REDUCING ENVIRONMENTS OF THE DEEP-SEA FLOOR
101
and Thomson, 1995). Vereshchaka and Vinogradov
(1999) found a similar aggregation of zooplankton
biomass on upper and lower layers of the plume over
vents on the Mid-Atlantic Ridge. Sedimentation rates
for chemosynthetically-derived organic carbon beneath
hydrothermal plumes can equal the flux of organic
carbon from the photic zone (Roth and Dymond,
1989).
ISSUES PECULIAR TO VENTS
The hydrothermal-vent ecosystem offers many unusual
features in the context of its location, dependencies,
featured adaptations and constituency. The reader is directed elsewhere for commentary on diversity (Jollivet,
1996), population genetics (Vrijenhoek, 1997), antiquity (McArthur and Tunnicliffe, 1998), biogeography
(Tunnicliffe et al., 1998), reproductive biology (Tyler
and Young, 1999) and ecology (Van Dover, 2000).
Juniper and Tunnicliffe (1997) have discussed the
limitations of current knowledge of this ecosystem. In
addition, recent summaries of vent-biology studies are
in the conference proceedings edited by Biscoito et al.
(1998). Here, we comment on two interesting features
of this habitat: stability and dispersion. The location
atop a changeable feature – the mid-ocean ridge-crest –
means that these communities are directly affected
by the volcanic and tectonic processes driving plate
tectonics. This irregular habitat is dispersed linearly.
Stability and predictability
Faunal communities of reducing habitats have evolved
to exploit an energy source that is discontinuous
in both space and time. High-frequency variation in
conditions for life is common in the hydrothermal-vent
environment. Habitat can alter drastically over periods
ranging from days to months, as mineral accretion
and tectonic events modify hydrothermal flow through
subsurface conduits (Fustec et al., 1987; Tunnicliffe
and Juniper, 1990; Sarrazin et al., 1997). Hydrothermal
vents tend to occur in clusters on the scale of 10–
100 m, or in fields with a common subsurface heat
source. Whereas individual orifices may be unstable,
vent fields are active for relatively long periods of
time. Sites in the eastern Pacific discovered 15 to
20 years ago have since evolved noticeably, but are
all still actively venting and supporting life (Hessler
et al., 1988; Desbruy` eres, 1998). Some vent sites
may be very long-lived – to tens of thousands of
years – as evidenced by the accumulation of some
large mineral deposits (Hannington et al., 1995; Lalou
et al., 1995). While the literature tends to emphasize the
ephemeral nature of vents, it is important to stress that
some sites appear quite stable, and others have likely
maintained continual venting in the general vicinity
even if individual vents are short-lived.
Volcanic eruptions can cause major perturbations
of hydrothermal systems and associated communities.
A submersible cruise to 9ºN on the East Pacific Rise
in 1991 discovered evidence of a very recent (days to
weeks) seafloor eruption, and the partial destruction of
vent communities. Subsequent recolonization was very
rapid, beginning with a burst of subsurface and seafloor
microbial growth (Haymon et al., 1993; Shank et al.,
1998). The sequence of microbial bloom and faunal
colonization of new vent fields was observed again
following a remotely-detected eruption at the CoAxial
Segment on the Juan de Fuca Ridge in 1993 (Juniper
et al., 1995; Tunnicliffe et al., 1997) and the Axial
Volcano which extruded lavas over tube-worm fields in
1998 (Embley et al., 1999) (Fig. 4.3). While seafloor
eruptions may be locally destructive, they represent
a renewal of the subsurface heat source driving the
hydrothermal system. At fast and superfast spreading
centres, where volcanic activity is most intense, a
high frequency of eruption in time and space also
has the effect of creating abundant habitat. Juniper
and Tunnicliffe (1997) have suggested that habitat
abundance may counter instability on these ridges in
supporting regional diversity. The same forces that
disturb venting at a particular location on the East
Pacific Rise (i.e. robust magma supply) will also assure
that venting is continuously present within a short
distance of the disturbed site.
Dispersal
Spatial distribution of the vent habitat can be likened to
that in island chains or the alpine of mountain ranges.
One may suppose that dispersal from one site to another
poses a particular adaptive challenge. The dispersal
stage of most vent inhabitants is a pelagic larva, as
is found in most marine invertebrates. Some marine
larvae can live for months in the water, and thus have
the potential for broad distributions. It is interesting
to note that few vent species transgress the different
regions shown in Fig. 4.1 (Tunnicliffe et al., 1998).
