Plankton are floating or weakly swimming organisms. All types of plankton are at the mercy of tides, currents, and waves for transportation. So they have evolved adaptions to float at least as long as it takes to reproduce themselves.
Individuals only live for a few hours to a few days.
The greater the surface area of an organism relative to its volume, the greater its friction as it moves through the water, and the slower it sinks. Some cells have a large cavity at their center, reducing their mass and density relative to their surface area. Another way to increase relative surface area is to grow spines and other projections help to distribute the organism s weight over a large surface area. In many phytoplankters, stores of oil and fat give them buoyancy. Consider possible mechanisms by which phytoplankton remain suspended in ocean water.