What Is The Benthic Zone?

The benthic zone is the ecological zone at the bottom of any body of water, from a shallow pond floor to the deepest ocean trench, including the sediment surface and the layer of water immediately above it. The organisms that live on or in the sediment are collectively called benthos, and they include corals, sea stars, sponges, crabs, clams, worms and a host of microbes. Benthic zones span every climate and depth zone on Earth.

The benthic zone is the bottom-most ecological layer of any marine or freshwater body, such as a river, ocean, lake or pond, and it includes the sediment surface. Benthic zones are found all across the world, wherever there is an appreciable water body.

Now, the benthic zone begins at the shore and extends to the bottom of the ocean. This implies that the benthic zone could be as shallow as a few centimeters, but may reach a depth of a few thousand meters. On account of the depths it can reach, the benthic zone is usually characterized by low temperature, high pressure and minimal sunlight.

Well, such conditions aren’t optimal for the sustenance of a vast amount of flora and fauna, but sediment layers in the benthic zone help in the recycling of nutrients, which in turn makes it possible for the sustenance of rich aquatic life in the upper water column. The benthic zone might not be brimming with vivid aquatic creatures, but many crustaceans, snails, sponges, sea stars etc. are found here.

Let’s now look into three key characteristics of the benthic zone.

Temperature

For benthic zones located close to the shore or with notably shallow depth, the temperature is warmer, but for those located hundreds of meters deep, temperatures can fall to 2°C to 3°C (near the abyssal zone). Aquatic life that can survive in such extreme cold temperatures is limited and those found there often move at a slow pace to conserve energy.

Pressure

Just like temperature, pressure also varies across the benthic zone. From being low for shallow benthic zones to very high for those located a few thousand meters deep. The Mariana Trench, which happens to be the deepest trench on Earth (its lowest point, the Challenger Deep, sits roughly 10,935 metres below sea level), is the extreme example for a very high-pressure benthic zone. There, the pressure is roughly 1,100 times the normal pressure at sea level (about 1,086 bar, or eight tonnes pressing on every square inch).

High-pressure benthic zones tend to be unusually homogeneous environments, and organisms living there often have distinctive traits. One peculiar pattern is deep-sea gigantism, where some species (giant isopods, giant tube worms, the colossal squid) grow far larger than their shallow-water counterparts. The cold, the slower metabolism, and (in polar deep waters) the higher dissolved-oxygen content are commonly cited as contributing factors, though the underlying cause is still debated among biologists.

Light

The intensity of light decreases sharply with depth. The well-lit euphotic zone, where photosynthesis can support primary producers, typically extends down to about 200 metres in clear ocean water. From roughly 200 to 1,000 metres lies the dysphotic (twilight) zone, where light is so dim that photosynthesis is barely possible. Below about 1,000 metres lies the aphotic zone, where sunlight is essentially absent, and the deep-sea benthic communities there rely on dead organic matter raining down from above (so-called marine snow) and, near hydrothermal vents, on chemosynthesis.

How Deep Is The Benthic Zone?

Here is the part that trips most people up: the benthic zone does not sit at one fixed depth. It is the floor wherever there is water, so it begins at the waterline and follows the seabed all the way down. To make sense of that enormous range, oceanographers slice the seafloor into bands and name each one by how far down it lies.

Closest to land is the intertidal (or littoral) zone, the strip of seabed that is uncovered at low tide and submerged at high tide. Beyond it, the sublittoral zone blankets the continental shelf, running from the low-tide line out to roughly 200 metres, where the shelf drops away. The bathyal zone then descends the continental slope from about 200 down to 4,000 metres. Below that, the abyssal zone covers the vast, flat abyssal plains between roughly 4,000 and 6,000 metres. Anything deeper than about 6,000 metres, almost all of it the floor of ocean trenches like the Mariana, belongs to the hadal zone. In other words, the benthic zone we have been describing is really a stack of very different worlds, from sunlit tide pools to crushing trench floors.

Benthos

Because the benthic zone can occur in varying environments, chemical and physical characteristics vary greatly and are often dependent on context. Nutrient availability in the deep sea is also scarce, so organisms need to adapt themselves accordingly to survive here.

