Movement of Light, Heat, and Chemicals in Water - Freshwater Ecology: Concepts and Environmental Applications - page 35

the same considerations apply to the thick-
ness of diffusion and flow boundary layers;
depressions in solid surfaces lead to thicker
diffusion boundary layers, and thinner lay-
ers occur where a solid surface protrudes
into flowing water and as water velocity
and turbulence increase.
The diffusion rate is so much slower
across the region where molecular diffusion
predominates than in regions with eddy dif-
fusion that it represents a rate-limiting
boundary layer to the passage of biologi-
cally active chemicals. Compounds that are
required for metabolic activity must cross
the diffusion boundary layer so metabolic
rate can be limited by the thickness of this
layer. The diffusion boundary layer con-
strains evolutionary pressures that shape
chemically mediated interactions among
organisms (Dodds, 1990; Brönmark and
Hansson, 2000). For example, the dilution
associated with transport diffusion makes it
unlikely that biologically active compounds
will be released into turbulent waters. The
evolutionary cost of purposefully releasing
chemicals that are costly to synthesize is
too great if chemicals are rapidly diluted
to concentrations so low they are not
effective.
Chemical diffusion rates are related to
the size and geometry of organisms. Surface
area to volume relationships are essential in
the design of aquatic organisms. Large ob-
jects have a greater volume relative to their
surface area compared to smaller objects.
Geometry can affect relative rates of diffu-
sion (of nutrients inward and metabolic
waste products outward), which can limit
metabolism. One way to conceptualize this
is to think of the average distance of all
points inside a sphere to the outside edge.
On average, the distance to the edge of a
larger sphere is greater. Fick's law states that
greater distance translates into lower diffu-
sion. Thus, metabolic processes in large
organisms are more likely to be limited by
diffusion.
Changes in form that increase surface
area to volume ratios are one way to over-
come limitations imposed by diffusion. Form

Sidebar 3.1.
Does Diffusion Select for Morphology of
Small Planktonic Organisms?

Factors crucial to survival of planktonic organ-
isms (organisms suspended in water) include:
(i) avoiding sinking into regions where light is
insufficient for survival (Reynolds, 1994), (ii) a
surface area to volume ratio that maximizes
nutrient and gas diffusion and increases com-
petitive ability (Reynolds, 1994), and (iii) devel-
opment of defensive appendages that lower
the probability of capture or ingestion by preda-
tors. The morphology (shape and size) of sin-
gle- or few-celled planktonic organisms can
influence all these factors.
Sinking can be affected by particle size (see
formula for F v and discussion of Stokes law in
Chapter 2). Small particles experience high vis-
cosity and sink more slowly. However, in-
creasing surface area also increases viscosity
F v . Therefore, spines or projections (e.g., Figs.
8.8, 9.4, and 19.4) lower sinking rates.
Increasing the surface area with spines or
projections also increases the surface area to
volume ratio. Assuming that the spines or pro-
jections can allow materials to move through
them to and from the cell, they also can increase
the biologically active surface area. A greater
biologically active surface area increases the
diffusion rates of incoming nutrients and outgo-
ing waste products. Thus, there can be natural
selection for planktonic organisms to increase
their biologically active surface area.
However, the sinking and diffusion rates may
not be the only selective factors acting on mor-
phology. Predation risk is decreased by defen-
sive appendages. Such spines or projections in-
crease handling times and unwieldy organisms
can clog mouthparts of planktonic predators. In
conclusion, three selective pressures converge,
and there are a wide variety of shapes of plank-
tonic organisms. The relative importance of
these three factors may vary with the organisms
considered and their habitats. Considering the
relationship between morphology and diffusion
is still necessary to describe the selective pres-
sures on shape of small planktonic organisms.