SEABED: CHARACTERISTICS, RELIEF, TYPES, FLORA AND FAUNA - ENVIRONMENT

The seabed is the portion of the earth's crust that is below the sea. The seabed is very diverse and can be classified through the use of multiple variables.

For example, we can classify them by the material that compose them and the size of their grains, but we should also specify the depth at which they are found, as well as the organisms that colonize them (plants and animals).

Figure 1. Scheme of the different ocean divisions. Divisions based on distance to shore and divisions based on depth are seen. Source: Oceanic divisions.svg: Chris huh, via Wikimedia Commons

The seabed is geologically distinct from the continents. It experiences a perpetual cycle of formation and destruction that shapes the oceans and controls much of the geology and geological history of the continents.

General characteristics

Geological processes sculpt the shoreline, determine the depth of the water, control whether the bottom is muddy, sandy, or rocky, create new islands and seamounts (which organisms colonize), and determine the nature of marine habitats in many ways.

geology

The geological distinction between the ocean and the continents is due to physical and chemical differences in the rock that constitutes the crust in each case.

The oceanic crust, which forms the seabed, consists of a type of mineral called basalt that has a dark color. Unlike this, the majority of continental rocks are of the granite type, with a different chemical composition than basalt and a lighter color.

Mid-Atlantic Ridge

The mid-Atlantic ridge is a structure that runs through a good part of the planet in a north-south direction and from which the seabed constantly forms, as a result of the separation of tectonic plates.

Figure 2. The mid-Atlantic ridge marks the tectonic plate boundary from which new seabed is generated. Source: originally uploaded on the English wikipedia: 14:51, 21 October 2003. JamesDay (Talk / contribs). 200 × 415 (21,177 bytes) (mid-Atlantic ridge map), via Wikimedia Commons

Due to this phenomenon, the ocean floor near the ridge is younger (geologically) than the bottom closest to the continents, since it has been generated more recently.

This phenomenon has consequences on the composition and size of particles (among other variables), which influence the different types of habitats and their inhabitants.

Geography

The oceans cover about 71% of the earth's surface, the seabed being one of the most extensive habitats in the world.

On the other hand, the oceans are not evenly distributed with respect to the equator. In the northern hemisphere, there are 61% of the oceans, while in the southern hemisphere about 80%. This simple difference means that there is a greater extension of the ocean floor in the southern hemisphere.

Classification of the oceans

The oceans are traditionally classified into four large basins:

The Pacific Ocean

It is the largest and deepest ocean, almost as large as all the others combined, at 166.2 million km ² and an average depth of 4,188 m.

The Atlantic ocean

At 86.5 million km ², it is slightly larger than the Indian Ocean (73.4 million km ²), but the two are similar in mean depth (3,736 and 3,872 meters respectively).

The arctic ocean

It is the smallest and shallowest ocean with about 9.5 million km ² and 1,130 m deep.

Several shallow seas, such as the Mediterranean Sea, the Gulf of Mexico, and the South China Sea, are connected to or marginal to the major ocean basins.

Connection between the oceans

Although we generally treat the oceans as separate entities, they are actually interconnected. The connections between the main basins allow seawater, materials and some organisms to move from one ocean to another.

The seabed could also be conceived as a large interconnected system. However, other variables such as the depth of the oceanic mass at a particular point, abrupt changes in relief, among others, establish true boundaries for much of the oceanic fauna.

Types of seabed

The classification of the seabed depends on different variables, such as its depth, the penetration of light, the distance to the coast, the temperature and the substrate that constitutes it.

The seabed can be classified into:

-Coastal background

The coastlines range from the highest tide limit to the limit that determines the euphotic zone (about 200 meters), where solar radiation penetrates (and photosynthesis occurs).

In the euphotic zone 99% of the radiation is extinguished, making it impossible for photosynthesis to occur in deeper areas.

Coastal bottom areas

A) The supralittoral area, which is not submerged but is highly influenced by the sea.

B) The eulitoral area that floods intermittently, from the low to high tide limit.

C) The sub-littoral zone, which is always submerged and which includes the zone from the low tide limit to the euphotic zone. This sub-coastal area is what is considered the seabed.

Types of coastline

On the other hand, the littoral bottom is also classified depending on its composition in:

Homogeneous bottoms : mainly made up of mud, sand, small ridges, gravel or rock.
Mixed funds: they are mixtures of the previous components in different proportions; They can be made up of sand-mud, sand-pebbles, or any of the possible combinations.
Diffuse bottoms: they are transitions between some of the previous types and they appear in places of confluence of currents, river deltas, among others.

