Factors such as temperature, sunlight, oxygen, and nutrients determine which organisms live in which area of the water.Aquatic ecosystems containseveral types of organisms that are grouped by their locationand by their adaptation.Three groups of aquatic organisms include plankton, nekton, and benthos.Plankton are the mass of mostly microscopicorganisms that float or drift freely in the water, and can be microscopic animals called zooplanktonor microscopic plants called phytoplankton.Nekton are all organisms thatswim actively in open water, independent of currents.Benthos are bottom-dwellingorganisms of the sea or ocean and are often attached to hard surfaces.Decomposers are also aquatic organisms.