Australia: The Land Where Time Began

A biography of the Australian continent 

Antarctic Krill – Their Importance in Biogeochemical Cycles

Antarctic krill (Euphausia superba) are swarming oceanic crustaceans that reach up to 2 inches (50.8 mm) long, and they are best known as a prey species for baleen whales and penguins, but they also have another important role in the ocean. As a result of thei large size, high biomass and their daily vertical migrations they transport and transform essential nutrients, stimulate primary productivity as well as influencing the carbon sink. They are also fished by the largest fishery in the Southern Ocean. This commercial fishing for krill is carried on while there is only poor understanding of the impact this has on nutrient fertilisation and the carbon sink in the Southern Ocean. The synthesis of Cavan et al. has shown that management of the krill fishery should consider the influential biogeochemical role played by Antarctic krill, larval as well as adult.

In the regulation of atmospheric carbon dioxide (CO₂) levels and in governing the nutrients available for the growth of phytoplankton the biogeochemical cycles of the ocean are paramount (Falkowski, Barber & Smetacek, 1998). As phytoplankton are essential components of most food webs in the ocean, biogeochemistry is also important in fuelling the productivity of fisheries (Pauley & Christensen, 1995). The central focus of many biogeochemical studies (e.g., Falkowski, 1994; Robinson, 2017), has been the role of phytoplankton in the drawdown of atmospheric CO₂ and the production of fish. In spite of evidence of their potential importance, however, higher organisms (metazoa) such as zooplankton (e.g., copepods and salps), nekton (e.g., adult krill and fish), seabirds and mammals (Steinberg et al., 2008; Belcher et al., 2016; Cavan et al., 2017; Steinberg & Landry, 2017; Ratnarajah, Nicol, Bowie, 2018; Davison, 2013; Belcher et al., 2017; Cavan et al., 2017), have received less attention concerning their roles in the biogeochemical cycles.

The biological pump (Falkowski, Barber & Smetacek, 1998) is one of the main mechanisms by which biogeochemical cycles can be influenced by metazoa. A suite of biological processes that ultimately sequester CO₂ from the atmosphere into the deep ocean on long timescales is described by the biological pump. Ocean phytoplankton produce organic matter during photosynthesis and a fraction (<40 %) sinks to the deeper waters (Buesseler & Boyd, 2009). It is estimated that 5-12 Gt of carbon is exported from the surface of the global ocean annually (Henson et al., 2011), and herbivorous metazoa contribute to the biological pump by their release of faecal pellets that sink rapidly, respiring carbon at depth that was originally assimilated in the surface layer of the ocean and by excreting nutrients near the surface promote further growth of phytoplankton (Cavan et al., 2017; Steinberg & Landry, 2017). Recognition of the role of metazoa in biogeochemical cycles is essential (Schmitz et al., 2013) to improve the mechanistic understanding of the environment of the present. Predictions of how biogeochemical cycles may change in the future will be enabled, as metazoa become impacted by multiple anthropogenic pressures such as fishing and climate change (Southward et al., 1995; Gruber et al., 2009).

In this study Cavan et al. focused on the role of krill, in particular Antarctic krill Euphausia superba, in biogeochemical cycles for 3 key reasons:

·        This single species has extraordinarily high biomass so it could have large impacts on biogeochemical cycles (Fielding et al., 2011);

·        They are among the largest pelagic crustaceans with commensurately high swimming speeds, and very high grazing capacity, large faecal pellets that sink rapidly and have the ability to migrate vertically;

·        And because little attention has been given to assessing the importance of krill in the biogeochemical cycles of the ocean.

