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makes seamounts ideal fishing grounds. Seamount-generated currents also cause
erosion of the seamounts (and Fe-Mn crusts) and move surface sediments, which
produces sand waves and ripples.
Seamount height, summit size, types of ambient currents, and energy of the tidal
currents determine which seamount-specific currents will be generated and their
longevity. It is clear that some seamounts in the equatorial Pacific have been swept clean
of sediment for most of 60 Ma, because that is the duration of crust growth in those areas,
whereas other seamounts may be capped by as much as 500 m of carbonate sediment and
therefore lack development of Fe-Mn crusts on the summit.
These physical processes also affect seamount biology. Seamount communities
vary from seamount to seamount, even communities from the same water depths on
adjacent seamounts. This is partly the result of the varying physical processes in the
water column. Most studies of seamount biology have concentrated on seamounts with
a sediment cap and on the biological communities living on (epifauna) and in (infauna)
that sediment (e.g., 117). Fewer studies have addressed communities dwelling on the rock
outcrops, which consist of mostly attached (sessile) organisms. A few studies have
looked at the types of organisms that live on the surface of Fe-Mn crusts, which consist
predominantly of agglutinated foraminifera (e.g., 118). The bacterial or microbiological
processes that may mediate the growth of Fe-Mn crusts and the concentration of trace
metals, have not been studied.
Seamount biological communities are characterized by relatively low density and
low diversity where the Fe-Mn crusts are thickest and cobalt-rich. This occurs because
the low oxygen contents in the OMZ decrease the abundance of consumer populations,
excludes most tolerant species from seafloor habitats, and can produce steep gradients in
seafloor communities 119. Above and below the OMZ, the populations may be greater and
more diverse. Levin and Thomas 120 found lower biological activity at the high-energy
summit margin (covered by both rock and sediment) of the central-Pacific Horizon
Guyot than at sediment-covered summit sites. In contrast, Genin et al. 121 found that
antipatharian and gorgonian corals are more abundant in areas of seamount summits
where flow acceleration is prominent. Thus, the make-up of the seamount communities
and population density and diversity are determined by current patterns, topography,
bottom sediment and rock types and coverage, seamount size, water depth, and size and
magnitude of the OMZ, which in turn is related to primary productivity.
70 International Seabed Authority