16
2. ENVIRONMENTALLY DRIVEN PLASTICITY
flow velocity U changes in time in response to convection (U · )U, spatial
variations in pressure VP, and viscous forces vv
2 U
Two important dimensionless parameters, characterizing the flow,
are:
Re = iii ,
v
Pe = iii
D
The Re parameter is the Reynolds number and is the ratio between the
inertial forces iii and the viscous forces, where Ii is the averag e flow velocity and I a characteristic length in the system (for example the height
of the organism). High values of Re indicate that the flow becomes turbulent and the flow will not attain a steady state (aUI at =0), while th e
flow will reach a steady state for low Re numbers (laminar flow).
The Pe parameter is the Peclet number and is the ratio between th e
inertial forces iii and the Brownian forces , where D is the diffusion coefficient. The value of D represents, for example, the diffusion coefficient
of a suspended food particle. Low Pe numbers indicate that particles are
mainly distributed by diffusion, while high Pe numbers indicate that food
particles are mainly dispersed through hydrodynamics.
Box 2.2 Underwater liglLt intensities
The change of light intensity, due to the attenuation in the water column,
is described for monochromatic light by the Lambert-Beer Law:
Id = Ioe- rd
(2.5)
where Id and 1 0 are respectively the light intensities at depth d and just
below the water surface. The attenuation coefficient r is the sum of the
absorption coefficient and the scattering coefficient. The light absorption depends upon the wavelength of the monochromatic light, minimal
absorption takes place in clear water at 465 nm, absorption of blue light
increases by the presence of soluble yellow humus-like substances in the
water. The scattering coefficient depends upon the amount of suspended
particles in the water.
The Lambert-Beer Lawgives the light intensity at a certain depth. The
amount of light received by a unit area can be calculated by considering
the angle of incidence of light, which gives a more detailed description
of the local light intensities at the surface of an organism. A (highly
simplified) light model (Foley et al. 1990) is described by the equation:
I =I . cos B
(2.6)
The light intensity I (W1m
2
) on a surface is determined by cos B, where
B is the angle of incidence between the direction of the light beam and
the surface normal and Is is the intensity of the light source (at a certain
depth). In this equation it is assumed that the light source is constant,
the light direction corresponds to the vertical, and there is no diffuse
reflection from the environment.
2. ENVIRONMENTALLY DRIVEN PLASTICITY
flow velocity U changes in time in response to convection (U · )U, spatial
variations in pressure VP, and viscous forces vv
2 U
Two important dimensionless parameters, characterizing the flow,
are:
Re = iii ,
v
Pe = iii
D
The Re parameter is the Reynolds number and is the ratio between the
inertial forces iii and the viscous forces, where Ii is the averag e flow velocity and I a characteristic length in the system (for example the height
of the organism). High values of Re indicate that the flow becomes turbulent and the flow will not attain a steady state (aUI at =0), while th e
flow will reach a steady state for low Re numbers (laminar flow).
The Pe parameter is the Peclet number and is the ratio between th e
inertial forces iii and the Brownian forces , where D is the diffusion coefficient. The value of D represents, for example, the diffusion coefficient
of a suspended food particle. Low Pe numbers indicate that particles are
mainly distributed by diffusion, while high Pe numbers indicate that food
particles are mainly dispersed through hydrodynamics.
Box 2.2 Underwater liglLt intensities
The change of light intensity, due to the attenuation in the water column,
is described for monochromatic light by the Lambert-Beer Law:
Id = Ioe- rd
(2.5)
where Id and 1 0 are respectively the light intensities at depth d and just
below the water surface. The attenuation coefficient r is the sum of the
absorption coefficient and the scattering coefficient. The light absorption depends upon the wavelength of the monochromatic light, minimal
absorption takes place in clear water at 465 nm, absorption of blue light
increases by the presence of soluble yellow humus-like substances in the
water. The scattering coefficient depends upon the amount of suspended
particles in the water.
The Lambert-Beer Lawgives the light intensity at a certain depth. The
amount of light received by a unit area can be calculated by considering
the angle of incidence of light, which gives a more detailed description
of the local light intensities at the surface of an organism. A (highly
simplified) light model (Foley et al. 1990) is described by the equation:
I =I . cos B
(2.6)
The light intensity I (W1m
2
) on a surface is determined by cos B, where
B is the angle of incidence between the direction of the light beam and
the surface normal and Is is the intensity of the light source (at a certain
depth). In this equation it is assumed that the light source is constant,
the light direction corresponds to the vertical, and there is no diffuse
reflection from the environment.