Oceanic swarms of Antarctic krill perform satiation sinking

Abstract

Antarctic krill form some of the highest concentrations of animal biomass observed in the world's oceans potentially due to their prolific ability to swarm. Determining the movement of Antarctic krill within swarms is important to identify drivers of their behaviour and their biogeochemical impact on their environment. We examined vertical velocity within approximately 2000 krill swarms through the combined use of a shipborne echosounder and an acoustic Doppler current profiler. We revealed a pronounced downward anomaly in vertical velocity within swarms of -0.6 cm s compared with vertical motion outside the swarm. The anomaly changed over the diel cycle, with smaller downward anomalies occurring at night. Swarms in regions of high phytoplankton concentrations (a proxy for food availability) also exhibited significantly smaller downward anomalies. We propose that the anomaly is the result of downward velocities generated by the action of krill beating their swimming appendages. During the night and in high phytoplankton availability, when krill are more likely to feed to the point of satiation, swimming activity is lowered and the anomaly is reduced. Our findings are consistent with laboratory work where krill ceased swimming and adopted a parachute posture when sated. Satiation sinking behaviour can substantially increase the efficiency of carbon transport to depth through depositing faecal pellets at the bottom of swarms, avoiding the reingestion and break-up of pellets by other swarm members.

Predatory zooplankton on the move: Themisto amphipods in high-latitude marine pelagic food webs

“…This was also visible in the acoustic record, where small swarms occurring between 50 and 100 m tended to descend after sunrise, to greater depths, sometimes to the shelf floor (Pakhomov & Froneman, 1999). A sinking behaviour towards deeper depths straight after feeding may also explain these descents, similar to the satiation sinking behaviour discovered for Antarctic krill (Tarling & Thorpe, 2017). In another study, nighttime abundance of T. gaudichaudii was consistently higher than day time levels on the Prince Edward Islands' shelf, and no vertical variation in distribution between size classes was observed (Pakhomov & Froneman, 1999).…”

The importance of Antarctic krill in biogeochemical cycles

“…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 likely owing to a combination of krill behaviour including pronounced vertical migrations 37,39,40 and the formation of large swarms that produce a 'rain' of fast-sinking faecal pellets that overwhelm detrital consumers 6,27,35,37,39 . In addition, short migrations (40 m) just below the mixed layer can occur multiple times during a night's feed, dependent on the satiation state of the krill 41,42 , which may increase the chance of faecal pellet export by shunting pellets deeper into the water column. Vertical migrations can also shunt carbon to depth when krill occupy deeper layers and respire carbon consumed at the surface, a process termed active carbon flux.…”

“…Krill can also mix nutrients; mass migrations of krill swarms from deep nutrient-rich water, particularly in localised, permanently or temporarily oligotrophic waters, could mix nutrients to the surface and stimulate phytoplankton growth 42,78 (Fig. 3).…”

Successful ecosystem-based management of Antarctic krill should address uncertainties in krill recruitment, behaviour and ecological adaptation

“…Other studies suggest that there has been a long-term southward contraction of krill distribution in the SW Atlantic sector, with populations becoming concentrated toward the Antarctic continental shelves 38,87 as a consequence of climatic-driven ecosystem changes. Whether or not krill will track their thermal niche southward is complicated because of the interaction with other habitat requirements that are important for krill, including water depth, ice regimes, and quantity of primary production 88 . Models of krill habitat under projected future changes in ocean temperature (increase), sea ice (decrease) and chlorophyll-a concentration (decrease) under the business-as-usual emission scenario could result in an 80% contraction of the available krill spawning habitat, with a complete disappearance of spawning grounds along the WAP by 2100 [89][90][91] .…”