Plankton composed of phytoplankton and zooplankton is a community of free-floating organisms living in oceans and fresh waters. They are the first steps in the food chain in aquatic systems. Elements, stable and/or radioactive are transferred from these first levels to successive trophic levels, and eventually some are incorporated into food items important to man. Thus, a knowledge of the chemical composition of plankton populations becomes essential, not only for an understanding of the passage of elements, beneficial and/or noxious through the food chain to man, but also for an understanding of the nutritional requirements of plankton with respect to the environment.