The littoral bottom is in general very fertile, as it receives a large contribution from the runoff waters of the continent, which are usually loaded with minerals and organic matter.

Fauna of the coastline

The fauna of the littoral bottom is very wide in the sub-littoral zone, decreasing the number of species as one advances towards the supralittoral zone (where the most resistant species to desiccation abound).

The variety of fauna includes from gastropods, crustaceans such as barnacles, sponges, nematodes, copepods, hydroids, anemones, bryozoans, sea squirts, polychaetes, amphipods, isopods, echinoderms (sea urchins), mollusks such as mussels and octopuses, crabs, shrimp and fish.

Corals, which are colonial animals that harbor microalgae in their bodies, are also present on the coastline and serve as a refuge for many other species. These animals require light to reach them so that their symbiotic microalgae can photosynthesize.

The reefs that make up the corals are called the "sea jungles", due to the great amount of diversity of species they host.

Flora of the coastline

Plants and algae are also present on the coastline.

In tropical and subtropical waters, the meadows of Thalassia (popularly called turtle grass), a marine phanerogam (flowering plant), are typical. This plant grows on soft, sandy bottoms.

The intertidal region (part of the coastline between the levels of maximum and minimum tides) can present plants such as the mangrove, adapted to grow on muddy bottoms that may lack oxygen (under anoxic conditions).

Figure 4. Nurse shark (Ginglymostoma cirratum) resting on a meadow of turtle grass (Thalassia testudinum). Source: NOAA CCMA Biogeography Team

Kelp forests

One of the most common sub-littoral habitats in the temperate regions of the world are the great “forests” or “beds” of Kelp, made up of groups of brown algae of the Laminariales order.

These communities are important because of their high productivity and the diverse invertebrate and fish communities they host. Mammals such as seals, sea lions, sea otters and whales are even considered associated with this type of habitat.

Figure 5. Map of the world distribution of Kelp forests. Source: Maximilian Dörrbecker (Chumwa), via Wikimedia Commons

Kelp forests also give rise to large amounts of drift algae, especially after storms, which settle on nearby beaches, where they provide a source of energy for communities.

Figure 6. Diver in a Kelp forest in California, USA. Source: Ed Bierman from Redwood City, USA, via Wikimedia Commons

Kelp forests that can extend up to 30 m or more above the substrate, give vertical structure to sub-littoral rock communities.

Sometimes these extensive forests can modify the light levels in the substrate below, reduce the impact of waves and turbulence, and vary the nutrients available.

Figure 7. A sea otter and her cubs feeding in a Kelp forest. Source: Ed Bierman from Redwood City, USA, via Wikimedia Commons

-Ocean bottom

Physicochemical properties

The deep sea extends across the globe vertically, that is, from the edge of the continental shelf to the floors of the deepest ocean trenches.

The physical and chemical properties of the body of water that fills this vast space vary throughout its depth. These properties have been used to define the characteristics of the seabed.

Hydrostatic pressure: hydrostatic pressure (water column pressure) increases with depth, adding the equivalent of 1 atmosphere (atm) for every 10 m.

Temperature: In most of the world, deep-sea temperatures are low (approximate range of -1 to +4 ° C, depending on depth and location), but extremely stable.

Most deep-sea organisms never experience large or rapid changes in environmental temperature, except for those that inhabit hydrothermal vents, where superheated fluids mix with low-temperature bottom water.

Salinity and pH: constant thermal conditions in most of the deep ocean, combine with a stable salinity and pH.

Flow of energy and matter on the ocean floor

The deep sea is too dark, so it does not allow photosynthesis to take place. Therefore, the primary production of green plants (which is the basis of practically all terrestrial, freshwater and shallow marine ecosystems) is absent.

In this way, the food webs of the seabed depend almost entirely on organic particles that sink from the surface.

The size of the particles varies from dead cells of phytoplankton, to the carcasses of whales. In regions without marked seasonality, the deep sea receives a constant drizzle of small particles (called “sea snow”).

Along the continental margins, underwater canyons can funnel large amounts of seagrasses, macroalgae, and land plant debris to the deep seabed.

Figure 8. Underwater canyon of the Congo River in South West Africa, showing about 300 km of the canyon Source: Mikenorton, from Wikimedia Commons

Particles can be consumed by midwater animals, or degraded by bacteria as they sink through the water column

The resulting sharp decline in available food as depth increases is perhaps the factor that most affects the structure of deep-sea ecosystems.

