Seamounts

Seamounts –  Seamounts are “a discrete (or group of) large isolated elevation(s), greater than 1,000 m in relief above the sea floor, characteristically of conical form” (IHO, 2008).  Seamounts are thus defined as peaks that rise over 1,000 m above the seafloor, calculated based on the SRTM30_PLUS model. We adhered strictly to the requirement that seamounts are “of conical form”, thus distinguishing “seamounts” (having a length/with ratio <2) from ridges (having a length/width ratio ≥2).  The criterion of a length/width ratio <2 for seamounts is consistent with the geomorphic analysis of Mitchell (2001).  Seamounts are, furthermore, distinguished from flat-topped guyots

Estimates of the numbers of seamounts in the world ocean and their distribution have been published by Agapova et al., (1979), Craig and Sandwell (1988) and Kitchingham and Lai (2004).  Agapova et al. (1979) based their assessment on existing bathymetric data and identified 7,080 seamounts over 1,000 m in elevation, including 302 flat-topped guyots.  Using satellite altimetry data, Craig and Sandwell (1988) identified 8,556 seamounts having a diameter of at least 15 km.  Kitchingman and Lai (2004)  estimated the number of seamounts to be of 14,287 based on an analysis of the US Natioanl Oceanographic and Atmospheric Agency (NOAA) ETOPO2 raster bathymetric data set[1].  These studies have mainly focussed on identifying individual seamount peaks.  Global seamount basal area was also estimated by Etnoyer et al., (2010) and by Yesson et al., (2011).  In the study by Harris et al. (2014), basal area of seamounts as well as summit morphology (i.e. distinguishing between ridges, guyots and seamounts) was mapped in order to produce a broad range of statistical measures of seamount geomorphology.

[1] ETOPO2 Global 2 minute elevations, 2001, National Geophysical Data Center, NOAA/NGDC, USA, www.ngdc.noaa.gov.

There are more seamounts in the Pacific Ocean than in the Atlantic, and their distribution can be described as comprising several elongate chains of seamounts superimposed on a more or less random background distribution (Craig and Sandwell, 1988).  Seamount chains occur in all three major ocean basins, with the Pacific having the most number and most extensive seamount chains.  These include the Hawaiian (Emperor), Mariana, Gilbert, Tuomotu and Austral Seamounts (and island groups) in the north Pacific and the Louisville and Sala y Gomez ridges in the southern Pacific Ocean.  In the North Atlantic Ocean, the New England Seamount Chain extends from the eastern coast of the United States to the mid-ocean ridge.  Craig and Sandwell (1988) noted that clusters of larger Atlantic seamounts tend to be associated with other evidence of hotspot activity, such as on the Walvis Ridge, Bermuda Islands and Cape Verde Islands.  The mid-Atlantic ridge and spreading ridges in the Indian Ocean are also associated with abundant seamounts (Kitchingman and Lai, 2004).  Otherwise, seamounts tend not to form distinctive chains in the Indian and Southern Oceans, but rather their distribution appears to be more or less random.

Statistics on seamounts (after Harris et al., 2014)

Ecologically, there may be no significant difference between seamounts and some other rocky seabed features that have a prominent vertical relief, such as knolls, ridges, pinnacles or escarpments.  These features all have locally steep slopes with rocky outcrop and the habitats might also be similar among them.  However, the isolated nature of seamounts, that plays a role in the evolution of endemic species (eg. Rogers, 1994), is not represented by ridges or escarpments (although knolls and pinnacles do have this factor in common with seamounts).  The smaller size of knolls, hills and pinnacles has significant implications for how they interact with the ocean currents flowing over them and this affects sedimentation on their flanks and, presumably, the dispersal of fauna as well as providing a smaller area for larvae to settle and colonise.  Thus the comparisons that we can make between seamounts and other rocky seabed features are limited and many ecological processes will be unique to different classes of feature.

A key factor that controls the occurrence and particularly the abundance of benthic animals is the water depth at the seamount’s summit.  Seamounts that reach to within 1,500 m of the ocean surface have much higher density of faunal coverage than deeper seamounts.  Faunal density reached 90% coverage at 1,000-1,100 m depth on 13 unfished seamounts surveyed by Williams et al. (2006), but coverage was less than 10% at 1,400-1,500 m.

