Seamounts are extinct submarine volcanoes which are not likely to erupt again or no longer have a magma supply. Just like some well-known mountains, seamounts form along cracks in the Earth surface, with the only exception that they are mostly found more than 1000m below the sea surface. Situated in the hostile and hard to study environment of the deep sea, seamounts have been rarely studied in details. A total of 33452 seamounts -among the 25 million estimated-  have been mapped up to date. These mountains of volcanic rock rise in the middle of the sandy deep sea floor, creating unique habitats that support wide diversity of organisms. Because of this and other unique characteristics, seamounts are considered to be among the main hotspots of life of the deep sea.

Multibeam echo-sound image of a 1.100m seamount, located 5.100 metres underneath the Pacific Ocean (from: Center for Coastal and Ocean Mapping/Joint Hydrographic Centre)

The oceanographical definition of seamount from GEBCO Sub-Committee on Undersea Feature Names is: “… an independent feature with a limited summit area that rises to at least 1000m above the seafloor”. They usually have very shear cliff sides (because lava solidifies 25 times more quickly in water than in air) and the peaks are mostly found at depths where little or no light penetrates. Seamounts are present in all Oceans in the world and -because of their volcanic origin- mainly near mid-ocean ridges (where new Ocean floor is created as the Earth’s tectonic plates move apart) and subduction zones (where one tectonic plate moves under another).

So, seamounts are tall, deep, steep extinct volcanos and there is no light where they are found, but…

What makes them oases of life?

First, seamounts provide to deep sea animals something which is very difficult to find at those depths: a hard surface for anchoring. The solid lava is a perfect structure for the attachment and growth of sessile animals (those who are permanently attached to a surface), such as gorgonians, cold water corals and sponges. As the deep-sea floor is mainly a sandy/muddy environment, the volcanic rock of seamounts host assemblages of species which are completely different from the surroundings. This result in a high abundance of species which are unique to seamounts (endemic species). Over 168 different species of animals were observed on Davidson Seamount, off the coast of California, and 7% of these have been only observed on this particular seamount.

Schematic drawing illustrating a seamount ecosystem. The upward arrow on the right side represents the upwelling current, the whirlpool on the left side illustrates the eddy (from: Aliza Vinzant)

Secondly, seamounts provide water movement (and so transport of nutrients) in an otherwise still (and nutrient-poor) environment. As they project upward from the Ocean floor, seamounts deviate the flow of water, causing strong upward-moving currents and eddies (swirling of water that creates a downstream flow running in opposite direction to the current). Vital nutrients for the growth of phytoplankton (photosynthesising unicellular algae at the base of all marine food webs) are transported upwards by the current causing an explosion in plankton abundance. The plankton is also brought upwards by the current and sustains numerous populations of diverse species of fish like tunas and mackerels, which gather around the top of the seamount to feed.

The majority of organisms found on seamounts are filter feeders (they feed off tiny food particles suspended in the water), such as sponges, sea-pens and corals. The strong currents flowing up along the steep sides of seamounts fall down as “marine snow“, which supplies these animals with constantly replenished food-rich water, delivering the food particles onto the feeding appendages of corals and into the openings of sponges.

Bubblegum coral on the crest of Davidson Seamount (from: NOAA / MBARI)

The bubblegum coral (Paragorgia arborea) can be found in several deep-sea habitats, but it grows nowhere else as abundant as on seamounts. Scientists still do not know why this species of coral is so abundant on seamounts, but they think it may be related to the strength of the water flow. The bubblegum coral can grow up to 6m and sustains a diverse community of organisms such as basket stars and other invertebrates. Bubblegum corals may live for hundreds of years but they are also very slow growing. This means that coral communities on seamounts can form over a very long period of time.

 

Together with nutrients, also soft sediments are transported upwards by the current, and deposited along the sides on seamounts. Animals like marine worms and sea slugs, colonise these patches of sand, enhancing the biodiversity of the seamount. In addition, seamounts have been observed to be relevant orienting points for migrating animals, in particular whales. Other large marine animals seem to use seamounts as aggregation points, probably to gather food.

Threatened oases

Seamounts act as hotspots of biodiversity for fish communities, and this makes them very attractive to fisheries. Overfishing is a serious threat to the ecology of seamounts, because target fish species are usually long lived and slow growing. Stocks of the orange roughy (Hoplostethus atlanticus) –which can live up to 149 years- have been already depleted around seamounts off the coasts of Australia. Fishing becomes an even bigger threat to seamounts when bottom trawling is involved, because it also damages the very slow growing communities of sessile animals living on the sides. As organisms like cold water corals and deep sea sponges are very slow growing and reach maturity late in life, they are very vulnerable to damage caused by trawling and their recovery -if taking place- can take thousands of years.

The ecological conservation of seamounts is a very difficult subject because of the lack of scientific informations available, but several management instruments (like Marine Protected Areas and gear restrictions) are already being applied to some seamounts, even if not at a wide scale. Further scientific research is needed in order to understand more about these mountains of the sea and so conserve their oases of life.

 

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