Southern Ocean used to be red with masses of krill | Polarjournal
The fluke of a diving baleen whale in the Weddell Sea. The now degraded ecosystem in Antarctica was even more productive than previously thought before the near extinction of the whales. (Photo: Stefan Hendricks)

The krill biomass in the Southern Ocean today has shrunk to a fraction of former stocks as researchers describe in the current issue of Nature. Formerly highly productive, diatom-dominated areas have changed since the 1930s to a classic iron-restricted, nutrient-rich, chlorophyll-poor, microbial-dominated state characteristic of much of the ocean surface today. This is described by researchers around Matthew Scott Savoca in the current issue of ‘Nature’ and Victor Smetacek, emeritus scientist of the Alfred Wegener Institute comments on the results in a “News and Views” article of the scientific journal.

Even today, occasional aggregations of krill (here “Red lobster krill” near New Zealand) and a formation of a red ‘cloud’ still occur. However, the last large surface swarms that turned the ocean red were described in the early 1980s. (Photo: Paul van Kampen)

The Antarctic krill, Euphausia superba, can reach enormous biomass. A recent ‘Nature’ study presents new calculations that the species sequestered up to a tenth of a gigaton (100 million tons) of carbon before its sharp decline, the equivalent of about a gigaton of live weight (i.e., 1 billion tons), about twice the mass of all humans or even all cows on Earth. Of this, the former whale population ate about 400 million tons of krill annually – in comparison, the global fish catch has been about 130 million tons for decades. However, after the near extinction of whales by humans, the krill also began a sharp decline: the last large surface swarms that turned the ocean red were described in the early 1980s.

Prof. Dr. Victor Smetacek (left) and Matthew Scott Savoca have been observing and studying krill populations in the southern Atlantic Ocean for many years. (Photo: AWI) (Photo: AWI)

“The removal of a predator is often accompanied by an increase in its prey, so this decline in krill may be surprising at first,” said Prof. Dr. Victor Smetacek, who has conducted research at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). “However, these observations are consistent with a model that describes whales recycling iron, which supports the growth of krill populations,” the expert continued. “However, these observations are consistent with a model that describes whales recycling iron, which supports the growth of krill populations,” the expert continued.

Smetacek had already published in Nature in 2005 that the biological productivity of the Southern Ocean is controlled by iron supply. In the current study, the authors led by Matthew Scott Savoca of Stanford University now support this hypothesis. By consuming iron-rich krill and releasing fecal plumes that are also rich in iron into surface waters, the whales significantly increased the growth of phytoplankton and thus the availability of food for krill. Iron is nearly insoluble in seawater, and most of this element, which is essential to productive ecosystems, is contained in living biomass. This allows krill populations to act as giant, mobile iron stores, which could support a much more productive ecosystem than we find today in the Southern Ocean with a greatly reduced whale population.

Source: AWI, Bremerhaven

Link to the study: Savoca, M.S., Czapanskiy, M.F., Kahane-Rapport, S.R. et al. Baleen whale prey consumption based on high-resolution foraging measurements. Nature 599, 85–90 (2021). https://doi.org/10.1038/s41586-021-03991-5

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