This review centred on E. superba as the largest krill fishery (Nicol & Foster, 2016) is located in the Southern Ocean, which is one of the largest carbon sinks in the world (Khatiwala, Primeau & Hall, 2009), and is a site of the formation of water mass that transports nutrients throughout the global ocean (Rintoul, 2018). As most crustacea that are fished are benthic organisms, such as crabs and prawns, E. superba are also one of the few pelagic crustacea that is fished commercially (Bondad-Reantaso et al., 2012). Most of the processes discussed in this paper are also relevant to other fisheries, particularly those harvesting small pelagic fish such as anchovy and sardines, which are also plankton feeders and maybe important in biogeochemical cycles (van der Lingen, 1998; Saba & Steinberg, 2012). Management of the E. superba fishery in the Southern Ocean is currently centred on the importance of E. superba in the support of populations of megafauna, such as seals, penguins and whales etc., and in maintaining a sustainable commercial fishery (Nicol & Foster, 2016). There is no consensus at present on the effect harvesting of large quantities of E. superba, or any other species, could have on global biogeochemical cycles, and therefore on atmospheric CO₂ levels. In this synthesis current knowledge of the importance of E. superba in the regulation of biogeochemical cycles in the ocean and consider the effect commercial harvesting of these krill could have on ocean biogeochemistry.

In terms of biomass E. superba are by far the dominant species of krill (Cuzin-Roudy et al., 128). They exhibit a circular distribution which coincides largely with the extent of winter sea ice (Siegal & Watkins, 2016). E. superba typically live 5-6 years in the wild, growing up to 65 mm long, and therefore are larger than any other species of krill some of which (e.g., Meganyctiphanes norvegica and Euphausia pacifica) which have a crucial role in northern marine ecosystems. These northern species are key members of ecosystems that are much more diverse, so only rarely dominate pelagic biomass in the way that E. superba does. The way E. superba are represented in food web models in the Southern Ocean, where they are parameterised as their own functional, species-resolved group while other euphausiids are combined with other species (Fulton, smith & Johnson, 2003). Krill are often regarded as plankton, though all krill are much larger than many planktonic species and the strong swimming abilities of adult krill are a characteristic of nekton (Fevolden & George, 1984). Some of the largest monospecific aggregations (swarms) in the animal kingdom (Tarling & fielding, 2016), which makes them a critical food item for whales, seals and seabirds, and the target of the largest fishery of the Southern Ocean. E. superba are a major grazer of primary production in the Southern Ocean (Whitehouse et al., 2009).

At 19 million km² the spatial distribution of adult E. superba is vast (Atkinson et al., 2009), resulting in great difficulty in conducting a synoptic survey of the entire population, and the result is an estimate of the biomass of krill that is highly uncertain. The method for the assessing of biomass of krill that is preferred is by hydroacoustics, but this involves methodical uncertainties and does not survey either surface (<20 m) or deep water (Nicol & Brierley, 2010). The best estimates obtained from the acoustics of the density of post-larval E. superba in the southwestern Atlantic sector of the Southern Ocean was 29 g m^-2 (biomass = 60 million tonnes) in 2000 (Fielding et al., 2011), and 5.5 g m^-2 (1996) (Nichol et al., 2000) and 23 g m^-2 (2006) (Jarvis et al., 2010) at 2 different sites in the Indian Ocean sector of the Southern Ocean.

The circumpolar estimates of the abundance of E. superba are based on net data, for which there is a much longer historical record of data (Atkinson et al., 2000) than for acoustics. There are also limitations on nets associated with animal avoidance, low sampling frequency and limited sampling of the water column, typically up to 200 m (Nichol & Brierley, 2010). Over several decades, data from multiple scientific net surveys estimate circumpolar post-larval biomass at 379 million tonnes, in which the Southwest Atlantic sector contained the highest biomass (Atkinson et al., 2009). When net and acoustic data are combined a slightly lower estimate of biomass is obtained of 215 million tonnes (Atkinson et al., 2009). The high biomass of E. superba in the Southern Ocean is confirmed further by the large number of predators that are dependent on E. superba. The high biomass of E. superba combined with body size means they a likely to be a significant vector for the recycling of nutrients and transporting carbon on large scales.

