Explanation:

Based on the their life form or location, aquatic organisms (both flora and fauna) are classified into five groups:

Neustons: Unattached organisms which live at air-water interface such as floating plants etc.
Periphytons: There are organisms which remain attached to stems and leaves of rooted plants or substances emerging above the bottom mud such as sessile algae.
Planktons: These are free floating species with limited powers of locomotion. This group includes microscopic plants like algae(phytoplankton) and animals like crustaceans and protozoans (zooplankton).
Nektons: This group includes animals which are swimmers. The animals range in size from the swimming insects to the largest animals, blue whale.
Benthos: These are found living in the bottom of the water mass.