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UC San Diego Study Highlights Biogeochemical Importance of Zooplankton Fecal Pellets

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Microscopic plants called phytoplankton have gained scientific fame for their key role in transferring carbon from the atmosphere to the ocean, but they now may need to share their spotlight with salps, the jelly-like organisms that feed on them.

The ocean is a major reservoir of carbon, absorbing large quantities of carbon dioxide in a way that effectively removes carbon from the atmosphere, a process called carbon export, for long periods of time. Scientists estimate the ocean has absorbed about one-third of human fossil fuel emissions, and plankton are key players in this process.

New research links fivefold increases in carbon absorption to salp prevalence in the uppermost reaches of the ocean. That efficiency ranks among the highest rates known anywhere in the world’s oceans.

“It can shift an ecosystem that has been fairly low in carbon export into being very efficient at delivering carbon to depth,” said zooplankton ecologist Moira Décima of the University of California San Diego’s Scripps Institution of Oceanography. Décima and 13 co-authors published their findings Feb. 2 in the journal Nature Communications.

When large salps “bloom,” or reproduce at exponential rates, they eat large amounts of phytoplankton. The feces they produce contribute to carbon export into the oceanic environment. Importantly, their feces sink rapidly so they are less likely to be consumed or microbially degraded in the upper ocean. The deeper these carbon-rich particles sink, the less likely carbon is to return to the atmosphere, thus becoming sequestered for relatively long periods of time.

If the salps bloom because they feast on phytoplankton, Décima said, “you potentially can have high export that is both because of the salps and also because of the regular phytoplankton processes that enhance carbon export.”

The process is one of many avenues by which carbon dioxide is removed from the atmosphere, converted into a solid form, and sequestered into the deep ocean. It is thus a key process existing in nature that mitigates the effects of fossil fuel use and other activities contributing to climate change.

Before launching the SalpPOOP (Salp Particle expOrt and Ocean Production) study, Décima’s take on salps was formed on a research cruise where she initially saw them as net-clogging pests that interfered with collecting crustaceans such as krill. She would also see them while swimming along the New Zealand coast, where large numbers of them seemed to turn the waters into Jell-O.

Salps reproduce both sexually and asexually. They bloom during asexual reproduction, by releasing chains of hundreds of tiny salps that grow quickly, creating a gelatinous soup within a few days. Although salps look like jellyfish, they are actually chordates, with a nerve chord similar to that of vertebrates. “Biologically, they’re just fascinating,” Décima said.

Décima began studying salps at the National Institute of Water and Atmosphere Research in Wellington, New Zealand. She collected the salp data in an area of the Southern Ocean east of New Zealand’s South Island during a 2018 cruise aboard research vessel Tangaroa.

The researchers mostly sampled one species, Salpa thompsoni, ranging in size from a fraction of an inch to more than five inches long. The study was an attempt to distinguish the effect of salps from a ‘normal’ baseline: How do salps alter carbon export patterns?

The region around New Zealand presents ideal conditions for this “natural experiment.” Warmer subtropical waters meet colder sub-Antarctic waters, and each are characterized by different types of phytoplankton and consequently carbon export potential. The researchers wanted to know how much salps increased carbon export, but also if this depended on the underlying phytoplankton community.

Typically, in the absence of salps and other large zooplankton, phytoplankton communities dominated by small cells are not as efficient at sinking to depth compared to those dominated by large phytoplankton, such as diatoms. Would salp blooms create an “expressway to the deep” for all types of cells, large and small?

“In our study area we found that pretty much independently of the underlying phytoplankton community or water mass type, the effect was the same: if you had high salp abundances they massively increased export [of carbon to depth], in both subtropical and sub-Antarctic waters,” Décima said. “And we found blooms in both areas with different types of microbial communities.”

The team found that salps account for a two- to eight-fold increase in carbon export, with an average of five-fold increase. It arrived at these figures by measuring the amount of carbon exported in water with salp blooms compared to non-salp blooms, by looking at the fecal pellets collected in sediment traps, and by measuring the salps’ grazing and calculating a fecal pellet production rate based on their abundance, how much they are consuming, and temperature.

According to Décima, they are not always present in high abundances, but when they are they can have large impacts on carbon export and the food web in general.

Salp abundance in the Southern Ocean was documented to have substantially increased in the southern area of their range between 1920-2000.

If the trend persists, “we can expect important changes in areas where salp blooms are recurrent,” wrote Décima and her co-authors.

The changes would come “in the dynamics of phytoplankton bloom formation and termination, in the absorption and sequestration of carbon dioxide by the ocean, and in the composition of exported plankton affecting both organic and inorganic carbon flux to the deep ocean,” according to the authors.