Krill and biogeochemical cycles

Carbon

According to Cavan et al. pelagic crustaceans such as krill can have a prominent role in the regulation of the magnitude of carbon that is stored in the ocean by way of the biological pump (Cavan et al., 2017; Steinberg & Landry, 2017; Steinberg et al., 2000). Unicellular phytoplankton transform dissolved inorganic carbon (DIC or CO₂) during photosynthesis, and a portion of this originates from the atmosphere, into organic carbon in their cells in the surface ocean (Steemann-Nielsen, 1957). Krill feed either directly on phytoplankton, or protists and invertebrates (mainly zooplankton) that have fed on phytoplankton. A large part of the carbon that is ingested by the krill is absorbed (estimates of this carbon that is ingested range from 42 to 94%, depending on the food type and availability (Atkinson et al., 2012), the remainder being egested in the faecal pellets. The carbon components that are absorbed are either catabolised to supply energy, which leads to the respiration of CO₂, excreted as dissolved organic carbon (Ruiz-halpern et al., 2011) or is incorporated into the tissues of the body and potentially transferred to their predators, krill.

Faecal pellets are an integral part of the biological pump (Steinberg & Landry, 2017), with some being dense, compact particles that are able to sink rapidly through the ocean. Krill produce large faecal pellets, as krill are some of the largest pelagic crustaceans, typically in strings of up to 1 cm in length, with sinking rates that are variable but often rapid (Atkinson et al., 2012; Wilson, Steinberg & Buesseler, 2008). The majority of the sinking particles, krill faecal pellets, that have been analysed in shallow (170 m) and deep water (1,500 m) in sediment traps in the Southern Ocean that have been deployed to the west of the Antarctic Peninsula and downstream of South Georgia respectively (Gleiber, Steinberg & Ducklow, 2012). The contribution of krill to organic carbon flux can be huge, as krill mostly swarm in vast numbers, and estimates span orders from 7 to 1,300 mg C m^-2 d^-1 (Clarke, Quetin & Ross, 1988; Pakhomov & Froneman & Perissinotto, 2002). Most rates that have been observed, however, tend to be at the lower end of that range, as those that have been reported in the marginal ice zone (e.g., 7-104 mg C m^-2 d^-1 at 100 m (Belcher et al., 2019)). For reference, values of total (all types of particle) particulate organic carbon flux at 100 m in the Southern Ocean, as determined by ²³⁴Th (thorium); ranges form 10 – 600 mg C m^-2 d^-1, with an average of between 100 and 150 mg C m^-2 d^-1 (Maiti et al., 2013)) across latitudes. In the Scotia Sea located in the Atlantic section of the Southern Ocean where there is a high biomass of krill (Fielding et al., 2011), at 100 m in summer total particulate organic flux is up to 90 mg C m^-2 d^-1, with the highest fluxes in the marginal/seasonal ice zone. Over the productive season in the marginal ice zone, the estimate that was modelled of the total export flux of krill faecal pellets at 100 m is 0.04 Gt C y^-1 (Belcher et al., 2019), which is equivalent to 42 mg C m^-2 d^-1 based on the mean area of the marginal ice zone.

As a result of scavenging and degradation the number of faecal pellets that are observed declines with depth (Steinberg et al., 2008; Belcher et al., 2016; Iversen & Poulsen, 2007). Some studies in the marginal and seasonal ice zones of the Southern Ocean indicate, however, that the faecal pellets of krill can be transferred extremely efficiently, with minimum attenuation with depth i.e., the amount of carbon in faecal pellets of krill in the surface is similar to that at depths of 100s of metres below (Belcher et al., 2016; Cavan et al., 2015; Belcher et al., 2017). It has not been observed in other regions of the ocean, or for other crustaceans, that there are such low rates of faecal flux attenuation, which suggests that krill play a disproportionally important role in the sinking of carbon to the deep ocean (Belcher et al., 2019). In ice regions low attenuation of krill pellets is believed to likely be the result of a combination of the behaviour of krill that includes pronounced vertical migrations (Cavan et al., 2015; Belcher et al., 2017; Wefer et al., 1988) and the formation of large swarms that produce a rain of fast-sinking faecal pellets that overwhelm consumption by detrital feeders (Belcher et al., 2016; Atkinson et al., 2012; Belcher et al., 2019; Cavan et al., 2015; Belcher et al., 2017). Also, just below the mixed layer short migrations of 40 m can occur multiple times during a nights feeding, which  is dependent on satiation state of the krill (Tarling & Johnson, 2006; Tarling & Thorpe, 2017), which may increase the chance of the export of faecal pellets deeper into the water column.

