The sea bottom It 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).
The seabed is geologically distinct from the continents. Experience a perpetual cycle of formation and destruction that shapes the oceans and controls much of the geology and geological history of the continents.
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Geological processes sculpt the shoreline, determine water depth, 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..
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 seafloor, consists of a type of mineral called basalt that has a dark color. Unlike this, most continental rocks are of the granite type, with a different chemical composition than basalt and a lighter color..
The mid-Atlantic ridge is a structure that runs through a large 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..
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..
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..
The oceans are traditionally classified into four large basins:
It is the largest and deepest ocean, almost as large as all the others combined, at 166.2 million kmtwo and 4,188 m of average depth.
With 86.5 million kmtwo, is slightly larger than the Indian Ocean (73.4 million kmtwo), but the two are similar in average depth (3,736 and 3,872 meters respectively).
It is the smallest and shallowest ocean with about 9.5 million kmtwo 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 major ocean basins.
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 a large part of the oceanic fauna..
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:
The coastlines are understood from the limit of the highest tide, up 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.
To supralittoral area, that is not submerged but very influenced by the sea.
B) The eulitoral zone that floods intermittently, from low to high tide line.
C) The area subcoastal, that it is always submerged and that it includes the zone from the limit of the low tide to the euphotic zone. This sub-coastal area is what is considered to be the seabed.
On the other hand, the littoral bottom is also classified depending on its composition in:
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..
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 (hedgehogs), 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 form the corals are called the "jungles of the sea", due to the great amount of diversity of species they host.
Plants and algae are also present on the coastline.
In tropical and subtropical waters, grasslands are typical Thalassia (popularly called turtle grass), a marine phanerogam (flowering plant). 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 mangroves, adapted to grow in muddy bottoms that can lack oxygen (in anoxic conditions).
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 order Laminariales.
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..
Kelp forests also give rise to large amounts of drift algae, especially after storms, which are deposited on nearby beaches, where they provide a source of energy for communities.
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..
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 parts 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 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 stable salinity and pH.
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 carcasses of whales. In regions without marked seasonality, the deep sea receives a constant drizzle of small particles (called “marine snow”).
Along the continental margins, underwater canyons can funnel large amounts of seagrasses, macroalgae, and land plant debris to the deep seafloor..
The particles can be consumed by mid-water 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".
The effects of darkness on body shape, behavior, and physiology in deep-sea organisms are most evident in animals that inhabit medium depths..
The zones mesopelagic (200-1000 m) and bathypelagic (1000-4000 m), together constitute more than 1 billion km3 of space inhabited by actively swimming fish, cephalopods, and crustaceans, along 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 desert of the ocean floor with a shortage of nutrients, hydrothermal vents and the carcasses of whales and large fish represent true oases of abundance.
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.
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 near 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 specialized pectoral fin rays, endowed with enlarged spinal nerves, allow them to detect changes around them, functioning as a matrix mechanosensitive.
The seabed also has scavenging fauna, which has also developed a keen sense of smell (fish, crabs, among others).
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 consisting mainly of monotonous, species-poor mud flats..
The seabed is the kingdom of animals mud eaters. 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 detritivores, 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, however, it has little nutritional value..
In the temperate and polar oceans, the 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..
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