The unseen World of the upper Waters

Most planktonic animal are capable of some limited swimming movements, but these are insufficient to overcome the Oceans' surface forces.

Nonetheless the swimming activity of theses diminutive life forms should not be ignored, since it is employed in a rather ingenious way. By the means of active swimming, most of the animals we haul up in plankton nets effect what is known as vertical migration.

This is seemingly pointless exercise enables the animals the battle against the watery forces and to maintain a relatively constant mean position

When you live in an ocean which directly connects two polar ice caps and several million cubic miles of tropical reef waters, it is rather important that you avoid being swept where you least wish to be!
How then does vertical migration overcome the problem?

Most moving layers of water, especially those at or near the surface, have sandwiched between them similar layers of waters, which tend to flow in the opposite direction.
Thus the top four fathoms might drifts east to west, the next six fathoms would drift west to east and the next three might move again east to west.
The planktonic animals `play` these layers like lumberjack uses his logs to his own advantage.

By swimming vertically up into an east-wester they become transported westwards. If say they do this at night, but descend to the lower layer by day, then every twenty four hours they will have moved in a square in this way, their mean position over the sea floor, will be remarkably constant day by day, and week by week.

So drifters theses animals may be, but their powers of modality are effective and vital to survival.