When krill occupy deeper layers and respire carbon that is consumed at the surface vertical migration can also shunt carbon to depth, a process that has been termed active carbon flux. This occurs especially in younger developmental stages of E superba (larvae and juveniles), which can undergo extensive diel (daily) vertical migrations (DVMs) (Marr et al., 1962; Hernández-León et al., 2001) as they travel to deeper depths than adults, often being below permanent thermoclines (Tarling & Johnson, 2006; Simmard, Lacroix & Legendre, 1986). During the night larval DVMs may follow a normal pattern of ascent during the night and descent during the day (Atkinson et al., 2014), or reverse pattern of ascent during the day and descent at night (Hernández-León et al., 2001). In adult krill DVM patterns are less clear, and they may exhibit a range of behaviours, which include normal and reverse DVM as well as remaining at particular depths throughout the diel cycle (Godlewska, 1996; Tarling et al., 2018), so they may have a different biogeochemical role which depends on the depths they inhabit or migrate to. In spite of this where DVM does occur in adults, they generally remain above the permanent thermocline, within the surface mixed layer (Schmidt et al., 2016). Evidence of the total contribution of this species to active carbon flux have not yet been resolved fully, as a result of difficulties of resolving the complex DVM of Antarctic krill (Tarling & Johnson, 2006; Tarling et al., 2018; Schmidt et al., 2011).

There are also additional mechanisms by which krill might contribute to the carbon sink. In the populations of adult E. superba, for instance, they appear to move to coastal basins (Reiss et al., 2017) in winter and studies that used underwater cameras and active acoustics have revealed that krill aggregate at greater depths in winter than in summer (Kane et al., 2018; Lascara et al., 1999). When residing in deeper waters in winter, metabolism of their lipid reserves to CO₂ releases surface-produced carbon to the deep ocean, as has been observed in copepods (Jónasdóttir et al., 2015). Termed the lipid pump, this process is significant in that it moves carbon to depth without depleting surface concentrations of nutrients over winter that are potentially limiting (e.g., nitrogen and phosphorus). The short phytoplankton-krill-whale food chain, where krill carbon is stored as biomass in baleen whales for decades, and upon death their carcasses sink rapidly to the deep sea floor, thereby facilitating the rapid transport of carbon to the deep ocean/sea floor (Haag, 2006). Finally, some E. superba also feed on detritus on the sea floor, often at great depth, which are then fed upon by benthic fish and invertebrates, the result being that carbon remains in the deep ocean (Schmidt et al., 2011). It is potentially significant, though remains unquantified, that all these processes contribute to the transport of carbon to the sea floor.

Iron

Primary productivity over large areas of the oceans, including much of the ice-free Southern Ocean, is limited by the low availability of iron, which is an important trace element (Martin, 1990; Boyd, 2004). The deep winter mixing to the surface waters of the Southern Ocean (Tagliabue, 2014) and the seasonal melting of sea ice (59). Further primary production depends increasingly on recycled iron (Tagliabue, 2014) following the pulse of the winter-spring iron (Tagliabue, 2014). An important role is played by E. superba in the cycling of oceanic iron (Schmidt et al., 2016; Schmidt et al., 2011; Nicol et al., 2010; Ratnarajah, 2014; Tovar-Sanchez et al., 2007) which is facilitated by the ingestion of iron-rich phytoplankton and lithogenic particles. The concentration of iron in an individual whole adult krill ranges from 4.4 to 190.5 mg kg^-1 (Schmidt et al., 2016; Schmidt et al., 2011; Nicol et al., 2010; Ratnarajah et al., 2014; Barbante et al., 2000; Ratnarajah, L. et al., 2016), with the >40-fold difference of the iron content in krill reflecting seasonal and regional differences in the content of dietary content (Ratnarajah et al., 2016). The iron retained in individual bodies can eventually be released back into surface waters when E. superba is consumed by baleen whales and other vertebrates and subsequently defecated (Nicol et al., 2010). Therefore, in the Southern Ocean where iron is limiting the iron that is recycled by krill and their predators is important for the stimulation of primary production.

