84
GORDON A. RILEY
ton distribution, but sampling for non-living organic matter has not
been conducted at sufficiently close intervals to reveal the fine detail
of vertical distribution. An earlier discussion of micro-distribution
cited the work of Paramonov et al. (1966) which demonstrated marked
vertical variations in transparency. These probably were associated
with thermal discontinuities. The decrease in organic carbon that has
been noted in the transition zone presumably is merely an indication
of a general trend, and the gradient is by no means a smooth one.
I n the surface layer aggregates constitute the bulk of the material.
Below the transition zone small particles and flakes are predominant.
This may be due to selective consumption of large particles in the
transition zone by animals, or it may be simply a matter of physical
sorting. Large particles with a rapid sinking rate will spread downward
through the layer more quickly than small ones, and their concentration per unit volume will be proportionately reduced.
The rate of decrease of organic matter in the transition zone cannot
be determined with any degree of generality, for few areas have been
sampled in sufficient detail to provide a representative spectrum of
vertical profiles. A sample analysis will be presented here, based on
data obtained by Riley et al. (1965) in the Sargasso Sea. The results
will be useful in a later discussion of biological problems in this area,
but it serves no other function except as an exposition of method.
I n this area the mean concentration of particulate organic carbon
was about 100pg/litre at 100 m and 30pgllitre at a depth of 900m.
These figures can be regarded as representing the upper and lower limits
of the transition zone. A first approximation to consumption is simply
the decrease in organic matter between the top and bottom of the
transition zone. This can be converted to a rate of consumption if the
residence time is known. Experiments suggest that the mean sinking
rate is of the order of 0.25 m/day, allowing for a decrease in sinking
rate with increasing viscosity, and this leads to an estimate of 0-022 pg
C.litre - 1.day- 1.
This kind of computation is applicable to a static situation ; however, in the present case we can assume that organic matter is imported
into the top of the transition zone faster than it is exported from the
bottom, so that total consumption is greater than is indicated by mere
decrease in concentration. If the concentration exists in a steady state,
the mean rate of decrease can be determined simply as the difference
between the two vertical fluxes. The flux F , directed positively downward, is given as
AC
F = d C - A - AX
GORDON A. RILEY
ton distribution, but sampling for non-living organic matter has not
been conducted at sufficiently close intervals to reveal the fine detail
of vertical distribution. An earlier discussion of micro-distribution
cited the work of Paramonov et al. (1966) which demonstrated marked
vertical variations in transparency. These probably were associated
with thermal discontinuities. The decrease in organic carbon that has
been noted in the transition zone presumably is merely an indication
of a general trend, and the gradient is by no means a smooth one.
I n the surface layer aggregates constitute the bulk of the material.
Below the transition zone small particles and flakes are predominant.
This may be due to selective consumption of large particles in the
transition zone by animals, or it may be simply a matter of physical
sorting. Large particles with a rapid sinking rate will spread downward
through the layer more quickly than small ones, and their concentration per unit volume will be proportionately reduced.
The rate of decrease of organic matter in the transition zone cannot
be determined with any degree of generality, for few areas have been
sampled in sufficient detail to provide a representative spectrum of
vertical profiles. A sample analysis will be presented here, based on
data obtained by Riley et al. (1965) in the Sargasso Sea. The results
will be useful in a later discussion of biological problems in this area,
but it serves no other function except as an exposition of method.
I n this area the mean concentration of particulate organic carbon
was about 100pg/litre at 100 m and 30pgllitre at a depth of 900m.
These figures can be regarded as representing the upper and lower limits
of the transition zone. A first approximation to consumption is simply
the decrease in organic matter between the top and bottom of the
transition zone. This can be converted to a rate of consumption if the
residence time is known. Experiments suggest that the mean sinking
rate is of the order of 0.25 m/day, allowing for a decrease in sinking
rate with increasing viscosity, and this leads to an estimate of 0-022 pg
C.litre - 1.day- 1.
This kind of computation is applicable to a static situation ; however, in the present case we can assume that organic matter is imported
into the top of the transition zone faster than it is exported from the
bottom, so that total consumption is greater than is indicated by mere
decrease in concentration. If the concentration exists in a steady state,
the mean rate of decrease can be determined simply as the difference
between the two vertical fluxes. The flux F , directed positively downward, is given as
AC
F = d C - A - AX