in the larvae places them in the phylum Chordata, and makes them the closest relatives to vertebrate animals. Salps are zooplankton, meaning that they float around in the oceans, filtering out and eating mostly tiny plant plankton.

The role played by salps in the biological carbon pump has now been investigated as part of the research program known as EXPORTS (EXport Processes in the Ocean from RemoTe Sensing). This is a four-year, multi-institutional field program funded by NASA, with the goal of combining shipboard and satellite observations to more accurately quantify the global impact of the biological pump. Co-authors of the salp study hail from marine institutes in Maine, Bermuda, California, Newfoundland, British Columbia, and Alaska.

Carbon dioxide (CO2) enters the oceans at the surface, where phytoplankton use the sunlight’s energy and the atmosphere’s CO2 in photosynthesis, to produce organic molecules. The phytoplankton are eaten by zooplankton, including salps, which then incorporate the carbon into their own tissues. When zooplankton excrete waste, or die, this carbon sinks to the ocean depths and may become sequestered – stored in the benthic sediments, sometimes for millions of years. This means that this carbon is effectively removed from the atmosphere for that time.

During a month-long EXPORTS expedition to the northeast Pacific Ocean in 2018, Dr. Deborah Steinberg of William & Mary’s Virginia Institute of Marine Science, and her colleagues, chanced upon a large bloom of poorly studied salps (Salpa aspera). These creatures have the ability to respond to favorable environmental conditions by reproducing very rapidly, forming massive blooms, called swarms. Salp swarms often go undetected because they do not last long – a salp may only live for a few weeks. The consequence is that the role of these “jelly barrels” in the movement of carbon is rarely included in measurements or models of the BCP.

“Salps follow a ‘bloom or bust’ life cycle, with populations that are inherently patchy in space and time. That makes it hard to observe or model their contribution to the export of carbon to the deep sea,” says Steinberg.

In the case of the current study, the researchers identified a salp bloom that covered more than 4,000 square miles (~11,000 km2), about the size of Connecticut. They spent eight days aboard the R/V Revelle, sampling the particulate matter in the water column, along with the density of salps and other zooplankton, right down to a depth of 1,000 m. They made use of a wide range of ocean-observation tools, from traditional plankton nets and sediment traps to underwater video recorders and sonar-based computer models. 

Sampling was done during both daytime and night-time, taking into account the fact that salps come to the surface at night to feed, but descend during the day to a depth of between 300 and 700 meters, in order to avoid their own predators. Moreover, by using two research vessels – the 277-ft Roger Revelle and the 238-ft Sally Ride – the scientists were able to observe conditions inside the salp bloom and compare them to conditions in the surrounding waters.

The results of the team’s unprecedented field campaign are published in the journal Global Biogeochemical Cycles. They show that, when present in dense swarms, salps have a massive effect on the amount of particulate organic carbon (POC) moving downwards in the water column, and improve the efficiency of the biological carbon pump significantly. 

“High salp abundances, combined with unique features of their ecology and physiology, lead to an outsized role in the biological pump,” says Steinberg. 

Salps produce relatively large, dense fecal pellets, that sink rapidly to the depths, giving bacteria little chance to decompose the organic matter on the way. The scientists found that 82 percent of the POC present was derived from salp fecal pellets. Furthermore, the daily migration from surface waters to deeper depths gives the pellets a headstart on their journey downwards. The researchers determined that salp-mediated carbon export markedly increased the biological carbon pump efficiency, increasing by 1.5-fold the proportion of net primary production exported as POC from the surface waters down towards the depths.  

The onboard experiments showed that salps are capable of exporting a daily average of 9 milligrams of carbon through each square meter at 100 meters below the bloom, meaning that the amount of carbon exported to the deep sea was about 100 metric tons per day. For comparison, a typical passenger car emits 4.6 metric tons per year. Comparing these values shows the carbon removed from the climate system each day by the salps is equal to taking 7,500 cars off the road. Adjusting these values using the team’s highest measured rate of salp-mediated export (34 mg of C per day) increases the carbon offset to more than 28,000 vehicles.

Moving forward, the team calls for increased recognition of the key role that salps play in global carbon export. “Blooms like the one we observed often go undetected, and their contributions to the biological pump are rarely quantified, even in some of the best-studied regions of the world’s oceans,” says Steinberg.

Incorporation of salp dynamics into a recent carbon-cycle model illustrates the potential of salp-mediated export. In this global model, salps and other tunicates exported 700 million metric tons of carbon to the deep sea each year, equal to the emissions from more than 150 million cars. 

“Greater use of new technologies, such as adding video imaging systems to autonomous floats, would help detect these salp blooms,” says Steinberg. “Our study serves as a ‘call to arms’ to better detect and quantify these processes, using technology and sampling schemes that enable their inclusion in measurements and models of the biological carbon pump.”.

By Alison Bosman, Staff Writer

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