A small proportion of dissolved iron (dFe<0.2 μm) (Schmidt, K. et al., 2016) greater than demand by E. superba is secreted, and excretion rates range from 0.2 to 5.5 nmol dFe ind^-1 d^-1 (Schmidt, K. et al., 2016). When the krill feed on diatoms the highest rates occur, which is consistent with the ability of some diatoms to acquire and store excess intracellular iron (Marchetti et al., 2006). E. superba may also release iron-binding ligands when phytoplankton are digested (e.g., porphyrin compounds) (Ratnarajah, Nicol & Bowie, 2018), which can complex with organic iron and thereby increase the concentration of iron that is available to phytoplankton (Schmidt, K. et al., 2016). Most (90%) of the iron in E. superba is released, however, by their rapidly sinking faecal pellets, which contain 3-4 orders of magnitude more iron than muscle tissue (Schmidt K. et al., 2016). The cycling of iron by krill is therefore linked closely to the fate of their faecal pellets, which may sink to great depth before it is consumed (Cavan et al., 2016; Belcher et al., 2017). It has been shown by a study on salps that iron was not readily leached from their faecal matter (Cabanes et al., 2017), and, if this is also true for krill, their pellets would need to be fragmented in order to release dFe into the water column as the pellet sinks. The feeding activity of the abundant E. superba as a whole nevertheless provides the basis for several pathways of dFe supply to phytoplankton – also involving microbes, zooplankton and krill predators – which, as well as the release of ligands, can benefit the growth of phytoplankton. Such fertilising processes that are mediated by krill may explain why blooms of phytoplankton downstream of the island of South Georgia last longer and are more intensive in years with high abundance of krill on-shelf (Schmidt, K. et al., 2016).

Macronutrient regeneration and grazing

Macronutrients such as ammonium is also released by krill, which can be particularly important in regions where iron is limited, as the iron demand of phytoplankton is reduced by ~30% (Raven, Wollenweber & Handley, 1992) by the use of ammonium instead of nitrate. Regions have been used where frequently high, though spatially variable density of E. superba as a series of natural experiments when examining the role of nutrient recycling and grazing is shaping the abundance and composition of phytoplankton. Grazing was sufficient at South Georgia to suppress the biomass of phytoplankton towards the east of the island (White et al., 2009), though a large fraction of the excretion of ammonium supplied the requirements to the ungrazed cells. At South Georgia the rates of excretion of ammonium have been measured and found to range from 12 to 273 nmol NH₄ ind^-1 h^-1 (Atkins & Whitehouse, 2000), with higher rates being measured further to the south off the Western Antarctic Peninsula (61-475 nmol NH₄ ind^-1 h^-1) (Lahette et al., 2012).

Also, it has been found that grazing and deep mixing by E. superba shifts the community of phytoplankton from diatoms to flagellates at the Antarctic Peninsula (Kopczynska, 1992). Cells of phytoplankton or other particulate matter that are grazed by krill can also be fragmented and release dissolved organic matter into the water (termed sloppy feeding (Iversen & Poulsen, 2007; Lampitt, Noji & Bodungen, 1990), which can be broken down further and remineralised by bacteria (termed microbial gardening) (Mayor et al., 2014). The flux of carbon to the deep ocean is reduced by this process, though so far it has not been shown explicitly for krill that there is a link between sloppy feeding and increased microbial activity. E. superba can thereby exert 2 opposing top-down controls on phytoplankton; they can graze blooms which decreases the biomass of phytoplankton while increasing the biomass of phytoplankton by the release of nutrients.

