Diel vertical migration in zooplankton—in which many taxa like copepods move downward into darkness during the day and back up toward surface in order to feed at night—has been described as the largest animal migration on the planet. But what really are the tradeoffs and behavioural rules that govern this, in the more complex form that vertical migration takes in polar oceans, where daily, seasonal, and interannual variations in light all play a role?
Hobbs et al. 2021, “A marine zooplankton community vertically structured by light across diel to interannual timescales” looks at this using a long mooring record from Kongsfjorden in Svalbard, showing that across all these timescales, a single isolume (depth-varying line of constant light intensity) forms a ceiling on the bulk of the vertical zooplankton distribution. This raises a lot of questions about how to model the population-level impact of changing light on zooplankton.