Zooplankton ascend to surface waters at night to feed on abundant phytoplankton, then descend to depth during the day to avoid visual predators. This behavior creates the largest animal migration on Earth by biomass and drives significant vertical energy and nutrient transport (active transport), moving carbon and sustaining deep-sea communities independent of sinking particles.
Use acoustic data to track the deep scattering layer throughout diel cycles. Measure gut content and energy reserves in zooplankton collected at different times and depths. Model predation risk and feeding benefits to explain migration patterns and amplitude.
Diel migration is not a simple day-night toggle; it is more nuanced (twilight-triggered, ontogenetic shifts) and varies with moon phase and local predation pressure. Not all zooplankton migrate; large copepods and some euphausiids show smaller amplitudes. Migration is energetically costly; it represents a trade-off between feeding and predation avoidance.
From your study of the photic zone, you know that sunlight penetrates only the upper 200 meters or so of the ocean, and that this well-lit layer is where nearly all photosynthesis — and therefore nearly all primary food production — occurs. From your study of zooplankton food web structure, you know that zooplankton are the crucial link between phytoplankton and higher trophic levels. The dilemma facing zooplankton is stark: the food is at the surface, but so are the predators. Diel vertical migration (DVM) is evolution's solution to this problem, and it constitutes the largest synchronized animal movement on Earth.
Every evening at twilight, vast populations of copepods, euphausiids (krill), and other zooplankton begin ascending from depths of 200–1,000 meters toward the surface. They feed on phytoplankton through the night, then descend back to depth before dawn. The trigger is light intensity — specifically the rate of change of light at twilight, not an absolute threshold. The logic is straightforward: visual predators like fish and seabirds hunt by sight. By occupying the dark mesopelagic zone during daylight hours, zooplankton become nearly invisible to these predators. The energy cost of swimming hundreds of meters twice daily is substantial — estimates suggest migration can consume 10–30% of a zooplankter's daily energy budget — but the survival benefit outweighs the cost in environments where predation pressure is high.
The migration is not uniform across species, sizes, or life stages. You might expect from the thermocline structure you studied that migrating through a sharp temperature gradient imposes metabolic costs — and it does. Larger, more conspicuous zooplankton tend to migrate deeper (they are more visible to predators), while smaller species may not migrate at all. Juvenile stages often migrate differently than adults, and migration amplitude varies with moon phase: on bright, full-moon nights, some species descend deeper or reduce migration because moonlight extends the visual hunting window for predators. This behavioral plasticity shows that DVM is not a rigid program but an adaptive response continually tuned to the local predation landscape.
The biogeochemical consequences of DVM are profound. When zooplankton feed at the surface and then descend to depth, they carry carbon with them — in their guts, in their bodies, and as fecal pellets released at depth. This active transport of carbon bypasses the slow sinking of dead organic particles (the passive biological pump) and delivers carbon directly to the mesopelagic and bathypelagic zones, where it can be sequestered for decades to centuries. Estimates suggest that DVM-driven active transport accounts for 15–40% of total downward carbon flux in some ocean regions. The migrators also excrete dissolved nutrients (ammonium, phosphate) at depth, fueling microbial communities far below the sunlit layer. Without this nightly conveyor belt, deep-ocean ecosystems would be significantly less productive, and the ocean's role in the global carbon cycle would be diminished.