Zooplankton are tiny aquatic animals that drift within water currents in oceans, lakes, and ponds. They form a fundamental part of aquatic food webs, serving as a food source for many larger organisms. A widespread behavior among these small creatures is Diel Vertical Migration (DVM), a daily up-and-down movement through the water column, occurring over a 24-hour cycle. This synchronized movement represents the largest biomass migration on Earth.
Avoiding Predators
Zooplankton undertake DVM primarily to evade predators. During daylight hours, the sunlit surface waters make zooplankton highly visible to visual predators like fish and birds. To minimize this risk, they descend to deeper, darker waters where they are much harder to detect.
As night falls, zooplankton ascend back towards the surface. The darkness provides a protective cover, allowing them to forage in relative safety. This daily cycle reflects a significant trade-off: deeper waters offer refuge from predators but typically contain less food. Therefore, zooplankton must balance the need for safety with the necessity of feeding.
Accessing Food
Surface waters, known as the photic zone, are rich in phytoplankton, the primary food source for many zooplankton. Phytoplankton are microscopic plant-like organisms that perform photosynthesis. Zooplankton migrate to these sunlit surface layers during the nighttime hours to graze on the abundant phytoplankton. This nocturnal feeding strategy is crucial for optimizing their energy intake.
They can maximize their feeding time in the phytoplankton-rich surface waters under the cover of darkness. This cyclical pattern of ascending to feed allows zooplankton to efficiently acquire nutrients while minimizing the risk of becoming prey themselves.
Conserving Metabolic Energy
Deeper waters are generally colder than surface waters. For zooplankton, whose body temperatures fluctuate with their environment, colder temperatures can significantly slow down metabolic processes. By spending their days in these cooler, deeper waters, zooplankton can reduce their metabolic rate. This leads to a conservation of energy.
This energy conservation strategy can potentially increase their lifespan or reproductive output. While not the primary driver for DVM, the metabolic benefits gained from residing in colder depths contribute to the overall survival and fitness of zooplankton populations. It represents an additional advantage of their daily vertical movements.
Environmental Cues
Light intensity is the most significant environmental cue triggering zooplankton vertical migration. As dusk approaches and light diminishes, zooplankton are prompted to ascend towards the surface. Conversely, the increasing light at dawn signals them to descend back into deeper waters.
Other environmental factors also influence these migration patterns. Temperature gradients can guide their movements. Chemical signals, such as kairomones released by predators, can also alert zooplankton to danger and induce a deeper descent. The presence of food can further modulate migration, although light remains the dominant cue.