Along with dissolved organic matter, POM drives the lower aquatic food web by providing energy in the form of carbohydrates, sugars, and other polymers that can be degraded.  POM in water bodies is derived from terrestrial inputs (e.g. soil organic matter, leaf litterfall), submerged or floating aquatic vegetation, or autochthonous production of algae (living or detrital).  Each source of POM has its own chemical composition that affects its lability, or accessibility to the food web.  Algal-derived POM is thought to be most labile, but there is growing evidence that terrestrially-derived POM can supplement the diets of micro-organisms such as zooplankton when primary productivity is limited.