28
David THISTLE
rate of settlement was significantly greater on the lower
nodules despite their exposure to much slower flows.
A complete understanding of such flow effects will
require simultaneous measurements of velocity and the
concentration of propagules (or food).
Flow effects also arise because of the vertical
gradient in horizontal velocity adjacent to the seabed
(Fig. 2.21). At the seabed, the horizontal velocity is
Fig. 2.21. An illustration of the decrease in horizontal speed in the
bottom boundary layer as the seabed is approached.
zero (Vogel, 1981). In the layer immediately above
the seabed, molecular viscosity suppresses turbulent
motions, and horizontal velocity increases slowly and
linearly with height. In the deep sea, this region of
slow flow (the viscous sublayer) may be a centimeter
thick (Jumars and Gallagher, 1982). Above the viscous
sublayer, turbulence is present and horizontal velocity
increases logarithmically with height. This “log” layer
can be 1–10 m thick in the deep sea (Jumars and
Gallagher, 1982). As the horizontal velocity in the
log layer increases, the viscous sublayer becomes
thinner, and turbulent eddies collide with the seabed
more frequently, disrupting or destroying the viscous
sublayer.
This vertical gradient in horizontal velocity can
be expected to influence the settlement location of
sessile suspension feeders, because most hard-bottom
suspension feeders extend a collecting apparatus into
the flow and depend on water motion to bring particles
to it. Assuming that there is no vertical gradient in
food concentration, the flux of food increases with
height above the bottom (and the rate of increase
rises rapidly with height as the viscous sublayer gives
way to the log layer). Therefore, there should be
strong selection for passive suspension feeders on hard
bottoms to extend their feeding apparatus as far from
the seabed as possible, and there is a large premium
on extension above the viscous sublayer. Accordingly,
the propagules of passive suspension feeders that are
small as adults may be expected to settle at locations
where the local horizontal velocity is great enough to
disrupt the viscous sublayer or reduce its thickness
(Jumars and Gallagher, 1982). For example, a variety
of foraminifers attach in such a way that they are
millimeters to centimeters above the seabed (Lutze and
Altenbach, 1988; Lutze and Thiel, 1989). This selection
pressure should also apply to passive suspension
feeders that are large as adults, because their juveniles
should benefit from extending out of the viscous
sublayer (Jumars and Gallagher, 1982). For example,
passive suspension feeders tend to be found on the
edges of rocks, where the flow can be expected to be
locally accelerated and the viscous sublayer thinned.
Also, the feeding polyps of horny corals (gorgonians)
and sea pens (pennatularians) are absent near the base
of the animal, where the flow is slowest, and feather
stars (comatulid crinoids) perch on other animals
well above the seabed (Tyler and Zibrowius, 1992).
Although the flux of propagules and the flux of food
may increase monotonically with increasing velocity,
the rate of larval settlement and the rate of food
capture by a suspension feeder may not. In particular,
as near-bottom velocity (more correctly, the vertical
gradient of horizontal velocity) increases, the shear
(horizontal) force exerted on the seabed increases. To
settle, a larva must attach with sufficient strength to
resist that force. At some velocity, the frequency of
successful settlement will begin to decrease, raising
the possibility that settlement success could be greatest
at intermediate-velocity locations (Crisp, 1955). Also,
when velocity exceeds some limit, the feeding rate
of some suspension feeders will begin to decline
because their feeding apparatus will be deformed by
the flow (Koehl, 1977) and become less effective.
The distribution of suspension feeders (predominantly
sessile foraminifers) on manganese nodules may reflect
such effects. They were more abundant in a band below
the summits of manganese nodules than on the tops of
the nodules, where the velocity (and shear stress) was
greatest (Mullineaux, 1989; see also Mullineaux and
Butman, 1990).
Flow is not the only variable that influences the
settlement location of propagules on hard substrata.
Mullineaux and Butman (1990) found that some
species would settle on plates coated with ferromanganese but would not settle on smooth control plates,
indicating that some propagules seek specific substrata
or textures (see also Mullineaux, 1988; Bertram and
Cowen, 1994, 1999).
