Diel Vertical Migration
One of the most characteristic behavioural features of plankton is a vertical migration that occurs with a 24-hour periodicity. This has often been referred to as diurnal or diel vertical migration. The vertical distance travelled over 24 hours varies, generally being greater among larger species and better swimmers. But even small copepods may migrate several hundred meters twice in a 24-hour period, and stronger swimmers like euphausiids and pelagic shrimp may travel 800 m or more. The depth range of migration may be inhibited by the presence of a thermocline or pycnocline. However, phytoplankton and zooplankton capable of diel vertical migration are often concentrated in the pycnocline. Furthermore, those marine organisms with swimming skills through thermocline or pycnocline may experience strong temperature and density gradients, as well as considerable pressure changes during the migration.
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