Transport of nutrients

As well as shunting carbon to deeper waters, krill also are involved in the vertical and lateral transport of other nutrients. Adult E. superba, for instance, moult up to once every 2 weeks, the time between moults depending on the temperature and the season (Buchholz, 1991), with the result that they moult many times over their life span of 5-6 years in the wild. Their moults sink at the rate of 50-1,000 m d^-1 (Nicol & Stolp, 1989), and this contributes to the carbon sink, as well as also to the release of other micronutrients to the water column as the moult sinks. The concentrations of fluorine in the exoskeletons of live E. superba, for instance, are 2,500 times higher, at least, than the surrounding waters (Adelung et al., 1987), and this fluoride is leached out during ecdysis (Nicol & Stolp, 1985) and degradation of the exoskeletons. There are also a range of other elements that are present in krill exoskeletons, e.g., the exoskeleton contains 47% of the phosphorus and 84% of the calcium concentrations of these minerals in the krill (Tou, Jaczynski & Chen, 2007). The speed at which these nutrients are released from the exoskeletons (moults) and their possible contribution to biogeochemical cycles has not yet been quantified. Krill can  also mix nutrients; mass migrations of krill swarms  from deep waters that are nutrient-rich, particularly in waters that are localised, permanently or temporarily oligotrophic, could mix nutrients to the surface and stimulate the growth of phytoplankton (Tarling & Thorpe, 2017; Houghton et al., 2018). Conversely, the carbon that the krill have transferred from the surface to below the mixed layer is subjected to remineralisation by bacteria and detritivores, which convert dissolved organic carbon to CO₂ (Belcher et al., 2016; Iversen & Poulsen, 2007). A crucial factor in determining the longevity of storage of CO₂ in the deep ocean, is the depth at which this mineralisation occurs, or the depth of krill respiration, i.e., whether the co₂ that has been released is mixed back to the surface (shallow remineralisation) or is stored for decades in the deep ocean (deep remineralisation) (Cavan et al., 2019). CO₂ will be subjected to seasonal physical mixing to the surface of the ocean and potentially re-exchanged with the atmosphere within a year following release from the krill, if the CO₂ that is released above the permanent thermocline (deeper winter mixed layer depth, globally < 750 m) (Hosoda et al., 2010). As a result of ocean circulation (Broecker, 1991) the length of time CO₂ (or nutrients) will remain in the deep ocean also depends on the water mass it enters.

For E. superba living to the south of the Antarctic Circumpolar Current (ACC, i.e., a substantial part of the population) (Henschke, Everett, Richardson & Suthers, 2016), any nutrients that are released from E. superba are likely to remain in the Southern Ocean. Nutrients released from an organism that is within the ACC, or at the northern boundary of the ACC, may be subducted into the Antarctic Intermediate Water. According to Cavan et al. it is not known at present whether nutrients that are released by organisms in the Southern Ocean make a significant contribution to production elsewhere.

Larval stages

The contribution of larval krill to biogeochemical cycles differs from that of adults as a result of their unique pattern of growth and development, smaller size and feeding ecology (Kawaguchi, 2016). Sea ice is used by larval E. superba as a feeding ground and shelter (Meyer et al., 2017) and play an important part of ice-pelagic coupling, as a result of their ingestion of ice biota and subsequent migration into the water column. The larvae of E. superba consume up to 26% of their body weight in carbon per day, and about 10% of this is egested as faecal pellets (Meyer, Atkinson, Blume & Bathmann, 2003). This equates to egestion by the larvae of ~4 μg C d^-1, which is ~ 1,000 times less than adults (Tarling & Johnson, 2006) though they can be up to 100 times more abundant than adults in the Scotia Sea (Brinton, 1985). This would equate to a larval contribution of an additional 1-10% of the faecal flux from adults, if these relative abundances hold across the wider section. Also, DVM in larval E. superba takes them considerably deeper than adults (400 m and 200 m respectively) (Marr, 1962; Tarling et al., 2018). Cavan et al. suggest that the pronounced DVM patterns of larval krill in the proximity of ice may be responsible for the low attenuation of krill faecal pellets with depth in the marginal ice zone of the Atlantic sector of the Southern Ocean (Belcher et al., 2016; Cavan et al., 2015), rather than the DVMs of adult krill. It may be that larvae are more likely to contribute to active transport of carbon via egestion and respiration at depth, though the mass and sinking potential of faecal pellets from larvae have not yet been characterised.

