The Benthic Zone is the bottom of the pond or lake and consists of organic sediments and soil.
This zone will increase as the body of water ages. It is considered the pond or lake’s digestive system. This zone is where bacteria decompose organic matter from dead algae, aquatic plants, and fish and animal waste.
Easily decomposed substances floating down through the profundal zone are partly mineralized while sinking.
The remainder of the organic debris, the dead bodies of plants and animals from the open water and decomposing plant matter from shallow water areas settles onto the bottom. This biomass, together with quantities of material washed in by inflowing water, make up the bottom sediments that are the habitat of the benthic organisms.
The bottom ooze is a region of great biological activity. So great that oxygen curves for lakes and ponds show a sharp drop in the profundal water just above the bottom.
When the amount of organic matter reaching the bottom is greater than that which can be utilized by the bottom fauna, an odiferous muck rich in hydrogen sulfide and methane results. Because this organic muck lacks oxygen nearly completely, the dominant organisms are anaerobic bacteria.
Thus, lakes with highly productive limnetic and littoral zones will have impoverished fauna on the profundal bottom.
Life in the bottom ooze is most abundant in lakes with deep hypolimnion in which oxygen is still available. Some of the bottom species such as the flatworm, rhabdocoela live on the surface of the ooze, but others burrow into the bottom mud to feed on decaying organic matter.
Among these are the rhizopods, some amoeba-like protozoans encased in shells, the clam psidium, small crustaceans such as isopods and cladocerans. Phantom midges which may rise to the surface at night and return to the bottom at dawn are also inhabitants.
Most organisms of the profundal bottom are not unique to this zone. They represent for the most part, a few species of the larger littoral bottom fauna that can tolerate severe stagnation. Even these organisms will be eliminated if subject to continuous stagnation and oxygen depletion.
As the water becomes more shallow, the benthos changes. The bottom materials are modified by the action of water, by plant growth, by drift materials, and by recent organic deposits. Increased oxygen, light and food result in a richness of species and abundance not found on the profundal bottom.
On the bottom of the littoral zone live, in addition to the tube worms, the midges and the waterbearers, numerous other plants and detritus feeders.
Closely associated with the benthic community are the Periphyton..
Periphyton is a complex mixture of algae, cyanobacteria, heterotrophic microbes, and detritus that is attached to submerged surfaces in most aquatic ecosystems.
These are the organisms that are attached to or move upon a submerged substrate but do not penetrate it. Small periphyton communities are found on the leaves of submerged aquatics, on sticks, rocks and other debris. Organisms found there depend upon the movement of water, kind of substrate, temperature and depth.
Periphyton found on living plants are fast growing, lightly attached and consist primarily of algae and diatoms. Because the substrate is so short lived, these rarely exist for more than one summer.
Unlike those on plants, the periphyton on the more substantial substrate are more persistent. Periphyton on stones, wood and debris form a more crust-like growth of blue-green algae, diatoms, water mosses and sponges.
Burrowing into and living up in this crust is a host of associated animals, such as rotifers, hydras, copepods, insect larvae and a wide variety of protozoans.
Periphyton found in moving waters such as currents and waves on the lake shore adhere tightly to the bottom.
Some periphyton organisms have gelatinous sheaths, tubes or cups and are attached to the substrate by gelatinous stalks or basal discs.
The various means of attachment of periphyton to the substrate are about as varied as the methods phytoplankton use to remain afloat.