Current-seamount interactions

The acceleration of currents over obstacles on the seabed is a function of their height in relation to the depth of water, as well as to their orientation with respect to the current for non-symmetrical features.  Therefore, the larger a feature is with respect to the depth of water in which it occurs, the more it will interact with currents and cause local flow acceleration (the absolute flow speed varying as a function of the speed of the ocean current being deflected).  Wind-driven, surface currents flowing around seamounts are affected by seamount size, shape, the proximity of other seamounts or continental margins, the Coriolis apparent force and stratification.  Seamounts may generate coastally trapped waves, internal waves and may amplify the ocean tide. Mixing in the waters over a seamount can therefore be 100 to 10,000 times greater than mixing rates in the surrounding ocean (Lueck and Mudge, 1997).  The resulting strong currents carry food to filter feeders, remove waste, and inhibit sedimentation.

Seamounts rising to within a few hundred meters of the sea surface will interact with waves and surface currents, deflecting flow around the obstacle and forming local eddies.  In some cases, an eddy may become trapped over the seamount to form a closed circulation cell known as a “Taylor column”.  These oceanographic features have been observed to persist over an individual seamount for 6 weeks and the turbulence generated by a Taylor column may induce upwelling and locally enhanced primary productivity in an otherwise oligotrophic oceanic regime (Rogers, 1994).

Biodiversity of seamounts

Seamounts are isolated habitats that have evolved slowly over millions of years and they support communities having a high degree of endemism (Rogers, 1994; Tyler, 1995). For example, 40-51% of the species found at the Nasca/Saa-y-Gomez chain off western South America (Parin et al., 1997) and 29-34% of those from seamounts in the Coral and Tasman Seas were new to science (Richer de Forges et al., 2000).  These properties make seamounts of special ecological significance and they are of particular interest to conservationists and environmentalists.

The biology of hard substrata has been studied on shallow seamount tops (within SCUBA diving range) and to a lesser extent in deeper waters using submersibles and remotely operated underwater vehicles (ROVs).  Due to clear water conditions that often occur in the open ocean, photosynthesis is possible a great depths.  For example, live coralline algae was recovered at 268 m from the San Salvador Seamount in the Caribbean Sea (Littler et al., 1986).  At greater depths the dominant species are suspension feeders: corals, gorgonians, actinarians, pennatulids and hydroids. These species require a hard substrate to anchor themselves and strong currents to provide food and remove waste products and, most importantly, to keep sediment away.  The densities of attached filter feeders is greatest on the peaks and upper flanks of seamounts, with decreasing abundances found at greater depths (Rogers, 1994).  Submersible observations on Axial Seamount on the Juan de Fuca ridge in the northeast Pacific revealed dense communities of organisms (up to 100 individuals per m²) on the vertical walls of a caldera where current speeds were measured at 25 cm/sec (Tunnicliffe et al., 1985).  Suspension feeder abundance is inversely correlated with soft sediment cover, and so their numbers decline around the base of seamounts, where sediment deposits and weaker bottom currents are also found.

Figure 4.6 Photographs taken by remotely operated vehicle (ROV) from the flanks of the Manning Seamount, one of the New England Seamounts located in the North Atlantic Ocean.  Photos taken at around 2,500 m water depth show: (A) black coral, whip-like bamboo corals, sponges, crinoids, and sea stars; (B) yellow Enallopsammia stony coral, whip-like bamboo corals, sponges and crinoids; (C) Enallopsammia stony coral, whip-like bamboo corals, sponges and crinoids; (D) close-up view of Keratoisis sea fan with crinoids; (E) Lophelia, Candidella, solitary cup corals, brittle stars, crinoids, and sponges; and (F) Lophelia, Enallopsammia stony coral, dead coral skeleton, ruffled sponges with purple Trachythela octocoral growing on dead coral skeleton (photos courtesy of Mountains in the Sea Research Team; the IFE Crew; and NOAA: http://www.oceanexplorer.noaa.gov/explorations/04mountains/welcome.html).

Enhanced primary productivity over seamounts attracts migratory birds and pelagic species, such as sharks, tuna, billfishes and cetaceans.  The relatively abundant benthos attracts and supports a range of demersal and epipelagic fish species.  The relative abundance of life on seamounts makes them the preferred aggregation and spawning grounds for deep sea species such as orange roughy (Bull et al., 2001).  These factors compound to make the abundance of life as much as 100 times greater on seamounts than occurs on the adjacent sediment-covered abyssal plain.  Amidst the vastness of the oceans, seamounts are indeed oases for marine life.

