All forms of plankton are more abundant in the shallow coastal waters of relatively low salinity.
It is on the plankton that a great part of the higher animal life of the sea ultimately depends for food.
Some of the animals that eat the plant plankton are also plankton.
The organisms constituting this plankton are mostly unicellular, of ten aggregated together in colonies, and the remarkable structure which they exhibit has added a new chapter to the story of adaptation to environment.
In general the plankton - and especially the phyto-plankton of the polar and temperate seas - is much more abundant than is that of the sub-tropical and tropical zones.