The intriguing codistribution of the copepods Calanus hyperboreus and Calanus glacialis in the subsurface chlorophyll maximum of Arctic seas

Studying the distribution of zooplankton in relation to their prey and predators is challenging, especially in situ. Recent developments in underwater imaging enable such fine-scale research. We deployed the Lightframe On-sight Keyspecies Investigation (LOKI) image profiler to study the fine-scale (1 m) vertical distribution of the copepods Calanus hyperboreus and C. glacialis in relation to the subsurface chlorophyll maximum (SCM) at the end of the grazing season in August in the North Water and Nares Strait (Canadian Arctic). The vertical distribution of both species was generally consistent with the predictions of the Predator Avoidance Hypothesis. In the absence of a significant SCM, both copepods remained at depth during the night. In the presence of a significant SCM, copepods remained at depth in daytime and a fraction of the population migrated in the SCM at night. All three profiles where the numerically dominant copepodite stages C4 and C5 of the two species grazed in the SCM at night presented the same intriguing pattern: the abundance of C. hyperboreus peaked in the core of the SCM while that of C. glacialis peaked just above and below the core SCM. These distributions of the same-stage congeners in the SCMs were significantly different. Lipid fullness of copepod individuals was significantly higher in C. hyperboreus in the core SCM than in C. glacialis above and below the core SCM. Foraging interference resulting in the exclusion from the core SCM of the smaller C. glacialis by the larger C. hyperboreus could explain this vertical partitioning of the actively grazing copepodite stages of the two species. Alternatively, specific preferences for microalgal and/or microzooplankton food hypothetically occupying different layers in the SCM could explain the observed partitioning. Investigating the observed fine-scale co-distributions further will enable researchers to better predict potential climate change effects on these important Arctic congeners. The book deals with Diel Vertical Migration (DVM) of zooplankton in oceans and lakes and is the first critical discussion of the literature in 100 years of research. The accent is on photo-response experiments that revealed the physiological fundament unifying migration behaviour in both biotopes. Accelerations in relative changes in light intensity of dawn and dusk are the stimuli that trigger a PhotoBehaviour Mechanisms (PBM) evolved to realise predator evasion and starvation prevention. Physiology and behaviour are tuned to these adaptive goals. A "set of ecological factors" is necessary and an algorithm shows the operation of the "set". However, not only the kinetic component of behaviour is based on light, also orientation but now the angular light distribution is responsible. Contrast orientation as in Daphnia may also hold for other animals, for example, Euphausia.The application of the PBM in lakes and oceans is demonstrated amongst other for the vertical movements of Sound Scattering Layers. These layers move faster, slower or as fast as an isolume which was a problem for the decennia long explanation that migrating animals followed an optimal light intensity. The enigma was solved. Using time series of changes in population size, egg ratios, development times and death rates due to predation by juvenile fish, the influence of DVM on population dynamics was analysed. Finally, covering the flow of matter in the traditional food web by a network of information transitions illustrates the controlling function of infochemicals, such as fish kairomones. The book deals with Diel Vertical Migration (DVM) of zooplankton in oceans and lakes and is the first critical discussion of the literature in 100 years of research. The accent is on photo-response experiments that revealed the physiological fundament unifying migration behaviour in both biotopes. Accelerations in relative changes in light intensity of dawn and dusk are the stimuli that trigger a PhotoBehaviour Mechanisms (PBM) evolved to realise predator evasion and starvation prevention. Physiology and behaviour are tuned to these adaptive goals. A "set of ecological factors" is necessary and an algorithm shows the operation of the "set". However, not only the kinetic component of behaviour is based on light, also orientation but now the angular light distribution is responsible. Contrast orientation as in Daphnia may also hold for other animals, for example, Euphausia. The application of the PBM in lakes and oceans is demonstrated amongst other for the vertical movements of Sound Scattering Layers. These layers move faster, slower or as fast as an isolume which was a problem for the decennia long explanation that migrating animals followed an optimal light intensity. The enigma was solved. Using time series of changes in population size, egg ratios, development times and death rates due to predation by juvenile fish, the influence of DVM on population dynamics was analysed. Finally, covering the flow of matter in the traditional food web by a network of information transitions illustrates the controlling function of infochemicals, such as fish kairomones Whatever theory may be advanced to explain diurnal migration, the underlying reactions involved must be demonstrated conc- sively in the laboratory before the explanation can be ?nally accepted George L. Clarke 1933 p. 434 In oceans and lakes, zooplankton often make diel vertical migrations (DVM), descending at dawn and coming up again in late afternoon and evening. The small animals cover distances of 10–40 m in lakes or even a few hundred metres in the open oceans. Although not as spectacular as migrations of birds or the massive movements of large mammals over the African savannas, the numbers involved are very large and the biomass exceed the bulk of the African herds. For example, in the Antarctic oceans swarms of "Krill" may cover kilometres across, with thousands of individuals per cubic metre. These Euphausiids are food for whales, the most bulky animals on earth. Zooplankton are key species in the pelagic food web, intermediary between algae and ?sh, and thus essential for the functioning of the pelagic community. Prey for many, they have evolved diverse strategies of survival and DVM is the most imp- tant one. Most ?sh are visually hunting predators and need a high light intensity to detect the often transparent animals. By moving down, the well-lit surface layers are avoided but they have to come up again at night to feed on algae. Introduction Materials and Methods Study area LOKI deployment Automatic identification of copepod taxa using machine learning Fluorescence, chlorophyll a and light Statistical analyses Results Sea ice and the surface phytoplankton bloom in the study area Copepodite stage composition and distribution Fine-scale vertical distributions of Calanus copepodite stages Discussion A south-north gradient in ecosystem maturity Predator Avoidance Hypothesis and the vertical distribution of Calanus Fine scale vertical habitat partitioning in the SCM Data Accessibility Statement Supplemental files Acknowledgements Funding information Competing interests Author contributions References Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Table 1 Table 2 Windows : an introduction Swimming in a strange biotope Light-induced, reactive swimming A decision-making mechanism Mechanistic models Light and temperature Optical orientations Considerations before going into the field Diel vertical migration in lakes Migrations in the marine environment The confrontation of experimental and field studies From the individual to the population and beyond Recapitulations and considerations.