In the vast expanse of the ocean, a remarkable daily phenomenon unfolds. Many marine creatures undertake synchronized journeys, moving vertically through the water column in what scientists refer to as diurnal vertical migration (DVM). This widespread behavior represents one of the largest animal migrations on Earth in terms of biomass, occurring every single day across all oceans. It involves a diverse array of life, from microscopic organisms to larger predators.
Defining Diurnal Vertical Migration
Diurnal vertical migration describes the synchronized, daily movement of marine animals between the surface and deeper layers of the ocean. The “diurnal” aspect refers to its occurrence over a 24-hour cycle, while “vertical” signifies movement up and down the water column. Typically, organisms ascend towards the surface waters as dusk approaches and remain there during the night. As dawn breaks, they then descend back into the deeper, darker parts of the ocean. This pattern generally involves movement between the epipelagic zone, the sunlit surface layer down to about 200 meters, and the mesopelagic zone, often called the “twilight zone,” extending from 200 to 1,000 meters deep where light rapidly diminishes. The timing of these migrations can adjust based on various environmental cues, with light intensity being a primary trigger.
Key Marine Migrators
A wide variety of marine organisms participate in diurnal vertical migration, spanning multiple trophic levels. Zooplankton, a diverse group of small marine animals, are prominent migrators and collectively represent the largest biomass involved in DVM. These often microscopic creatures, such as copepods and krill, move from deep waters during the day to feed on phytoplankton near the surface at night. Copepods are a significant component of this daily movement due to their abundance. Krill, another important group of crustaceans, also undergo DVM.
Beyond zooplankton, other invertebrates also exhibit DVM. Various species of jellyfish engage in these daily vertical movements. Squid, which are molluscs, are also known to participate in DVM, ascending to shallower waters at night.
Small fish species, often residing in the mesopelagic zone, participate in DVM. Lanternfish and hatchetfish are common examples of these mesopelagic fish that undertake daily migrations. Larger, commercially important fish like Atlantic herring, sardines, and big-eye tuna also display DVM patterns, moving up to surface waters to feed at night and returning to deeper areas during the day. Even some larger marine predators, including certain shark species, perform a version of DVM, likely following their prey that are also migrating. This migratory behavior connects different parts of the marine food web, from primary consumers to apex predators.
Driving Forces of Migration
The primary reasons marine creatures undertake diurnal vertical migration revolve around balancing the need to find food with the necessity of avoiding predators. One significant driver is predator avoidance. During the day, the sunlit surface waters are teeming with visually-oriented predators such as fish, whales, and seabirds that are effective hunters in bright conditions. By descending into the darker, deeper mesopelagic zone during daylight hours, migrating organisms like zooplankton and small fish find refuge and reduce their risk of detection.
Foraging opportunities also strongly influence DVM patterns. The surface waters, particularly the epipelagic zone, are rich in phytoplankton, microscopic plants that form the base of the marine food web. Phytoplankton require sunlight for photosynthesis, so they are concentrated near the surface. Migrators ascend at night to feed on these abundant food sources when the risk from visual predators is minimized. This allows them to graze in relative safety, optimizing their energy intake.
Beyond predation and foraging, other factors can contribute to DVM behavior. Energy conservation is one such factor, as the deeper waters are typically colder, which can help reduce the metabolic rates of some organisms. The migration can also facilitate dispersal, enabling organisms to utilize different currents at varying depths to travel or maintain their position. Light intensity serves as the most prominent environmental cue, though internal biological clocks also play a role in regulating this behavior.
Impact on Marine Ecosystems
Diurnal vertical migration profoundly influences the structure and function of marine ecosystems on a global scale. It plays a fundamental role in energy transfer through the food web. DVM effectively connects surface and deep-sea ecosystems, allowing energy captured in the sunlit surface waters by primary producers to be transported to organisms living in the perpetually dark depths. As migrating organisms feed at the surface and then return to deeper waters, they become a food source for deeper-dwelling predators, facilitating the flow of energy across different ocean layers.
DVM also has a significant impact on nutrient cycling and the ocean’s biological pump, which is the process of transporting carbon from the surface to the deep ocean. When organisms consume organic matter in surface waters and then descend, they actively transport carbon to depth. Their waste products, such as fecal pellets, and their own bodies upon death, sink, further contributing to the downward flux of carbon. This process helps sequester carbon in the deep ocean, influencing global carbon cycles and climate regulation.
The movement of organisms can also redistribute nutrients across different water layers, affecting the availability of essential elements for phytoplankton growth and overall marine productivity. The vast scale of DVM underscores its importance for ocean health and marine life.