Dead cell aggregates attached to mucous substances and zooplankton fecal pellets sink rapidly, accumulating on the seabed as visible deposits of "phytodetritus".

Ocean floor fauna

The effects of darkness on body shape, behavior, and physiology in deep-sea organisms are most evident in animals that inhabit medium depths.

The mesopelagic (200-1000 m) and bathypelagic (1000-4000 m) zones together constitute more than 1 billion km ³ of space inhabited by actively swimming fish, cephalopods and crustaceans, together with a wide variety of gelatinous zooplankton (jellyfish, siphonophores, tenophores, larvaceans, salps and other groups).

Deep sea organisms show biochemical adaptations to counteract the effects of high pressure on the function of enzymes and cell membranes. However, darkness and food shortages are the factors that most affect the body and animal behavior.

For example, many organisms on the seabed have a slow metabolism, which in some cases manifests itself in a very long life expectancy.

In the nutrient-deficient desert of the ocean floor, hydrothermal vents and the carcasses of whales and large fish represent true oases of abundance.

Bioluminescence

More than 90% of the animal species in this environment (at depths well below the maximum penetration of sunlight) produce light. In some cases, this light production is due to symbiotic associations with luminescent bacteria.

Many fish and cephalopods have complex accessory structures (photophores) that reflect, refract or filter the emitted light, despite keeping their eyes functional

The abundance of bioluminescent organisms decreases considerably with increasing depth.

Touch and smell

In contrast to the large amount of bioluminescence in the deep water column, very few benthic organisms (bottom inhabitants) produce light. Some groups of fish that live close to the seabed have reduced eyes and are believed to have more developed other senses, such as touch.

The tiny eyes of the tripod fish (Bathypterois) may be of little use, but the rays of the specialized pectoral fins endowed with enlarged spinal nerves, allow them to detect changes around them, functioning as a mechanosensitive matrix.

Figure 9. A fish of the genus Bathypterois atricolor. Large number of modified appendages are observed. Source: NOAA Office of Ocean Exploration and Research, 2015 Hohonu Moana

The seabed also has scavenger fauna, which has also developed a keen sense of smell (fish, crabs, among others).

Diversity of the seabed

It is estimated that there are hundreds of thousands to more than 1 million benthic (deep-sea) species.

Such high levels of diversity are unexpected in a habitat that consists primarily of monotonous, species-poor mud flats.

Detritivores and the seabed

The seabed is the kingdom of mud-eating animals. Sponges, crinoids, and other filter feeders are found in areas where water currents increase the flow of suspended particles.

On the other hand, the vast abyssal plains are dominated by detritivore animals, which extract organic matter from bottom sediments.

Deep-sea sediment as a food source has the advantage of being in unlimited quantities and is very accessible, yet it has little nutritional value.

In temperate and polar oceans, phytodetritus (decomposing remains of plant organisms) provides a seasonal "windfall" for the seafloor ecosystem. However, the amount of phytodetritus that arrives is unpredictable and its distribution is often irregular.

The large and abundant holothurids (sea cucumbers) are detritivores of the abyssal depths. These present a variety of strategies for exploiting this ephemeral food source.

Figure 10. Cucumber or sea cucumber, a common inhabitant of the seabed. Source: Frédéric Ducarme, from Wikimedia Commons

References

Beaulieu, S. (2002). Accumulation and fate of phytodeffitus on the sea floor. Oceanography and Marine Biology: an Annual Review 40, 171-232.
Bergquist, DC Williams, FM and Fisher, CR (2000). Longevity record for deep-sea invertebrate. Nature. 403, 499-500.
Corliss BA-1., Brown, CW, Sun, X. and Showers, WJ (2009). Deep-sea benthic diversity linked to seasonality of pelagic productivity. Deep-Sea Research Part I 56, 835-841.
Glover, AG and Smith, CR (2003). The deep-sea floor ecosystem: current status and prospects of anthropogenic change by the year 2025. Environmental Conservation. 30, 219-241.
Levin, LA (2003). Oxygen Minimum Zone benthos: adaptation and community response to hypoxia. Oceanography 'and Marine Biology: an Annual Review 41, 1-45.
Thiel, H. (1975). The size structure of the deep-sea benthos. Internationale Revue der Gesamten Hydrobiologie. 60, 575-606.

SEABED: CHARACTERISTICS, RELIEF, TYPES, FLORA AND FAUNA - ENVIRONMENT - 2025