Benthic Zone

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What is the Benthic Zone?

Benthic zone is one of the ecological regions found at the lowest level of a water body including lake, ocean or stream. This zone also includes the sediment surface as well as some subsurface layers of the water body. An integral part of the benthic zone is the benthic boundary layer that comprises the bottom layer of water and the uppermost sediment layer which is directly influenced by the overlying water. This benthic boundary layer influences the biological activities that take place over there. Rocky outcrops, coral, bay mud and sand bottom are some examples of the contact soil layers.


Characteristics of Benthic Zone

Some of the important characteristics of benthic zone are as follows:

  • Temperature

The benthic zone temperature depends upon the benthic zone depth; it ranges from warmer temperature at shallow depth due to close proximity to the water surface and may further drop to 2-3 degree centigrade at the most extreme depths of the abyssal zone. Very few organisms can survive at lower depths and the ones which can grow there move at a very slow rate.

  • Pressure

The pressure in the benthic region varies from low to high depending upon the depth of the zone. The pressure is lower at shallow depths as compared to hundreds of metres of depth. An example of a very high pressure benthic zone is the Mariana Trench which has a pressure 1000 times more than the normal pressure. An organism of benthic zone living at a higher pressure region is very large in size. Also, at greater depths, there is higher dissolved oxygen and it results in the enlargement of bethos size.

  • Light

Different Benthic zone depths have different light intensities and it is such that as there is increase in the benthic zone depth, the intensity of the light increases. The intensity of light disappears quickly between 250-1000 meters and it is known as the disphotic zone. As a result, photosynthesis process is difficult to take place in this region and beyond 1000 meters there is no light availability and hence photosynthesis doesn’t take place here.

 

Benthic Zone Diagram

By observing the below benthic zone diagram, we can segregate a variety of other zones present near a water body. It includes benthic zone, euphotic zone, littoral zone and limnetic zone.

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What is Benthos?

  • Benthos are the living organisms found in the benthic zone and it includes microorganisms like bacteria, fungi and also larger invertebrates like crustaceans and polychaetes. Crustaceans are large arthropod taxon group organisms that include crabs, crayfish, lobsters, prawns, woodlice, shrimps, krill,  etc. Polychaetes are the bristle worms that belong to the class of annelid worms and each of their body segments consists of a pair of fleshy protrusions known as parapodia having many bristles (chaetae) and made up of chitin. Here, we discuss some of the benthos facts. 

  • Benthos generally live in close relationship with the substrate and most of them are permanently attached to the bottom layer or the benthic boundary layer. 

  • Most of the benthos lack a backbone and are referred to as invertebrates and may include sea anemones, sponges, corals, sea stars, worms, crabs, sea urchins, and many others.

  • Being the lowest level of a marine or freshwater system, it is often characterized by low temperatures and low sunlight.

  • Benthic habitats in the oceanic environments can be zoned by its depth. Different zones from the shallowest to the deepest of these include the epipelagic which is less than 200 meters, the mesopelagic which is 200-1,000 meters, the bathyal which is 1,000 to 4,000 meters, the abyssal which is 4,000 to 6,000 meters and the hadal is below 6,000 meters which is the deepest one. 

  • Benthic zone is important for the health of aquatic ecosystems where tiny microscopic organisms live. A healthy benthic environment serves as a source of food for bottom feeding animals. Benthos or the organisms living in this zone are good indicators of the water quality of the marine ecosystem.

  • Benthos depend upon each other for food and generally feed on worms, crabs, lobsters, sponges and other tiny organisms. Depending on the behaviour of food consumption, they can be divided as filter feeders and deposit feeders. 

  • Unlike the benthic zone, the littoral zone of a lake is the area near the shore area where sunlight can penetrate all the way to the sediment and enable the aquatic plants or macrophytes to survive.

FAQ (Frequently Asked Questions)

1. How Does the Nutrient Flow Take Place in the Benthic Zone?

The benthic zone organisms regulate the nutrient flow between the sediments layer and the water column. The detritivores feeding on the dead and decaying matter store the nutrients in their bodies for a longer period of time and therefore help in decreasing the flow of nutrients.


Microalgae which are found in the shallow benthic zones undergo photosynthesis by utilising carbon dioxide and micronutrients since sunlight is adequate in this region. The consumed carbon is then released as an extra polymeric substance known as slime.


This sticky slime holds the sediment particles together and stabilizes dissolved oxygen levels. The slime is then metabolized by the bacteria present on the ocean bed because it is the source of nutrients for bacteria. Thus, the extra polymeric substance is important in maintaining the local food web and the sediment structure of the marine ecosystem.

2. What are the Benthic Zone Examples?

Benthic zone is the near bottom of a water body like lake, ocean or river and includes some of the sub-surface layers and surface layers of the sediment. The sediment includes sand, mud, coral, rocks, and other substances. Some of the examples of the living organisms living in the benthic zone called benthos are seagrasses, mussels, sea urchins, sea stars, macroalgae, sponges, crustaceans, lobsters, polychaete worms, echinoderms, etc.

3. How do Benthos Survive at the Bottom of Water Bodies?

Benthos depend upon each other and are divided into filter feeders and deposit feeders. They eat smaller organisms including sponges, crabs, aquatic worms, and tiny other organisms.