When assessing the effectiveness of the ocean’s biological carbon pump (BCP), which transports carbon from surface layers to the deepest depths, few people consider the role of salps. This is probably because these creatures, also called ascidians, are mostly small, gelatinous and transparent, and their patchy distribution in the ocean makes them difficult to study. But they have certain characteristics that could make them important players in the ocean carbon cycle. They could even play an important role in curbing global warming.
Salps are reminiscent of small jellyfish, but are tunicates and more closely related to humans than to jellyfish. Their larvae have a notochord – a strong, flexible rod that protects the central nerve cord – that runs down their back. Although adults lose these notochords, their presence in larvae places them in the phylum Chordata, making them the closest relatives of vertebrates. Salps are zooplankton, meaning they swim in the oceans and filter and eat mostly tiny plant plankton.
The role that salps play in the biological carbon pump has now been investigated as part of the EXPORTS (EXport Processes in the Ocean from RemoTe Sensing) research program. This is a four-year, NASA-funded, multi-agency field program designed to combine shipboard and satellite observations to more accurately quantify the overall impacts of the biological pump. Co-authors of the Salp study come from marine institutes in Maine, Bermuda, California, Newfoundland, British Columbia and Alaska.
Carbon dioxide (CO2) enters the oceans at the surface, where phytoplankton use solar energy and CO2 from the atmosphere in photosynthesis to produce organic molecules. Phytoplankton are eaten by zooplankton, particularly salps, which then incorporate the carbon into their own tissues. When zooplankton excrete waste or die, this carbon sinks to the depths of the ocean and can be stored there, sometimes for millions of years, in benthic sediments. This means that this carbon is effectively removed from the atmosphere during this period.
During a month-long EXPORTS expedition to the Northeast Pacific in 2018, Dr. Deborah Steinberg of William & Mary’s Virginia Institute of Marine Science and her colleagues looked at a large, rarely studied salp flower (Salpa aspera). These creatures have the ability to respond to favorable environmental conditions by multiplying very quickly and forming huge flowers called swarms. Swarms of salps often go unnoticed because they do not survive for long: a salp can only live for a few weeks. The result is that the role of these “jelly barrels” in carbon movement is rarely considered in PCA measurements or models.
“Salps follow a ‘bloom or bust’ life cycle, with populations naturally patchy in space and time. “It is therefore difficult to observe or model their contribution to carbon export to the deep sea,” explains Steinberg.
In the case of the current study, researchers identified a salp bloom that spanned more than 4,000 square miles (~11,000 km2), the size of Connecticut. They spent eight days aboard the R/V Revelle, sampling particles from the water column as well as the density of salps and other zooplankton down to a depth of 1,000 m. They used a wide range of marine observation tools, from traditional plankton nets and sediment traps to underwater -video recorders and sonar-based computer models.
Sampling was carried out day and night, taking into account that the salps come to the surface at night to feed, but sink to a depth of between 300 and 700 meters during the day to avoid their own predators. In addition, scientists were able to use two research vessels – the 277-foot Roger Revelle and the 238-foot Sally Ride – to observe conditions within the salp and compare them to conditions in surrounding waters.
The results of the team’s unprecedented field campaign are published in the journal Global Biogeochemical Cycles. They show that salps, when present in dense swarms, have a massive impact on the amount of particulate organic carbon (POC) that sinks into the water column, significantly improving the efficiency of the biological carbon pump.
“The high abundance of salps, combined with the unique characteristics of their ecology and physiology, leads to an outsized role in the biological pump,” says Steinberg.
Salps produce relatively large, dense fecal pellets that quickly sink to the depths, giving the bacteria little chance of breaking down organic material. Scientists found that 82 percent of the POC present came from salp fecal pellets. Additionally, the daily migration of surface water to deeper depths gives the pellets a head start on their downward journey. The researchers found that salt-mediated carbon export significantly increased the efficiency of the biological carbon pump, increasing the proportion of net primary production exported as POC from surface waters by 1.5-fold.
Onboard experiments showed that salps are capable of exporting an average of 9 milligrams of carbon per day per square meter 100 meters below the bloom, meaning the amount of carbon exported to the deep sea was about 100 tons per day. For comparison: a typical car emits 4.6 tons per year. Comparing these values shows that the carbon removed from the climate system by Salps every day is equivalent to removing 7,500 cars from the road. Adjusting these values based on the highest Salp-mediated export rate measured by the team (34 mg C per day) increases the carbon offset to more than 28,000 vehicles.
Going forward, the team calls for greater recognition of the key role that salps play in global carbon export. “Blooms like the ones we observed often go unnoticed, and their contribution to the biological pump is rarely quantified, even in some of the most studied marine regions in the world,” says Steinberg.
Incorporating salt dynamics into a current carbon cycle model highlights the potential of salt-mediated export. In this global model, salps and other tunicates exported 700 million tons of carbon to the deep sea each year, equivalent to the emissions of more than 150 million cars.
“Increased use of new technologies, such as adding video imaging systems to autonomous floating devices, would help detect these salp blooms,” says Steinberg. “Our study serves as a call to better detect and quantify these processes using technologies and sampling schemes that enable their inclusion in measurements and models of the biological carbon pump.” »
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By Alison Bosman, editor of Species-threatened.fr