PARTICULATE ORGANIC MATTER I N SEA WATER
83
Particles smaller than 5 p in diameter will be taken less efficiently
or not at all by indiscriminate grazers, and they are not likely to be transferred downward very effectively either by ordinary sinking or by the
mediation of convective downdrafts. A thin layer of water spreading
laterally from these downdrafts will not remain in contact with the
thermocline very long and is unlikely to lose an appreciable quantity
of particles that have sinking rate of 0.1 m/day or less. The analogy
with phytoplankton dynamics is inapplicable, and small particles may
be subject to other processes, such as aggregation into larger particles,
which cannot be estimated quantitatively.
As indicated earlier, the amount of organic carbon in the various
size categories cannot be stated precisely, and this creates a major
difficulty in arriving at a quantitative treatment of this subject. Some
approximations will be presented in a later section as to likely rates of
transfer into deep water, and these will be compared with independent
estimates of rates of utilization.
(b) T h e transition zone. I n certain respects deep ocean conditions
begin at the top of the thermocline. Here convective flux ceases to be
a factor. Further transport into deep water will be essentially a sinking
phenomenon, somewhat augmented by vertical eddy diffusivity, for
there is usually a negative gradient in particle concentration. Bubbles
at this depth are most unlikely, and any formation of new particles
should proceed according to processes described in experiments on
unbubbled samples.
However, there is a zone underlying the mixed layer which differs
from the deep sea in several important respects. This transition zone
is best defined as that part of the vertical column in which there is a
marked decrease with depth in particle number and total organic
carbon. The maximum limit of this zone has been found at depths of
as little as 200 m (Menzel and Goering, 1966) or as much as 900 m in
some other sets of observations (Riley et aZ., 1965). Below this depth
variations tend to be smaller and of a random nature, or occasionally
there may be larger variations associated with particular water masses,
but there are no systematic negative gradients.
Particles enter the top of the transition zone a t a rate that is at
least equivalent to observed sinking rates of near-surface material and
may be somewhat larger, if there is any validity in the hypothesis
presented above with regard to speeds of convective transfer. If this
rate of transfer between the mixed layer and the transition zone is
greater than the natural sinking speed, there will be an increase in the
concentration of particulate matter in the upper part of the thermocline. This kind of distribution is well known in the case of phytoplank-
83
Particles smaller than 5 p in diameter will be taken less efficiently
or not at all by indiscriminate grazers, and they are not likely to be transferred downward very effectively either by ordinary sinking or by the
mediation of convective downdrafts. A thin layer of water spreading
laterally from these downdrafts will not remain in contact with the
thermocline very long and is unlikely to lose an appreciable quantity
of particles that have sinking rate of 0.1 m/day or less. The analogy
with phytoplankton dynamics is inapplicable, and small particles may
be subject to other processes, such as aggregation into larger particles,
which cannot be estimated quantitatively.
As indicated earlier, the amount of organic carbon in the various
size categories cannot be stated precisely, and this creates a major
difficulty in arriving at a quantitative treatment of this subject. Some
approximations will be presented in a later section as to likely rates of
transfer into deep water, and these will be compared with independent
estimates of rates of utilization.
(b) T h e transition zone. I n certain respects deep ocean conditions
begin at the top of the thermocline. Here convective flux ceases to be
a factor. Further transport into deep water will be essentially a sinking
phenomenon, somewhat augmented by vertical eddy diffusivity, for
there is usually a negative gradient in particle concentration. Bubbles
at this depth are most unlikely, and any formation of new particles
should proceed according to processes described in experiments on
unbubbled samples.
However, there is a zone underlying the mixed layer which differs
from the deep sea in several important respects. This transition zone
is best defined as that part of the vertical column in which there is a
marked decrease with depth in particle number and total organic
carbon. The maximum limit of this zone has been found at depths of
as little as 200 m (Menzel and Goering, 1966) or as much as 900 m in
some other sets of observations (Riley et aZ., 1965). Below this depth
variations tend to be smaller and of a random nature, or occasionally
there may be larger variations associated with particular water masses,
but there are no systematic negative gradients.
Particles enter the top of the transition zone a t a rate that is at
least equivalent to observed sinking rates of near-surface material and
may be somewhat larger, if there is any validity in the hypothesis
presented above with regard to speeds of convective transfer. If this
rate of transfer between the mixed layer and the transition zone is
greater than the natural sinking speed, there will be an increase in the
concentration of particulate matter in the upper part of the thermocline. This kind of distribution is well known in the case of phytoplank-