page_title: Krill
section_title: Biogeochemical cycles
hierarchical_section_title: Krill / Biogeochemical cycles
context_page_description: Krill are small crustaceans of the order Euphausiacea, and are found in all the world's oceans. The name "krill" comes from the Norwegian word krill, meaning "small fry of fish", which is also often attributed to species of fish.
  Krill are considered an important trophic level connection – near the bottom of the food chain. They feed on phytoplankton and zooplankton, yet also are the main source of food for many larger animals. In the Southern Ocean, one species, the Antarctic krill, Euphausia superba, makes up an estimated biomass of around 379,000,000 tonnes, making it among the species with the largest total biomass. Over half of this biomass is eaten by whales, seals, penguins, squid, and fish each year. Most krill species display large diel vertical migrations, thus providing food for predators near the surface at night and in deeper waters during the day.
  Krill are fished commercially in the Southern Ocean and in the waters around Japan. The total global harvest amounts to 150,000–200,000 tonnes annually, most of this from the Scotia Sea. Most of the krill catch is used for aquaculture and aquarium feeds, as bait in sport fishing, or in the pharmaceutical industry.
context_section_description: The Antarctic krill is an important species in the context of biogeochemical cycling and in the Antarctic food web. It plays a prominent role in the Southern Ocean because of its ability to cycle nutrients and to feed penguins and baleen and blue whales.
caption: Role of Antarctic krill in biogeochemical cycles Krill (as swarms and individuals) feed on phytoplankton at the surface (1) leaving only a proportion to sink as phytodetrital aggregates (2), which are broken up easily and may not sink below the permanent thermocline. Krill also release faecal pellets (3) whilst they feed, which can sink to the deep sea but can be consumed (coprophagy) and degraded as they descend (4) by krill, bacteria and zooplankton. In the marginal ice zone, faecal pellet flux can reach greater depths (5). Krill also release moults, which sink and contribute to the carbon flux (6). Nutrients are released by krill during sloppy feeding, excretion and egestion, such as iron and ammonium (7, see Fig. 2 for other nutrients released), and if they are released near the surface can stimulate phytoplankton production and further atmospheric CO2 drawdown. Some adult krill permanently reside deeper in the water column, consuming organic material at depth (8). Any carbon (as organic matter or as CO2) that sinks below the permanent thermocline is removed from subjection to seasonal mixing and will remain stored in the deep ocean for at least a year (9). The swimming motions of migrating adult krill that migrate can mix nutrient-rich water from the deep (10), further stimulating primary production. Other adult krill forage on the seafloor, releasing respired CO2 at depth and may be consumed by demersal predators (11). Larval krill, which in the Southern Ocean reside under the sea ice, undergo extensive diurnal vertical migration (12), potentially transferring CO2 below the permanent thermocline. Krill are consumed by many predators including baleen whales (13), leading to storage of some of the krill carbon as biomass for decades before the whale dies, sinks to the seafloor and is consumed by deep sea organisms.[39]
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