Poor swimmers have developed ways of moving efficiently through the water.
Planktonic organisms are technically ‘those which are made to drift’ by water movement. Some of the larger animal plankton might be able swim reasonably well, but even so they are very much at the mercy of the ocean currents.
Daily Vertcal Movement
Even very poor swimmers can manage to move up and down in the water column. Many phytoplankton move down during the night, and then back towards the light during daytime to allow photosynthesis. Most zooplankton (animals) move up at night to feed, and then return to deeper water during daylight hours to avoid predators (or at least to make it harder for predators to find them!).
Daily Horizontal Movement
Ocean currents typically flow in different directions as you move vertically through the water column. This means that any animal that has the ability to descend during the day will come up into a new part of the sea next evening – giving it new pastures to graze. Going up and down regularly (relatively short distances) can lead to an efficient zigzag movement through new feeding areas.
Seasonal Movements
Plants need nutrients and sufficient sunlight to photosynthesise. Away from the tropics, as day-length increases the phytoplankton ‘blooms’ and populations of zooplankton feeding on these plants explode. When the nutrients have been used up many of the phytoplanctonic organisms sink and enter a resting period – waiting for upwelling currents to replenish the surface waters and for sufficient daylight to return.
Many of the animals also move away from the surface until next season (Antarctic Krill, for example, hide away under the pack-ice.)