In summary, many biogeochemical cycles, including carbon, nitrogen and iron, are influenced by E. superba, from larval through to adult life stages, as well as having a diverse, multifaceted role within these individual elemental cycles. Given the current lack of knowledge and uncertainty in estimates of biomass, (Box 1) it is difficult to quantify its complete role in these cycles, while there has been some focus on the contribution by E. superba to cycles of organic carbon and iron. Nonetheless, the substantial biomass, diurnal vertical migrations and broad horizontal distributions of E. superba suggest a significant contribution. According to Cavan et al. these rates of quantification, as well as estimates of krill biomasses that are constrained better are crucial to providing meaningful data so that modellers can parameterise sufficiently the influence that E. superba has on nutrient cycles. Assessment of the impact of human activities, particularly fishing, on biogeochemical cycles and help in identifying management approaches that will minimise these impacts, will be allowed by better understanding of krill-nutrient interactions.

Implications of declining E. superba biomass

As a result of the complex biogeochemical roles of E. superba the harvesting of krill could have a variable and potentially opposing effect on the biogeochemistry of the ocean. Cavan et al. detail here the possible impacts the harvesting of krill could have on the carbon sink of the Southern Ocean given current knowledge. Also, they discuss briefly the biogeochemical implications of potential changes in the biomass of the krill owing to the recovery of the populations of whales and to climate change.

As has been discussed, large krill faecal pellets that sink fast can form a large proportion of total particulate organic carbon flux in the Southern Ocean (Belcher et al., 2016; Cavan et al., 2015). This flux of faecal pellets will decrease if krill are removed from the system.

Summary

According to Cavan et al. E. superba has a prominent role in the cycling of nutrients in the Southern Ocean because of its large size, high biomass and its swarming behaviour, as well as physiological traits, e.g., large faecal pellets and excretion of these pellets into waters that are limited in nutrients. It is shown by the vertical migratory habit throughout the water column, a trait that is particularly prominent in larvae, though it is more complex in adults, that they can influence the deep carbon sink as well as stimulating primary production at the surface. The cycling of carbon, carbon and ammonium by E. superba has a role that is particularly significant compared with krill in other regions, as the Southern Ocean plays a role that is disproportionally important in the global carbon sink, and productivity is limited in areas that are limited by the depletion of iron. The life history traits of all krill such as large body size, swimming ability which means, potentially, that other species of krill are important in biogeochemical cycles.

It has been shown by Cavan et al. that the role of E. superba in biogeochemical cycles is significant, though uncertain: this uncertainty extends to the times and locations when there is the most intense biogeochemical activity and to the magnitude of the role of E. superba. Ongoing efforts, that are particularly crucial, are continuing to determine the absolute biomass of E. superba and determine their depths of residence and patterns of migration, including that of larvae.

The lack of knowledge regarding the true extent of the ability of krill to regulate biogeochemical cycles is a concern as E. superba are targeted by the largest fishery in the Southern Ocean. There has been no active consideration of the biogeochemical role of krill by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) or to the knowledge of Cavan et al. the biogeochemical role of any other managed fishery, though the E. superba fishery is managed and regulated by CCAMLR. On the global scale measures to maintain biomass and productivity of stocks of fished species help indirectly to preserve their biogeochemical role. The management of the  fishery needs to consider the influence harvesting has on biogeochemical cycles, E. superba biomass and their biogeochemical role that are both likely to be impacted by the activity of fisheries and climate change, with implication for biogeochemical cycles in the future that are not certain.

Sources & Further reading  

Cavan, E. L., et al. (2019). "The importance of Antarctic krill in biogeochemical cycles." Nature Communications 10(1): 4742." Nature Communications 10(1): 4742.