David THISTLE
rate of settlement was significantly greater on the lower
nodules despite their exposure to much slower flows.
A complete understanding of such flow effects will
require simultaneous measurements of velocity and the
concentration of propagules (or food).
Flow effects also arise because of the vertical
gradient in horizontal velocity adjacent to the seabed
(Fig. 2.21). At the seabed, the horizontal velocity is
Fig. 2.21. An illustration of the decrease in horizontal speed in the
bottom boundary layer as the seabed is approached.
zero (Vogel, 1981). In the layer immediately above
the seabed, molecular viscosity suppresses turbulent
motions, and horizontal velocity increases slowly and
linearly with height. In the deep sea, this region of
slow flow (the viscous sublayer) may be a centimeter
thick (Jumars and Gallagher, 1982). Above the viscous
sublayer, turbulence is present and horizontal velocity
increases logarithmically with height. This “log” layer
can be 1–10 m thick in the deep sea (Jumars and
Gallagher, 1982). As the horizontal velocity in the
log layer increases, the viscous sublayer becomes
thinner, and turbulent eddies collide with the seabed
more frequently, disrupting or destroying the viscous
sublayer.
This vertical gradient in horizontal velocity can
be expected to influence the settlement location of
sessile suspension feeders, because most hard-bottom
suspension feeders extend a collecting apparatus into
the flow and depend on water motion to bring particles
to it. Assuming that there is no vertical gradient in
food concentration, the flux of food increases with
height above the bottom (and the rate of increase
rises rapidly with height as the viscous sublayer gives
way to the log layer). Therefore, there should be
strong selection for passive suspension feeders on hard
bottoms to extend their feeding apparatus as far from
the seabed as possible, and there is a large premium
on extension above the viscous sublayer. Accordingly,
the propagules of passive suspension feeders that are
small as adults may be expected to settle at locations
where the local horizontal velocity is great enough to
disrupt the viscous sublayer or reduce its thickness
(Jumars and Gallagher, 1982). For example, a variety
of foraminifers attach in such a way that they are
millimeters to centimeters above the seabed (Lutze and
Altenbach, 1988; Lutze and Thiel, 1989). This selection
pressure should also apply to passive suspension
feeders that are large as adults, because their juveniles
should benefit from extending out of the viscous
sublayer (Jumars and Gallagher, 1982). For example,
passive suspension feeders tend to be found on the
edges of rocks, where the flow can be expected to be
locally accelerated and the viscous sublayer thinned.
Also, the feeding polyps of horny corals (gorgonians)
and sea pens (pennatularians) are absent near the base
of the animal, where the flow is slowest, and feather
stars (comatulid crinoids) perch on other animals
well above the seabed (Tyler and Zibrowius, 1992).
Although the flux of propagules and the flux of food
may increase monotonically with increasing velocity,
the rate of larval settlement and the rate of food
capture by a suspension feeder may not. In particular,
as near-bottom velocity (more correctly, the vertical
gradient of horizontal velocity) increases, the shear
(horizontal) force exerted on the seabed increases. To
settle, a larva must attach with sufficient strength to
resist that force. At some velocity, the frequency of
successful settlement will begin to decrease, raising
the possibility that settlement success could be greatest
at intermediate-velocity locations (Crisp, 1955). Also,
when velocity exceeds some limit, the feeding rate
of some suspension feeders will begin to decline
because their feeding apparatus will be deformed by
the flow (Koehl, 1977) and become less effective.
The distribution of suspension feeders (predominantly
sessile foraminifers) on manganese nodules may reflect
such effects. They were more abundant in a band below
the summits of manganese nodules than on the tops of
the nodules, where the velocity (and shear stress) was
greatest (Mullineaux, 1989; see also Mullineaux and
Butman, 1990).
Flow is not the only variable that influences the
settlement location of propagules on hard substrata.
Mullineaux and Butman (1990) found that some
species would settle on plates coated with ferromanganese but would not settle on smooth control plates,
indicating that some propagules seek specific substrata
or textures (see also Mullineaux, 1988; Bertram and
Cowen, 1994, 1999).