Decline over time of krill stocks and increase in salp populations in the Southern Ocean

Antarctic krill (Euphausia superba) and salps (such as Salpa thompsoni) are two main grazers in the Southern Ocean. As any other species, the distribution of these two species is profoundly linked to environmental conditions and is sensible to them.

Antarctic krill is a small crustacean that lives in platforms, fronts and sea ice edges, where nutrients and oceanographical conditions promote primary production (the phytoplankton of which it feeds on). The distribution of krill is correlated to chlorophyll concentrations and extent of ice. This is due to the fact that during winter krill feeds on algae that grows on ice, a food resource highly important for krill larvae recruitment and success of the new generation during the following year. Since the 1970s, a decrease in krill stocks is being registered, in particular in the SW Atlantic sector of the Southern ocean, where krill is abundant.

Salps are planktonic tunicates that typically tolerate warmer waters than krill and are resistant to low productivity areas, in contrast with krill which requires higher productivity areas. The southern ocean has more areas of low productivity than areas with high productivity, for what salps have a larger distribution than krill. Still, salp numbers have been increasing and its distribution is spreading, in particular to the south where colder water are warming over the years.

Salp (Salpa thompsoni)

Antarctic Krill (Euphausia superba)

In certain areas, the decrease in krill (a species that requires high food abundance) and the increase in salps (which resists in water with low food abundance) coincide. On long-term, these variations coincide with increase in deep water temperature and (since the 1970s) with sea ice decrease. However, mechanisms underlying these variations are still uncertain and future predictions should be careful.

Krill is a key species in the food web of southern ocean, for what the decrease in krill populations and substitution by salp populations over time has the potential to have deep effects in the food web, in particular on krill predators such as penguins, fur seals and albatrosses. 

Variations in Krill densities over time (a) and geographic variations in Krill and salp densities (b and c respectively).

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Atkinson A, Siegel V, Pakhomov EA, Rothery P (2004) Long-term decline in krill stock and increase in salps within the Southern Ocean. Nature 432:100–103. doi: 10.1038/nature02950.1.

Author: Nadja Velez