Vertical distribution and diel vertical migration of krill beneath snow-covered ice and in ice-free waters

Abstract

A bottom mounted upward looking Simrad EK60 120-kHz echo sounder was used to study scattering layers (SLs) and individuals of the krill Meganyctiphanes norvegica. The mooring was situated at 150-m depth in the Oslofjord, connected with an onshore cable for power and transmission of digitized data. Records spanned 5 months from late autumn to spring. A current meter and CTD was associated with the acoustic mooring and a shore-based webcam monitored ice conditions in the fjord. The continuous measurements were supplemented with intermittent krill sampling campaigns and their physical and biological environment. The krill carried out diel vertical migration (DVM) throughout the winter, regardless of the distribution of potential prey. The fjord froze over in mid-winter and the daytime distribution of a mid-water SL of krill immediately became shallower associated with snow fall after freezing, likely related to reduction of light intensities. Still, a fraction of the population always descended all the way to the bottom, so that the krill population by day seemed to inhabit waters with light levels spanning up to six orders of magnitude. Deep-living krill ascended in synchrony with the rest of the population in the afternoon, but individuals consistently reappeared in near-bottom waters already <1 h after the ascent. Thereafter, the krill appeared to undertake asynchronous migrations, with some krill always being present in near-bottom waters even though the entire population appeared to undertake DVM.

Wintertime water dynamics and moonlight disruption of the acoustic backscatter diurnal signal in an ice‐covered Northeast Greenland fjord

“…The observed patterns of the ADCP‐derived VBS and vertical velocity below the land‐fast ice during winter 2013–2014 were consistent with diel vertical migration (DVM) of zooplankton, during which zooplankton moved toward the surface during the evening, and descends the next morning. Without zooplankton sampling, it was not possible for us to specify the exact species involved in DVM [ Cottier et al ., ; Falk‐Petersen and Hopkins , ; Falk‐Petersen and Kristensen , ; Falk‐Petersen et al ., ; Vestheim et al ., ], and hence we follow those studies that use only acoustical methods to determine and interpret the patterns of the DVM signal [e.g., Fischer and Visbeck , ; Berge et al ., ].…”

“…ADCP velocity observations detect the presence of relatively fast moving animals with vertical velocities of 1.5 cm/s that migrate to 80 m or deeper ( Figure 6). Krill (Euphausiacea) and Themisto are known to move down to 100-300 m depth in DVM at 708N [Falk-Petersen and Hopkins, 1981;Vestheim et al, 2014]. The estimated vertical migration speed fits nicely into the range of 1-6 cm/s characteristic for copepods and euphausiids [Heywood, 1996].…”

“…Nevertheless, the acoustic backscatter has been found to be correlated to zooplankton biomass [Flagg and Smith, 1989;Brierley et al, 1998;Fielding et al, 2004;Berge et al, 2009;Hamilton et al, 2013;Bozzano et al, 2014]. It was noted, however, that ADCPs, unlike echo-sounders, are limited in deriving accurate quantitative estimates of biomass due to calibration difficulties because their acoustic beams are narrow and are inclined to the vertical [Sato et al, 2013;Vestheim et al, 2014;Brierley et al, 1998]. With application of beam geometry correction, ADCPs are commonly used for qualitative studies of biomass, as they can provide information on zooplankton behavior [e.g., Pinot and Jans a, 2001;Chenghao et al, 2013].…”

Impact of tidal dynamics on diel vertical migration of zooplankton in Hudson Bay

“…ADCPs, unlike echo sounders (Lemon et al, 2012(Lemon et al, , 2001, are limited in deriving accurate quantitative estimates of biomass due to calibration difficulties because their acoustic beams are narrow and inclined from the vertical (Brierley et al, 1998;Lemon et al, 2008;Sato et al, 2013;Vestheim et al, 2014). But with the application of beam geometry correction, ADCPs are commonly used for qualitative studies, as they can provide information on zooplankton presence and behavior (Hobbs et al, 2018;Last et al, 2016;Petrusevich et al, 2016).…”

Individual‐based modeling explains the contrasted seasonality in size, growth, and reproduction of the sympatric Arctic (Thysanoessa raschii) and Nordic krill (Meganyctiphanes norvegica) in the St. Lawrence Estuary, eastern Canada

“…Both species perform extensive diel vertical migrations (DVM) all year round in the EGSL (Sourisseau et al ; Plourde et al ). M. norvegica individuals have been observed elsewhere performing DVM during all seasons of the year (Tarling ), while continuing to feed within the surface layer (Vestheim et al ). The timing and extent of these DVM are essentially controlled by external factors such as photoperiod (Tarling ; Sourisseau et al ), vertical profile of light scattering in the water column (Cohen and Forward ), the temperature (Plourde et al ), and the availability of food (Lass et al ).…”