26
2. ENVIRONMENTALLY DRIVEN PLASTICITY
A final example of the effects of flow on organismal metabolism is how
uptake of phosphorus by coral reef communities is mediated by hydrodynamic processes (Atkinson and Bilger 1992). Using the St number approach
outlined above, phosphorus uptake by coral reef benthos (corals , algal turfs,
and macroalgae) arranged in a flume over a range of flowspeeds indicated that
uptake was mass transfer limited and occurred through turbulent boundary
layers. Furthermore, Atkinson and Bilger concluded that rates of phosphorus uptake are 6-7 times higher than predicted from theory and this might
result , in part, from the fractal nature of coral reef surfaces and their high
surface area (of organisms) per planar area .
FLow, FORM, AND FUNCTION. The examples given above illustrate the cornplex relationships between organism morphology, the physical environment,
and physiological function (and presumably growth). For seaweeds, some
morphologies perform better in particular flow environments with shape
and form resulting from a trade-off between the positive and negative effects of water motion. However, the morphology is not constrained entirely
by genetics and phenotypic changes in morphology are common.
Size may also have important effects on mass transfer. As the size of
an organism increases, the flow environment that it experiences changes.
Larger benthic organisms generally extend further from the substratum and
away from the benthic boundary layer and experience higher flow that may
influence rates of mass transfer. Conversely, larger organisms present more
surface area over which local boundary layers can develop, perhaps negating
any beneficial effects of growing out of the benthic boundary layer. Combined
with the lower SAjV of larger organisms (assuming isometry), the overall
ability of larger organisms to take up materials from the surrounding fluid
relative to the demand for these materials, may be inversely related to size.
Clearly, the interplay between water motion, seaweed morphology and size,
and the resulting effects on mass transfer are complex. Attempts to model the
growth and form of seaweeds must incorporate, as far as possible, the effects
of the flow environment on organismal function, short-term, flow-induced
changes in morphology, and the numerous and intimate interactions between
morphology and the physical environment.
Particle Capture
Suspension-feeding invertebrates rely, to varying degrees, on the movement
of water for the delivery of plankton and other particulate matter to their
feeding surfaces. As is the case for the processes of mass flux and momentum
transfer, the delivery of food is often strongly affected by the characteristics of
water moving past the organism's surface, which in turn can be significantly
modified by the organism's shape and position within the benthic boundary
layer. Furthermore, particulate capture is also dependent on the size, density,
and, in the case of zooplankton, behavior of the food item being captured.
The general theories underlying the mechanisms of particle capture in
the marine environment have been thoroughly examined by previous reviews
(Rubenstein and Koehl 1977, LaBarbera 1984, Shimeta and Iumars 1991, Wildish and Kristmanson 1997). These mechanisms are often divided into general
categories, each reflecting the relative importance of factors such as the relative size of the particle and the filtering apparatus, the density (and thus
2. ENVIRONMENTALLY DRIVEN PLASTICITY
A final example of the effects of flow on organismal metabolism is how
uptake of phosphorus by coral reef communities is mediated by hydrodynamic processes (Atkinson and Bilger 1992). Using the St number approach
outlined above, phosphorus uptake by coral reef benthos (corals , algal turfs,
and macroalgae) arranged in a flume over a range of flowspeeds indicated that
uptake was mass transfer limited and occurred through turbulent boundary
layers. Furthermore, Atkinson and Bilger concluded that rates of phosphorus uptake are 6-7 times higher than predicted from theory and this might
result , in part, from the fractal nature of coral reef surfaces and their high
surface area (of organisms) per planar area .
FLow, FORM, AND FUNCTION. The examples given above illustrate the cornplex relationships between organism morphology, the physical environment,
and physiological function (and presumably growth). For seaweeds, some
morphologies perform better in particular flow environments with shape
and form resulting from a trade-off between the positive and negative effects of water motion. However, the morphology is not constrained entirely
by genetics and phenotypic changes in morphology are common.
Size may also have important effects on mass transfer. As the size of
an organism increases, the flow environment that it experiences changes.
Larger benthic organisms generally extend further from the substratum and
away from the benthic boundary layer and experience higher flow that may
influence rates of mass transfer. Conversely, larger organisms present more
surface area over which local boundary layers can develop, perhaps negating
any beneficial effects of growing out of the benthic boundary layer. Combined
with the lower SAjV of larger organisms (assuming isometry), the overall
ability of larger organisms to take up materials from the surrounding fluid
relative to the demand for these materials, may be inversely related to size.
Clearly, the interplay between water motion, seaweed morphology and size,
and the resulting effects on mass transfer are complex. Attempts to model the
growth and form of seaweeds must incorporate, as far as possible, the effects
of the flow environment on organismal function, short-term, flow-induced
changes in morphology, and the numerous and intimate interactions between
morphology and the physical environment.
Particle Capture
Suspension-feeding invertebrates rely, to varying degrees, on the movement
of water for the delivery of plankton and other particulate matter to their
feeding surfaces. As is the case for the processes of mass flux and momentum
transfer, the delivery of food is often strongly affected by the characteristics of
water moving past the organism's surface, which in turn can be significantly
modified by the organism's shape and position within the benthic boundary
layer. Furthermore, particulate capture is also dependent on the size, density,
and, in the case of zooplankton, behavior of the food item being captured.
The general theories underlying the mechanisms of particle capture in
the marine environment have been thoroughly examined by previous reviews
(Rubenstein and Koehl 1977, LaBarbera 1984, Shimeta and Iumars 1991, Wildish and Kristmanson 1997). These mechanisms are often divided into general
categories, each reflecting the relative importance of factors such as the relative size of the particle and the filtering apparatus, the density (and thus