While little is known of the basic biology of most
101
and Thomson, 1995). Vereshchaka and Vinogradov
(1999) found a similar aggregation of zooplankton
biomass on upper and lower layers of the plume over
vents on the Mid-Atlantic Ridge. Sedimentation rates
for chemosynthetically-derived organic carbon beneath
hydrothermal plumes can equal the flux of organic
carbon from the photic zone (Roth and Dymond,
1989).
ISSUES PECULIAR TO VENTS
The hydrothermal-vent ecosystem offers many unusual
features in the context of its location, dependencies,
featured adaptations and constituency. The reader is directed elsewhere for commentary on diversity (Jollivet,
1996), population genetics (Vrijenhoek, 1997), antiquity (McArthur and Tunnicliffe, 1998), biogeography
(Tunnicliffe et al., 1998), reproductive biology (Tyler
and Young, 1999) and ecology (Van Dover, 2000).
Juniper and Tunnicliffe (1997) have discussed the
limitations of current knowledge of this ecosystem. In
addition, recent summaries of vent-biology studies are
in the conference proceedings edited by Biscoito et al.
(1998). Here, we comment on two interesting features
of this habitat: stability and dispersion. The location
atop a changeable feature – the mid-ocean ridge-crest –
means that these communities are directly affected
by the volcanic and tectonic processes driving plate
tectonics. This irregular habitat is dispersed linearly.
Stability and predictability
Faunal communities of reducing habitats have evolved
to exploit an energy source that is discontinuous
in both space and time. High-frequency variation in
conditions for life is common in the hydrothermal-vent
environment. Habitat can alter drastically over periods
ranging from days to months, as mineral accretion
and tectonic events modify hydrothermal flow through
subsurface conduits (Fustec et al., 1987; Tunnicliffe
and Juniper, 1990; Sarrazin et al., 1997). Hydrothermal
vents tend to occur in clusters on the scale of 10–
100 m, or in fields with a common subsurface heat
source. Whereas individual orifices may be unstable,
vent fields are active for relatively long periods of
time. Sites in the eastern Pacific discovered 15 to
20 years ago have since evolved noticeably, but are
all still actively venting and supporting life (Hessler
et al., 1988; Desbruy` eres, 1998). Some vent sites
may be very long-lived – to tens of thousands of
years – as evidenced by the accumulation of some
large mineral deposits (Hannington et al., 1995; Lalou
et al., 1995). While the literature tends to emphasize the
ephemeral nature of vents, it is important to stress that
some sites appear quite stable, and others have likely
maintained continual venting in the general vicinity
even if individual vents are short-lived.
Volcanic eruptions can cause major perturbations
of hydrothermal systems and associated communities.
A submersible cruise to 9ºN on the East Pacific Rise
in 1991 discovered evidence of a very recent (days to
weeks) seafloor eruption, and the partial destruction of
vent communities. Subsequent recolonization was very
rapid, beginning with a burst of subsurface and seafloor
microbial growth (Haymon et al., 1993; Shank et al.,
1998). The sequence of microbial bloom and faunal
colonization of new vent fields was observed again
following a remotely-detected eruption at the CoAxial
Segment on the Juan de Fuca Ridge in 1993 (Juniper
et al., 1995; Tunnicliffe et al., 1997) and the Axial
Volcano which extruded lavas over tube-worm fields in
1998 (Embley et al., 1999) (Fig. 4.3). While seafloor
eruptions may be locally destructive, they represent
a renewal of the subsurface heat source driving the
hydrothermal system. At fast and superfast spreading
centres, where volcanic activity is most intense, a
high frequency of eruption in time and space also
has the effect of creating abundant habitat. Juniper
and Tunnicliffe (1997) have suggested that habitat
abundance may counter instability on these ridges in
supporting regional diversity. The same forces that
disturb venting at a particular location on the East
Pacific Rise (i.e. robust magma supply) will also assure
that venting is continuously present within a short
distance of the disturbed site.
Dispersal
Spatial distribution of the vent habitat can be likened to
that in island chains or the alpine of mountain ranges.
One may suppose that dispersal from one site to another
poses a particular adaptive challenge. The dispersal
stage of most vent inhabitants is a pelagic larva, as
is found in most marine invertebrates. Some marine
larvae can live for months in the water, and thus have
the potential for broad distributions. It is interesting
to note that few vent species transgress the different
regions shown in Fig. 4.1 (Tunnicliffe et al., 1998).
While little is known of the basic biology of most