Plankton are organisms that drift in water (or air) but are unable to actively propel themselves against currents (or wind). [1][2] marine plankton include drifting organisms that inhabit the saltwater of รขโ‚ฌยฆ Plankton, marine and freshwater organisms that, because they are nonmotile or too small or weak to swim against the current, exist in a drifting state.