Seamounts and Darwin’s atoll theory

The largest seamounts rise above the ocean surface forming a volcanic island.  As tectonic forces move the ocean crust laterally, the seamount moves away from the hotspot (that is fixed within the earth’s mantle) and the source of lava is eventually left behind.  Over geologic time, the crust cools and subsides, the seamount sinks and another seamount is formed over the same hotspot.  This process, of lateral movement of ocean crust over a single stationary hotspot, can give rise to the formation of a chain of seamounts such as the Hawaiian Islands.  Each volcano is formed in succession over the same hotspot, and then carried away by the gradual movement of ocean crust; the oldest volcano lies furthest from the hotspot and most likely it is deeply submerged beneath the ocean surface whereas the youngest, erupting volcano, may rise above sea level to form a volcanic island.  No seamount has been found in the ocean that is older than the ocean crust upon which it rests (Rogers, 1994), which supports the overall model of seamount formation outlined above.

Charles Darwin noted that seamounts that rise above the ocean surface making a volcanic island in tropical waters (like Hawaii) have a fringing coral reef around the island perimeter.  Darwin surmised that coral atolls evolve as a result of reef growth in combination with the subsidence of the volcanic island (Darwin, 1842).  As the island sinks, the reef grows upwards, keeping pace with subsidence until eventually the volcanic peak is completely submerged and all that remains is the circular coral atoll. This hypothesis was tested and proven correct 110 years after Darwin’s book was published by reef drilling in 1952 on Eniwetok Atoll; the drill cores penetrated up to 1400 m of shallow-water, reef carbonate before encountering basalt (Shepard, 1963).  What Darwin could not have known is that, after periods of sustained rapid subsidence that exceeds the pace of reef growth, the atoll may become deeply submerged beneath the ocean surface, forming a flat-topped seamount known as a guyot.  Guyots can also be formed by erosional processes, in which the volcanic peak is removed by wave action and eventual subsidence makes a flat-topped seamount.

Connectivity between seamounts

Factors that might influence the dispersal of larvae and colonisation of one seamount from another include the distance between seamounts, seamount size, the speed and direction of prevailing currents, the occurrence of Taylor columns and the depth of seamount peaks.  As mentioned in Chapter 2, the Theory of Island Biogeography has some direct applications to seamounts and it does predict that seamount size and spacing are factors in colonisation (and for example, the maintenance of metapopulations).   The theory also predicts that larger seamounts are expected to host larger and more diverse communities than smaller seamounts.  The depth of the seamount peak is also a factor: those having peaks located below the euphotic zone are clearly not able to host the same range of species as those seamounts having peaks within the euphotic zone.

The current regime plays a major role in larvae dispersal and hence the orientation of seamounts relative to the prevailing direction of flow determines whether one seamount is effectively downstream of another (and hence a potential site for colonisation).  The speed of flow determines how long larvae will have to be able to survive as it is carried along passively with the plankton from one seamount to the next.  For these reasons we might expect recruitment to be infrequent and episodic, punctuated with hiatuses of non-recruitment periods.

In some situations, an eddy may become established over the top of a seamount, establishing Taylor columns (as described above).  The persistence of Taylor columns over seamounts implies that it may be difficult for larvae to be transported by currents to another seamount site within the normal larval lifespan.   Seamounts prone to having Taylor columns are therefore not only geographically isolated, but they are oceanographically disconnected from surrounding areas, limiting the seamount’s ability to send or receive colonisers.

The comparatively low density of seamounts in the Southern Ocean, which is only one fifth that of the Indian and Atlantic Oceans and only one tenth that of the Pacific, has implications for the conservation of seamount biodiversity in that ocean.  The comparatively greater distance between Southern Ocean seamounts suggests that colonization is less likely to occur and that recruitment must be lower, compared with the more closely spaced seamounts in the other oceans.  It may be deduced, therefore, that Southern Ocean seamount communities are less well able to recover from disturbances such as deep sea trawling activities than those in the other ocean basins.

Back-arc basins

In some regions, two separate oceanic crustal plates collide.  The overriding plate is raised up to form a concave “bulge” with a basin located behind (Arculus, 1994).  These are termed back-arc basins because they are bounded by volcanic island arcs and can occur in association with both ocean-ocean crust and ocean-continental crust collision zones. Examples of ocean-ocean back arc basins include the Mariana, Tonga, Kermadec New Hebrides, Scotia and Lesser Antilles Arcs.  Ocean-continental examples are the Kuril, Japan, Ryukyu, Banda and Hellenic Arcs.  Biologically, back arc basins are associated with volcanism and hydrothermal vent communities.  For example, in the Tonga-Kermadec arc (comprising the fastest moving pieces of ocean crust on earth; Bevis et al., 1995), about 30 submarine volcanos were hydrothermally active, out of 70 that have been investigated in the region.

References

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