Diel vertical migration (DVM) is the daily mass movement of ocean animals between deep water and the surface, and it is the largest migration on Earth. Every evening, vast numbers of zooplankton, fish, and squid swim upward hundreds of meters to feed in nutrient-rich surface waters under the cover of darkness. At dawn, they descend again to the relative safety of the deep ocean. This cycle moves an estimated one quadrillion grams of carbon every single day.
How the Migration Works
The basic pattern is straightforward: organisms spend daylight hours at depth and nighttime hours near the surface. The upward journey typically begins at dusk and the return trip starts at dawn. Most migrating plankton swim at roughly 0.03 meters per second, a pace they sustain for about three hours, covering a vertical distance of around 300 meters. Some species travel much farther. Acoustic monitoring has tracked migrating layers from 500 meters all the way down to 1,600 meters below the surface.
The timing is remarkably precise. At shallower depths (above 650 meters), migrating plankton track the solar day length almost exactly. Deeper down, their perceived “day” shortens because less light penetrates. At 1,600 meters, plankton experience a day length of only about 7.5 hours, roughly 63% of the actual solar day at the surface.
What Triggers the Movement
Light is the primary cue. Migrating animals appear to follow specific light levels, called isolumes, that act like invisible depth markers in the water column. The strongest migrations occur when organisms track light intensities between 0.0001 and 0.01 watts per square meter, dim enough to reduce visibility for predators but bright enough to serve as a reliable signal for timing.
Light alone doesn’t explain the full picture, though. Research on Antarctic krill has shown that an internal biological clock also plays a role. When krill were kept in constant darkness in a lab, they continued to migrate on a rhythmic schedule, though the cycle shortened from 24 hours to roughly 12 hours. Their oxygen consumption (a proxy for metabolic activity) also continued to cycle rhythmically. This means DVM is not purely a reaction to changing light. The animals carry an internal timer that keeps the behavior going even without environmental cues, likely an adaptation for survival in polar regions where daylight patterns are extreme.
Why Animals Migrate Vertically
The driving force behind DVM is predator avoidance. Surface waters are where the food is, since that’s where sunlight fuels the growth of phytoplankton. But surface waters during the day are also dangerous. Fish and other visual predators hunt by sight, and being visible near the surface in daylight is a serious risk. By feeding at the surface only at night and retreating to dark depths during the day, migrating animals dramatically reduce their chances of being spotted and eaten.
This comes with a cost. Deeper water is colder, darker, and food-poor. Animals that spend their days at depth grow more slowly than they would if they stayed at the surface full time. But the trade-off works in their favor: the reduction in predation risk more than compensates for the lost feeding time. Even a moderate decrease in predation rate is enough to offset the fitness costs of spending half the day in cold, unproductive water.
The Metabolic Bonus of Cold Water
Retreating to cold, deep water turns out to have a hidden benefit beyond predator avoidance. Cold temperatures slow metabolism, meaning animals burn fewer calories while they’re resting at depth. This alone would seem like a disadvantage, since slower metabolism usually means slower growth. But experiments on Daphnia (small freshwater crustaceans often used as stand-ins for marine zooplankton) revealed something surprising.
Animals that alternated between warm and cold water, mimicking a DVM cycle, grew larger than expected. They didn’t just split the difference between warm-water growth and cold-water growth. They actually exceeded the average, reaching sizes bigger than a simple temperature calculation would predict. The reason appears to be that organisms acquire nutrients more efficiently in cold water. Under nutrient-limited conditions, the temperature cycling of DVM may help migrating animals maintain population growth that would otherwise stall. In other words, the “cost” of migrating to cold depths is significantly lower than scientists originally assumed.
Reverse Migration
Not all species follow the standard pattern. Some perform what’s called reverse DVM: staying near the surface during the day and descending at night. This sounds counterintuitive, since it means being visible to predators in daylight. But modeling work on copepod communities has shown that reverse migration tends to occur in intermediate-sized organisms, roughly 0.4 to 0.8 centimeters long. In the same environment, the smallest copepods don’t migrate at all (staying near the surface), while the largest ones perform normal DVM.
The logic behind reverse migration involves a kind of predator shell game. By associating with dangerous areas or species, like staying near large predators that threaten their own predators, small copepods can gain protection. It’s a risky strategy that only works under specific ecological conditions, but it has been directly observed in Norwegian fjords.
How DVM Shapes the Carbon Cycle
DVM plays a major role in moving carbon from the surface ocean to the deep sea, a process scientists call the biological pump. Here’s how it works: migrating animals feed on carbon-rich plankton at the surface, then descend to depth where they respire, excrete waste, and sometimes die. All of that releases carbon hundreds of meters below where it was consumed, effectively transporting it away from the atmosphere.
Modeling studies estimate that DVM increases the total carbon export from surface waters by about 14%. Without the migration, the global carbon export flux would be roughly 5.7 petagrams of carbon per year. With DVM included, that number rises meaningfully. Given that the ocean’s biological pump is one of the planet’s major carbon sinks, DVM is not just an ecological curiosity. It is a significant component of the global carbon cycle.
How Scientists First Discovered DVM
The story begins during World War II, when Navy technicians using newly invented sonar noticed something strange. The seafloor appeared to be much shallower than charts indicated, and its apparent depth changed throughout the day. What they were seeing wasn’t the seafloor at all. It was an enormous, dense layer of marine organisms, so tightly packed that sonar bounced off them like a solid surface. Scientists named it the deep scattering layer. Once researchers realized the layer rose at night and sank during the day, they had discovered diel vertical migration on a massive scale.
Threats From a Changing Ocean
Climate change is reshaping the ocean in ways that could disrupt DVM. One of the most significant threats is the expansion of oxygen minimum zones, regions of the ocean where dissolved oxygen levels are too low for most animals to survive. In coastal upwelling systems off northern Chile, the upper boundary of the oxygen minimum zone has been rising at a rate of 1.3 to 2.0 meters per year. As this low-oxygen layer creeps upward, it squeezes the habitable depth range for migrating zooplankton. Species with low tolerance for low-oxygen conditions face physiological stress and increased mortality, potentially disrupting both the migration itself and the food webs and carbon cycling that depend on it.