42
J. Galéron
communities in the deep-sea water column and on the ocean floor. This organic
matter decreases as depth increases and also varies according to the level and rate of
primary production in the surface waters. It can be supplemented by lateral supplies
from continents, in particular near to submarine canyons cut into ocean margins,
and can drain large quantities of matter from land.
In general terms, the deep sea is characterised by very low nutrient availability
(1–10 g of organic carbon per square metre per year), which may vary on a seasonal
basis, in particular in temperate regions but also on a multiannual or erratic basis
according to the specific climate conditions. These abyssal environments are known
as oligotrophic. There are however exceptions to this rule: hot fluid release areas
on mid-ocean ridges, back-arc basins, volcanic arcs, active intraplate volcanoes
and cold fluid emission areas on ocean margins. In these specific cases, the energy
source is not solar light, but chemical compounds discharged by the fluids, used by
micro-organisms to synthesise organic matter to feed exuberant animal communities, often known as an “oasis of life” in what appears to be an abyssal “desert”.
Life in the Abyssal ‘Desert’
The abyssal “desert” is mainly composed of plains covered in fine sediment that
stretch across the majority of the ocean floor. The low nutrient flow to the seabed
only allows very moderate biological productivity; deep-sea biological communities
are characterised by a low biomass, which decreases as the depth increases. Large organisms (megafauna), visible to the naked eye, are rarely found. These environments
were therefore long believed to be barren until the 1960s, when sediment sampling
tools were developed; this revealed the presence of a living world, invisible to the naked eye, still poorly known today, composed of micro-organisms and animals measuring a few dozen microns (meiofauna) to a few hundred microns (macrofauna).
Although rare in the deep-sea environment, larger animals are nevertheless present; a few dozen to a few hundred per hectare can be found on abyssal plains in the
Atlantic or Pacific Ocean. This fauna is composed mainly of animals that crawl
along the ocean floor and feed on detritus particles settled there; this is the case for
echinoderms (holothuroids (Fig. 2.1), asteroids, ophiuroids and echinoids) which
often form the majority of this fauna. We also find organisms fixed in the sediment
or on small hard substrates (protozoa, sponges, cnidaria) which feed on particles
in suspension in the overlying water, animals that live in the sediment—worms
(Fig. 2.2), bivalve molluscs, certain crustaceans—as well as a few fish and crustaceans that swim next to the bottom (Figs. 2.3, 2.4 and 2.5).
The macrofauna is mainly composed of polychaete annelid worms (Fig. 2.2),
small peracarid crustaceans (Fig. 2.6) and gastropod and bivalve molluscs; a few
dozen to a few hundred individuals can typically be found per square metre. The
meiofauna is generally dominated by nematode worms ( > 90 % metazoa) and by
foraminifera; their abundance can reach up to a few dozen per square centimetre.
These communities mainly live in the first 5–10 cm of sediment, with decreasing
J. Galéron
communities in the deep-sea water column and on the ocean floor. This organic
matter decreases as depth increases and also varies according to the level and rate of
primary production in the surface waters. It can be supplemented by lateral supplies
from continents, in particular near to submarine canyons cut into ocean margins,
and can drain large quantities of matter from land.
In general terms, the deep sea is characterised by very low nutrient availability
(1–10 g of organic carbon per square metre per year), which may vary on a seasonal
basis, in particular in temperate regions but also on a multiannual or erratic basis
according to the specific climate conditions. These abyssal environments are known
as oligotrophic. There are however exceptions to this rule: hot fluid release areas
on mid-ocean ridges, back-arc basins, volcanic arcs, active intraplate volcanoes
and cold fluid emission areas on ocean margins. In these specific cases, the energy
source is not solar light, but chemical compounds discharged by the fluids, used by
micro-organisms to synthesise organic matter to feed exuberant animal communities, often known as an “oasis of life” in what appears to be an abyssal “desert”.
Life in the Abyssal ‘Desert’
The abyssal “desert” is mainly composed of plains covered in fine sediment that
stretch across the majority of the ocean floor. The low nutrient flow to the seabed
only allows very moderate biological productivity; deep-sea biological communities
are characterised by a low biomass, which decreases as the depth increases. Large organisms (megafauna), visible to the naked eye, are rarely found. These environments
were therefore long believed to be barren until the 1960s, when sediment sampling
tools were developed; this revealed the presence of a living world, invisible to the naked eye, still poorly known today, composed of micro-organisms and animals measuring a few dozen microns (meiofauna) to a few hundred microns (macrofauna).
Although rare in the deep-sea environment, larger animals are nevertheless present; a few dozen to a few hundred per hectare can be found on abyssal plains in the
Atlantic or Pacific Ocean. This fauna is composed mainly of animals that crawl
along the ocean floor and feed on detritus particles settled there; this is the case for
echinoderms (holothuroids (Fig. 2.1), asteroids, ophiuroids and echinoids) which
often form the majority of this fauna. We also find organisms fixed in the sediment
or on small hard substrates (protozoa, sponges, cnidaria) which feed on particles
in suspension in the overlying water, animals that live in the sediment—worms
(Fig. 2.2), bivalve molluscs, certain crustaceans—as well as a few fish and crustaceans that swim next to the bottom (Figs. 2.3, 2.4 and 2.5).
The macrofauna is mainly composed of polychaete annelid worms (Fig. 2.2),
small peracarid crustaceans (Fig. 2.6) and gastropod and bivalve molluscs; a few
dozen to a few hundred individuals can typically be found per square metre. The
meiofauna is generally dominated by nematode worms ( > 90 % metazoa) and by
foraminifera; their abundance can reach up to a few dozen per square centimetre.
These communities mainly live in the first 5–10 cm of sediment, with decreasing