2.1. THE PHYSICAL ENVIRONMENT
2.1.1 Growing and Flowing
Marine sessile organisms, such as macroalgae and colonial animals, risk
being dislodged or broken by ambient water currents and waves,yet they also
depend on that moving water for transport (reviewed in Koehl 1982, 1986,
1999, Denny 1988, 1999, Vogel 1994). Ambient water motion is responsible
for dispersing the spores or larvae, and in many cases the gametes, of sessile
organisms. Waste products and sediments are also carried away by moving
water. Attached algae and animals depend on moving water for the transport
of dissolved materials such as nutrients and gases, while suspension-feeding
animals depend on ambient currents to bring particulate food to them, and in
many cases to ventilate their filters. Body designs that enhance an organism's
interaction with the water flowing around it enhance not only transport, but
also hydrodynamic forces.
The first step in studying how sessile organisms interact with the water flowing around them is to determine their hydrodynamic microhabitats
(e.g, Koehl 1977a, Denny 1988). Many benthic organisms in deep water and
in protected bays and estuaries encounter unidirectional currents or tidal
currents that flow in one direction for several hours, and then in the opposite direction. Attached organisms in shallow coastal habitats are also
exposed to waves. When a wave passes over an organism on the substratum
where water depth is less than one half of the crest-to-crest distance between
waves, the water flow along the bottom is back-and-forth with a period of
seconds. When fluid flows along a solid surface, such as the substratum
or the surface of an organism's body, the layer of fluid in contact with the
surface does not slip with respect to it. Therefore, a velocity gradient (the
boundary layer) develops in the fluid between the surface and the freestream
flow. The greater the distance a fluid flows across a surface, the thicker the
boundary layer becomes. In marine habitats, the benthic boundary layer
can be a meter or more thick, although the steepest velocity gradient occurs within a few centimeters of the substratum (reviewed in [umars and
Nowell 1984).Although a thin sublayer (mm's thick) of laminar flow occurs
along the substratum, water flow in the benthic boundary layer is turbulent, so mass and momentum are mixed between the freestream flow and
the bottom by swirling eddies. Since it takes time for a boundary layer to
build up when water begins to flow over a surface, the benthic boundary
layer in the back-and-forth sloshing water of waves is much thinner than
in unidirectional flow (e.g. Denny 1988). Local topography and neighboring organisms can have a profound effect on the water flow encountered by
a benthic organism, hence the hydrodynamic microhabitat of an attached
animal or plant can be very different from the freestream flow over the
site where it occurs (Koehl 1977a, Koehl and Alberte 1988). As a sessile organism grows, it can encounter more rapid water movement as it sticks
up higher in the benthic boundary layer and becomes larger relative to its
neighbors.
Biomechanical studies have shown that general physical rules that apply
across taxa can permit us to understand and predict how organisms interact with their physical environments. Such an approach provides a useful
framework for considering the consequences of shape and size on the hydrodynamic forces and on the transport experienced by sessile animals and
macrophytes as they grow.
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2.1.1 Growing and Flowing
Marine sessile organisms, such as macroalgae and colonial animals, risk
being dislodged or broken by ambient water currents and waves,yet they also
depend on that moving water for transport (reviewed in Koehl 1982, 1986,
1999, Denny 1988, 1999, Vogel 1994). Ambient water motion is responsible
for dispersing the spores or larvae, and in many cases the gametes, of sessile
organisms. Waste products and sediments are also carried away by moving
water. Attached algae and animals depend on moving water for the transport
of dissolved materials such as nutrients and gases, while suspension-feeding
animals depend on ambient currents to bring particulate food to them, and in
many cases to ventilate their filters. Body designs that enhance an organism's
interaction with the water flowing around it enhance not only transport, but
also hydrodynamic forces.
The first step in studying how sessile organisms interact with the water flowing around them is to determine their hydrodynamic microhabitats
(e.g, Koehl 1977a, Denny 1988). Many benthic organisms in deep water and
in protected bays and estuaries encounter unidirectional currents or tidal
currents that flow in one direction for several hours, and then in the opposite direction. Attached organisms in shallow coastal habitats are also
exposed to waves. When a wave passes over an organism on the substratum
where water depth is less than one half of the crest-to-crest distance between
waves, the water flow along the bottom is back-and-forth with a period of
seconds. When fluid flows along a solid surface, such as the substratum
or the surface of an organism's body, the layer of fluid in contact with the
surface does not slip with respect to it. Therefore, a velocity gradient (the
boundary layer) develops in the fluid between the surface and the freestream
flow. The greater the distance a fluid flows across a surface, the thicker the
boundary layer becomes. In marine habitats, the benthic boundary layer
can be a meter or more thick, although the steepest velocity gradient occurs within a few centimeters of the substratum (reviewed in [umars and
Nowell 1984).Although a thin sublayer (mm's thick) of laminar flow occurs
along the substratum, water flow in the benthic boundary layer is turbulent, so mass and momentum are mixed between the freestream flow and
the bottom by swirling eddies. Since it takes time for a boundary layer to
build up when water begins to flow over a surface, the benthic boundary
layer in the back-and-forth sloshing water of waves is much thinner than
in unidirectional flow (e.g. Denny 1988). Local topography and neighboring organisms can have a profound effect on the water flow encountered by
a benthic organism, hence the hydrodynamic microhabitat of an attached
animal or plant can be very different from the freestream flow over the
site where it occurs (Koehl 1977a, Koehl and Alberte 1988). As a sessile organism grows, it can encounter more rapid water movement as it sticks
up higher in the benthic boundary layer and becomes larger relative to its
neighbors.
Biomechanical studies have shown that general physical rules that apply
across taxa can permit us to understand and predict how organisms interact with their physical environments. Such an approach provides a useful
framework for considering the consequences of shape and size on the hydrodynamic forces and on the transport experienced by sessile animals and
macrophytes as they grow.
17