Salps fertilise Southern Ocean more effectively than krill | Polarjournal
The body shell of salps is mostly colourless and transparent and consists of cellulose. Salpa thompsoni is found in the southern hemisphere, with higher abundances in waters with temperatures between 3°C and 5°C. Photo: Jan Michels

Experts at the Alfred Wegener Institute have, for the first time, experimentally measured the release of iron from the fecal pellets of krill and salps under natural conditions and tested its bioavailability using a natural community of microalgae in the Southern Ocean. In comparison to the fecal pellets of krill, Antarctic phytoplankton can more easily take up the micronutrient iron from those produced by salps. Observations made over the past 20 years show that, as a result of climate change, Antarctic krill are increasingly being supplanted by salps in the Southern Ocean. In the future, salps could more effectively stimulate the fixation of the greenhouse gas carbon dioxide in Antarctic microalgae than krill, as the team of researchers report in the journal Current Biology.

In many parts of the Southern Ocean, iron is the primary limiting resource for the growth of phytoplankton. Accordingly, the amount of available iron has a major impact on how much CO2 the microalgae can fix and, in turn, how much biomass is available at the base of the food web. Studies clearly show that, as climate change progresses, Antarctic krill, the key species in the Southern Ocean, will increasingly be supplanted by salps.

Salps are tunicates that resemble jellyfish in appearance, but are chordates related to vertebrates. They are barrel-shaped, usually colorless and transparent, and the animals, which range in size from eight millimeters to 19 centimeters, can form very long chain-like colonies thanks to their rapid asexual reproduction. The Antarctic species Salpa thompsoni was found in the 1960s mainly between the 45th (subtropical convergence) and 60th southern latitudes (near the ice edge). They occur much less frequently at water temperatures below 3°C. However, as the Southern Ocean warms, they are increasingly observed in large numbers at higher latitudes.

Diatoms and other microalgae form the basis of the food web on which both salps and krill feed. One of the most important nutrients they need to grow is iron. Photo: Julia Hager, Alfred Wegener Institute

“We investigated what a dominance shift from krill to salps would mean for primary production,” explains Dr Scarlett Trimborn from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). As head of the AWI research group EcoTrace, during an expedition with the Research Vessel Polarstern she and her colleagues conducted experiments with natural phytoplankton populations in the Southern Ocean near Elephant Island. As a source of iron, the researchers offered the microalgae communities fecal pellets from krill and salps, since a dominance shift between the two species would mean higher feces production by salps in the future.

“We were surprised to find that, compared to krill, the fecal pellet material from salps released more iron per microgram of carbon. In addition, we determined that the iron released by the salps’ fecal pellets was more bioavailable for phytoplankton than the iron from krill pellets,” reports Sebastian Böckmann from the EcoTrace group and first author of the study. The phytoplankton communities were able to take up as much as five times more iron from the salps’ fecal pellets than from the krill feces. This improved uptake could be due to ligands, which enhance the iron’s bioavailability for the algae. This aspect could result in significantly increased CO2 fixation among the phytoplankton.

In the fecal pellets of zooplankton, the shells of the previously eaten algae are usually still visible. Photo: Julia Hager, Alfred Wegener Institute

The Southern Ocean is extremely important for the future of our climate, as its vast expanses of water can potentially absorb or release large quantities of CO2 from or into the atmosphere. In some regions, e. g. surrounding the Antarctic Peninsula, climate change is affecting the sea-ice cover. When the ocean is ice-free, more sunlight penetrates the upper water layers, providing an energy source for photosynthesis. That being said, the availability of the resource iron is what chiefly determines CO2 uptake in microalgae. “Although we know from which sources iron is transported into the Southern Ocean, it’s still completely unclear how much of the iron the microalgae can take up, especially with regard to its release through recycling on the part of grazers like salps and krill. Our study makes an important contribution to modelling biogeochemical cycles in the Southern Ocean of tomorrow,” Trimborn summarises.

The results of the study show that salps virtually fertilize their food themselves by providing readily available iron. One might assume that the stronger growth of microalgae caused by salps would be equally beneficial for krill, which also feed on phytoplankton. However, salps are extremely effective filter feeders and, as researchers reported in the 1990s, can consume up to one hundred percent of the daily primary production in various regions of the Southern Ocean. Thus, there is not much algae left for krill in ice-free waters in these regions.

Like salps, Antarctic krill(Euphausia superba) is also a filter feeder, but a less effective one. In contrast, however, it is also able to graze on microalgae growing on the underside of sea ice. Photo: Uwe Kils, Wikipedia

A regime shift in the Southern Ocean from krill to salps may make more nutrients available for phytoplankton growth and thus fix greater amounts of CO2 , but for many species of penguins, seals, whales, and seabirds that depend on krill as their primary food source, such a fundamental change would be catastrophic. Even if these animals were flexible enough to add salps to their diet, they could not survive on a salp-dominated diet. This is because they contain much less energy than krill, which is rich in proteins and fats. In a previous study, it was determined that more energy is used to warm such watery prey as salps to the predator’s body temperature than the salps provide. Thus, in this freezing environment, displacement of krill by salps would have dramatic consequences for endothermic animals at the top of the food web.

Press release Alfred Wegener Institute for Polar and Marine Research with additions by Julia Hager, PolarJournal

Link to the study: Sebastian Böckmann, Florian Koch, Bettina Meyer, Franziska Pausch, Morten Iversen, Ryan Driscoll, Luis Miguel Laglera, Christel Hassler and Scarlett Trimborn: Salp fecal pellets release more bioavailable iron to Southern Ocean phytoplankton than krill fecal pellets. Current Biology (2021), DOI: 10.1016/j.cub.2021.02.033

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