254
8 Transport in the Oceans and Coastal Zone
particles are placed in a fluid influenced by turbulence, the associated diffusion
due to turbulence is known as turbulent diffusion, in distinguishing it from the
molecular diffusion. Thrbulent diffusion is different from molecular diffusion
due to random molecular motion.
When particles are subjected to molecular or turbulent diffusion, their initial
concentration will change in space and time. The determination of particle
concentration at a given point, and at a given instance of time, is of fundamental
importance for biological life in the ocean. The concentration of organic and
non-organic particles or chemical substances strongly influences many elements
of biological life in the ocean, such as metabolic processes. On the other hand,
concentration of pollutants determines the degree of threat to marine plants
and organisms. We will resume this discussion in Chap. 13.
Molecular and turbulent diffusions are not the only means of transport of
matter in the ocean environment. Particles and substances can also be transported by convection, where convection includes transport by either the bulk
movement of a fluid relative to marine organisms or by movement of an organism relative to the fluid. In the next section we will examine these physical
mechanisms of transport. We will start with the basic information on diffusion.
8.2 Basics of Diffusion
8.2.1 Randomness of Diffusion
Consider a fluid in which a number of particles are suspended. If the particles
move around at random, transfer of kinetic energy of particles will occur. This
is a consequence of the interaction of neighbouring particles through the occurrence of occasional collisions. The direction taken by a particle after a collision
is random. If it hits another particle, it may return back to where it came
from. Therefore, subsequent positions of the particle are random, and as such
it can be treated as a random variable which satisfies some laws of probability.
Because of the randomness of this mechanism, Okubo (1980) defined diffusion
to be a basically irreversible phenomenon by which matter, particle groups, population, etc. spreads out within a given space according to individual random
motion. The diffusion is irreversible in the sense that the probability of returning to the original state decreases, when the number of diffusing particles
increases.
It should be noted that diffusion was defined as the dispersion of some property of the fluid, for example salinity, without any net mass transfer of the fluid
itself. In the oceans we distinguish diffusive transfer due to molecular motion
of the water particles (molecular diffusion) and due to turbulence of the water
mass (turbulent diffusion).
8 Transport in the Oceans and Coastal Zone
particles are placed in a fluid influenced by turbulence, the associated diffusion
due to turbulence is known as turbulent diffusion, in distinguishing it from the
molecular diffusion. Thrbulent diffusion is different from molecular diffusion
due to random molecular motion.
When particles are subjected to molecular or turbulent diffusion, their initial
concentration will change in space and time. The determination of particle
concentration at a given point, and at a given instance of time, is of fundamental
importance for biological life in the ocean. The concentration of organic and
non-organic particles or chemical substances strongly influences many elements
of biological life in the ocean, such as metabolic processes. On the other hand,
concentration of pollutants determines the degree of threat to marine plants
and organisms. We will resume this discussion in Chap. 13.
Molecular and turbulent diffusions are not the only means of transport of
matter in the ocean environment. Particles and substances can also be transported by convection, where convection includes transport by either the bulk
movement of a fluid relative to marine organisms or by movement of an organism relative to the fluid. In the next section we will examine these physical
mechanisms of transport. We will start with the basic information on diffusion.
8.2 Basics of Diffusion
8.2.1 Randomness of Diffusion
Consider a fluid in which a number of particles are suspended. If the particles
move around at random, transfer of kinetic energy of particles will occur. This
is a consequence of the interaction of neighbouring particles through the occurrence of occasional collisions. The direction taken by a particle after a collision
is random. If it hits another particle, it may return back to where it came
from. Therefore, subsequent positions of the particle are random, and as such
it can be treated as a random variable which satisfies some laws of probability.
Because of the randomness of this mechanism, Okubo (1980) defined diffusion
to be a basically irreversible phenomenon by which matter, particle groups, population, etc. spreads out within a given space according to individual random
motion. The diffusion is irreversible in the sense that the probability of returning to the original state decreases, when the number of diffusing particles
increases.
It should be noted that diffusion was defined as the dispersion of some property of the fluid, for example salinity, without any net mass transfer of the fluid
itself. In the oceans we distinguish diffusive transfer due to molecular motion
of the water particles (molecular diffusion) and due to turbulence of the water
mass (turbulent diffusion).