Organisms living in the benthic zone are called benthos. Benthos have specially adapted themselves to live on the bottom substrate in deep-water bodies with elevated pressure and cold temperatures. In fact, organisms that inhabit the deep-water pressure areas cannot survive in the upper parts of the water column. Most of these benthos are detritivores.

Due to the scarcity of light, the source of energy for benthos is often in dead organic matter from the organisms higher up in the water column that settle on the benthic belt after death. This dead organic matter provides nutrition to benthos and completes the aquatic food chain and nutrient recycling.

Based on whether they make their home on the ocean floor or beneath the ocean floor, benthos can be categorized into two types. Creatures living on or near the surface of the ocean floor are called epifauna, whereas those that live burrowed under it are called infauna. Certain extremophiles (organisms thriving in extreme environments) that can handle high-pressure ambience also live on the benthic floor.

What Animals And Plants Live In The Benthic Zone?

We just saw that benthos split into epifauna, which sit on the surface, and infauna, which burrow into it. Put names to those categories and the benthic zone starts to feel a lot more crowded. The epifauna include sea stars, brittle stars, sea urchins, sea cucumbers, sponges, sea anemones, corals, sea squirts and crawling crustaceans such as crabs and lobsters. The classic infauna are burrowing bivalves such as clams and cockles, along with polychaete (bristle) worms tunnelling through the mud.

Biologists also sort these creatures by size rather than lifestyle. The macrofauna are the animals you can see with the naked eye (larger than about 1 millimetre), the meiofauna are tiny animals such as nematodes, foraminiferans and tardigrades wedged between sand grains, and the microfauna are microscopic, mostly single-celled protozoans.

What about plants? Here the depth bands from the previous section matter, because plants need light. In shallow, sunlit benthic zones you find rooted flowering plants (the seagrasses), large macroalgae and kelps, and a thin film of microscopic algae called microphytobenthos, mostly benthic diatoms, coating the sediment. Once you drop below the sunlit layer, true plants disappear entirely. The deep benthic zone has no producers of its own and survives on the rain of dead organic matter from above and, around hydrothermal vents, on chemosynthesis.

Nutrient Flow

Benthos play an important role in regulating the nutrient flow between the water column and sediment layers. Benthos, which mostly consist of detritivores subsisting on dead and decaying matter, help in decelerating the flow of nutrients by storing the nutrients in their body for long periods of time. This acts as a buffer that is useful in preventing an excessive influx of elements. For instance, micro algae buffer nutrients that prevent phytoplankton overexploitation, which would otherwise lead to eutrophication.

In shallow benthic zones that receive manageable amounts of sunlight, micro algae are ubiquitous. They can photosynthesize by taking up CO₂ and a handful of micronutrients. Most of the carbon consumed by them is later released as extracellular polymeric substances, commonly called slime. EPS is sticky organic matter comprised of biofilms and facilitates the attachment of cellular substances to the sediment surface. The stickiness of EPS holds sediment particles together so they can avoid resuspension, which helps in stabilizing the dissolved oxygen levels. Moreover, the bacteria on the ocean bed can rapidly metabolize this EPS, as it serves as a source of nutrients for them. In this way, EPS plays a useful role in the marine ecosystem by contributing to both the sediment structure and the local food web!

The Benthic Zone Of A Lake

The benthic zone is not just an ocean thing. Every lake, pond and river has one too, and it is simply the floor of that water body together with the organisms living on and in the sediment. Limnologists (scientists who study inland waters) usually describe a lake in terms of three zones, and the benthic floor runs beneath all of them.

The shallow littoral zone hugs the shoreline, where sunlight reaches the bottom and rooted plants, snails, insect larvae and small fish flourish. The open, sunlit surface water is the limnetic zone. Below the depth that light can penetrate, and usually below the thermocline (the layer where temperature drops sharply), lies the cold, dark profundal zone. Down there the lake bed is muddy and effectively vegetation-free.

Life on a lake's deep benthic floor faces a familiar problem: little light and often little oxygen. The standout residents are the larvae of chironomid midges, often called bloodworms, and oligochaete worms. Both carry hemoglobin-like pigments that let them pull oxygen out of poorly oxygenated bottom water, and both feed on detritus drifting down from the sunlit layers above. Because these benthic invertebrates are sensitive to pollution and low oxygen, freshwater scientists routinely use them as living indicators of a lake's water quality.