Benthic zone

The benthic zone is the area at the very bottom of any body of water, like a stream, river, lake, or ocean. It includes not just the surface of the sediment, but also a few layers just underneath. This zone is named after a word from Ancient Greek, βένθος (bénthos), meaning "the depths of the ocean".

Living in the benthic zone are special creatures called benthos, or bottom dwellers. These include tiny microorganisms like bacteria and fungi, as well as larger animals such as invertebrates like crustaceans and polychaetes. Many of these animals stay close to the bottom, sometimes even stuck to it.

One important part of the benthic zone is the benthic boundary layer. This is the thin layer where the water meets the sediment. It affects much of the life in the zone. Different kinds of bottoms can be found here, such as sand bottoms, rocky areas, coral, and bay mud.

Physical description

The benthic zone is the bottom part of any body of water, like oceans, lakes, or rivers. In the ocean, it starts at the shoreline and goes all the way down to the deepest parts. This area changes a lot — some parts are shallow and get sunlight, while others are very deep and dark.

In lakes, the benthic zone is simply the lake floor, which can be covered with bits of plants and animals that have sunk down over time. Above the benthic zone is the water column, which may have plants and animals that need sunlight to live.

Benthos

Benthos is the community of organisms that live in the benthic zone, which is on, in, or near the bottom of a stream, river, lake, or ocean. These organisms live in or near sedimentary environments, from tidal pools along the foreshore out to the continental shelf, and down to the abyssal depths.

The term benthos comes from the Greek word for "depth of the ocean." It is used in freshwater biology too, to refer to organisms at the bottom of streams, rivers, and lakes. Light is an important energy source for shallow benthic systems, but in deep ocean water, the energy source is often organic matter that drifts down from higher up in the water column. This dead and decaying matter sustains the benthic food chain, and most organisms in the benthic zone are scavengers or detritivores.

Many organisms in the deep sea have adapted to live under high pressure, which can be significant — about one atmosphere for every 10 metres of water depth. The benthic zone offers diverse habitats, with variations in light, warmth, depth, and sediment type. This diversity supports a huge range of species, far more than in the open water above.

Ecological roles

Benthic communities get food from the water above them. This food includes bits of dead plants and animals, called detritus, and tiny living things. This material, often called marine snow, falls to the ocean floor and provides food for many bottom-dwelling creatures.

The main food for these bottom dwellers comes from tiny plants in the water called phytoplankton and from organic matter that settles on the bottom. In places near land, extra food comes from runoff. Tiny animals and bacteria help break down this organic matter, returning important nutrients like nitrate and phosphate to the water. The type of bottom dwellers depends on water depth, temperature, salt levels, and the local environment. Some, like sponges and clams, filter food from the water, while others, like certain worms, eat from the bottom itself. Fish and other sea animals also rely on these bottom dwellers for food.

Threats and mitigation

See also: Bottom trawling

Many sea creatures that live on the ocean floor face dangers from human activities. Fishing, pollution, and trash can harm them. Other threats include deep-sea mining, oil and gas work, tourism, shipping, invasive species, and climate change. Climate change can make the ocean more acidic, warmer, and change water movement, which also affects these creatures.

One fishing method, called bottom trawling, can be especially harmful. It makes up about 25% of the fish caught worldwide. This method can damage the ocean floor in two main ways. It stirs up the sand and mud, which can hurt the plants and animals living there. It can also change the mix of species, often removing larger animals and leaving mostly smaller ones. Because of these problems, some countries have banned or limited bottom trawling in the waters they control.