Abstract:
Vertical migration is a geographically and taxonomically widespread behaviour among zooplankton that spans across
diel and seasonal timescales. The shorter-term diel vertical migration (DVM) has a periodicity of up to 1 day and was
first described by the French naturalist Georges Cuvier in 1817. In 1888, the German marine biologist Carl Chun
described the longer-term seasonal vertical migration (SVM), which has a periodicity of ca. 1 year. The proximate control
and adaptive significance ofDVM have been extensively studied and are well understood. DVM is generally a behaviour
controlled by ambient irradiance, which allows herbivorous zooplankton to feed in food-rich shallower waters during the
night when light-dependent (visual) predation risk is minimal and take refuge in deeper, darker waters during daytime.
However, DVMs of herbivorous zooplankton are followed by their predators, producing complex predator–prey patterns
that may be traced across multiple trophic levels. In contrast to DVM, SVM research is relatively young and its
causes and consequences are less well understood. During periods of seasonal environmental deterioration, SVM allows
zooplankton to evacuate shallower waters seasonally and take refuge in deeper waters often in a state of dormancy. Both
DVM and SVM play a significant role in the vertical transport of organic carbon to deeper waters (biological carbon
sequestration), and hence in the buffering of global climate change. Although many animal migrations are expected to
change under future climate scenarios, little is known about the potential implications of global climate change on zooplankton
vertical migrations and its impact on the biological carbon sequestration process. Further, the combined influence
of DVM and SVM in determining zooplankton fitness and maintenance of their horizontal (geographic)
distributions is not well understood. The contrasting spatial (deep versus shallow) and temporal (diel versus seasonal) scales
over which these two migrations occur lead to challenges in studying them at higher spatial, temporal and biological resolution
and coverage. Extending the largely population-based vertical migration knowledge base to individual-based
studies will be an important way forward. While tracking individual zooplankton in their natural habitats remains a
major challenge, conducting trophic-scale, high-resolution, year-round studies that utilise emerging field sampling and
observation techniques, molecular genetic tools and computational hardware and software will be the best solution to
improve our understanding of zooplankton vertical migrations.
