Krill are small, shrimp-like marine crustaceans belonging to the order Euphausiacea, found in all the world’s oceans. Despite their modest size, generally growing to about two inches long, the Antarctic krill (Euphausia superba) commands an estimated biomass of around 379 million tonnes, making it one of the largest single-species animal masses on the planet. These tiny organisms are disproportionately important to the global ecosystem, particularly in the Southern Ocean. Krill are foundational to the health of the planet’s oceans, playing a role in the marine food web, the global carbon cycle, and the health of numerous species.
The Foundation of the Marine Food Web
Krill occupy a central position in the Antarctic ecosystem, serving as the primary mechanism for transferring energy from the ocean’s microscopic producers to its largest consumers. They graze directly on phytoplankton, efficiently converting the energy stored in these minute organisms into a form that higher-level predators can consume. The energy that krill provide sustains almost the entire Antarctic food web, which would face collapse without this abundant food source.
Baleen whales, including the massive blue whale, feed almost exclusively on krill, consuming up to four tonnes daily during their feeding season. Without these dense aggregations, it would be energetically impossible for the world’s largest animals to maintain their bulk. Other marine mammals, such as crabeater seals and fur seals, rely on krill for a large portion of their diet.
Seabirds and penguins are similarly dependent on krill availability for survival and reproduction. Adélie and chinstrap penguins, for example, time their breeding cycles to coincide with the seasonal krill bloom to feed their young. Numerous fish species and many squid also consume krill, reinforcing their status as the linchpin of the Southern Ocean’s food chain. Their unique behavior of forming gigantic swarms makes them an accessible and concentrated meal for many predators.
Carbon Sequestration and Nutrient Cycling
The ecological importance of krill extends beyond their role as a food source, encompassing a significant function in regulating the global climate through the biological pump. As krill consume vast quantities of carbon-rich phytoplankton, they incorporate this carbon into their bodies and excrete it as dense fecal pellets. These pellets are heavy enough to sink rapidly through the water column, transporting the carbon from the surface waters down to the deep ocean floor.
This process, known as carbon sequestration, locks the carbon away from the atmosphere for centuries, making krill a biological mechanism for mitigating atmospheric carbon dioxide levels. Krill contribute a substantial amount of carbon flux into the deep sea, rivaling the carbon storage capacity of certain coastal ecosystems.
Krill also play a role in nutrient recycling that supports the growth of the phytoplankton they consume. When krill molt their exoskeletons, they release dissolved iron and other essential micronutrients into the surface waters. This recycling acts as a natural fertilizer, encouraging more phytoplankton growth, which in turn draws more carbon dioxide from the atmosphere, further enhancing the biological pump’s effectiveness.
Commercial Harvesting and Human Health
The nutrient-rich composition of krill, especially their high concentration of omega-3 fatty acids, has driven a commercial fishery, primarily in the Southern Ocean. Krill oil, extracted from the crustaceans, is marketed globally as an omega-3 supplement for human health, offering an alternative source to traditional fish oil. The oil is favored because the omega-3s are bound to phospholipids, which are believed to enhance absorption in the human body.
Krill are also harvested on a large scale for use in aquaculture feed, pet food, and as bait in sport fishing, making them a globally traded commodity. The demand for these products places a spotlight on the management of the krill fishery to prevent over-exploitation. The Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) is the international body responsible for regulating krill harvesting. CCAMLR employs an ecosystem-based management approach, setting precautionary catch limits far below the estimated total krill biomass to ensure sufficient food remains for predators.
Regulatory Measures
A specific “trigger level” is set for the main fishing area, which, if reached, mandates the closure of the fishery. This measure prevents localized depletion and protects the foraging grounds of seals, whales, and penguins. This regulatory framework acknowledges that the long-term ecological function of krill far outweighs short-term commercial gain.
Climate Change and Krill Population Threats
Krill populations are under increasing pressure from environmental shifts, particularly those linked to climate change. The survival of Antarctic krill is tightly linked to the presence of sea ice, as juveniles rely on the algae growing on the underside of the ice for food during the winter months. A loss of winter sea ice, a trend observed in parts of the Antarctic Peninsula, directly reduces the habitat and food source needed for krill larvae to survive.
Long-term studies in the southwest Atlantic have connected decreased krill abundance with a reduction in winter sea ice coverage, highlighting the sensitivity of their life cycle to warming ocean temperatures. Furthermore, the increasing absorption of atmospheric carbon dioxide by the oceans is causing ocean acidification, which poses another threat to krill. This change in ocean chemistry can impair the ability of krill larvae to develop properly and affects the formation of their exoskeletons.
The combined effects of sea ice reduction and ocean acidification have the potential to significantly shrink the krill’s habitat and lower their reproductive success. A substantial decline in krill numbers would have cascading effects throughout the entire Southern Ocean ecosystem, directly impacting the recovery of baleen whale populations and the stability of seabird and penguin colonies.