Seabed foraging by Antarctic krill: implications for stock assessment, bentho-pelagic coupling, and the vertical transfer of iron

Abstract

A compilation of more than 30 studies shows that adult Antarctic krill (Euphausia superba) may frequent
benthic habitats year-round, in shelf as well as oceanic waters and throughout their circumpolar range. Net and
acoustic data from the Scotia Sea show that in summer 2–20% of the population reside at depths between 200 and
2000 m, and that large aggregations can form above the seabed. Local differences in the vertical distribution of
krill indicate that reduced feeding success in surface waters, either due to predator encounter or food shortage,
might initiate such deep migrations and results in benthic feeding. Fatty acid and microscopic analyses of stomach
content confirm two different foraging habitats for Antarctic krill: the upper ocean, where fresh phytoplankton is
the main food source, and deeper water or the seabed, where detritus and copepods are consumed. Krill caught in
upper waters retain signals of benthic feeding, suggesting frequent and dynamic exchange between surface and
seabed. Krill contained up to 260 nmol iron per stomach when returning from seabed feeding. About 5% of this
iron is labile, i.e., potentially available to phytoplankton. Due to their large biomass, frequent benthic feeding,
and acidic digestion of particulate iron, krill might facilitate an input of new iron to Southern Ocean surface
waters. Deep migrations and foraging at the seabed are significant parts of krill ecology, and the vertical fluxes
involved in this behavior are important for the coupling of benthic and pelagic food webs and their elemental
repositories.

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