Page 29 - living-ice

PHOTO: Joan Costa
Zooplankton in the elevator
As marine zooplankton and sh migrate up and down in the water column, they play an active role in transporting food between surface and deeper waters. Some Arctic sh and zooplankton species in the water column make daily trips up from the dark deep ocean and down again. They swim up to the surface at night to eat in the dim light when they are safe from visual predators, but return to greater depths before predators become active again in the daytime.
By making these daily trips, these vertical migrators bring food from the surface down to the deep ocean. They also dispose of organic matter and excrements that sink down to the seabed. While travelling several hundreds of meters up and down every day may not sound like a big deal, remember that a typical copepod is around ve millimeters long. The equivalent for a man who is two meter tall would be walking around hundred kilometers every day. This daily marathon occurs in all oceans on earth, which makes it the largest synchronized movement of living organisms on the planet. The daily migration is different from seasonal migration. Seasonal migrators use the productive summer period to eat as much as possible before migrating to a depth of about one thousand meter to spend the winter. Down there, they burn energy very slowly.