Data from: Oceanic swarms of Antarctic krill perform satiation sinking

Antarctic krill form some of the highest concentrations of animal biomass observed in the world’s ocean 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 (ADCP). We revealed a pronounced downward anomaly in vertical velocity within swarms of -0.6 cm.s-1 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 breakup of pellets by other swarm members.南极磷虾（Antarctic krill）是全球海洋中动物生物量浓度最高的类群之一，这一现象可能源于它们极强的集群能力。明确南极磷虾在集群内的运动特征，对于解析其行为驱动因素以及其对周边环境的生物地球化学影响至关重要。本研究结合船载回声测深仪与声学多普勒流速剖面仪（acoustic Doppler current profiler, ADCP），对约2000个磷虾群内的垂直速度开展了观测。研究发现，与集群外的垂直运动相比，集群内部的垂直速度存在显著的-0.6 cm·s⁻¹向下异常。该异常随昼夜周期发生变化，夜间的向下异常幅度更小。在浮游植物浓度较高（作为食物可获得性的替代指标）的区域内的磷虾群，其向下异常幅度也显著更低。我们推测，该异常源于磷虾摆动游泳附肢所产生的向下水流速度。在夜间以及食物充足、磷虾易达到饱食状态的情况下，其游泳活动会减弱，垂直速度异常也随之降低。本研究结果与实验室观测结果一致：饱食后的磷虾会停止游泳，并采取降落伞姿态。饱食状态下的下沉行为可通过将粪便颗粒沉积在集群底部，避免其被其他集群成员重新摄食或分解，从而大幅提升碳向深海输送的效率。