Aquatic ecosystems contain two main ecological categories of organisms: free-floating phytoplankton, strongly swimming zooplankton.

Aquatic ecosystems' life zone boundaries are determined by salinity, depth of water, temperature, pH and the presence or absence of waves and currents. 

Freshwater ecosystems include free flowing ecosystem like  streams and rivers, standing-water ecosystems like ponds and lakes and freshwater wetlands like marshes and swamps

Large lakes has three basic zones: the littoral, limnetic, and profundal zones. 

Freshwater wetlands are transitional between aquatic and terrestrial ecosystems.

Organisms in wider slow-moving streams and rivers depend greatly on the velocity of the current.

Freshwater wetlands include marshes, hardwood bottom-land forest, prairie pot holes and peat moss bogs.

The large and complex marine environment is subdivided into several zones: intertidal zone,  the benthic (ocean floor) environment and the pelagic (ocean water) environment.

The pelagic environment is divided into two provinces: the limnetic province and oceanic province.

The abyssal zone is part of the pelagic environment. 

Seagrass do not occur in polar waters. 

Kelp forest are the most diverse of all marine environments and contains hundreds and thousands of species of fishes and invertebrates such as giant clams, sea urchins, sea stars, sponges, sea fans, and shrimps. 

Phytoplankton, such as diatoms in cooler waters and dinoflagellates in warm waters produce food by photosynthesis and are the base of food webs.

Kelp beds support a diversity of life that is almost comparable to those found in coral reefs. 

Coral reefs are built from accumulated layers of calcium carbonate that are found  in warm, shallow seawater.