102
GORDON A. RILEY
value of <0.001 quoted by Hamilton et al. (1968) for deep water. The
daily uptake would be less than 10% of the carbon content of the
organisms (judging the latter by ATP analyses). This consumption
is small compared with metabolic rates that are commonly reported
for bacteria, but it is in accord with the low levels of bacterial activity
associated with long term laboratory experiments, as well as with the
experiments on heterotrophic utilization by natural populations.
Presumably both particulate and dissolved organic matter provide
carbon sources for these heterotrophs, and the latter is the larger
potential supply, although not all of it is likely to be assimilable.
Nevertheless, these rates have some bearing on the question of the
residence time of dissolved organic matter in the deep sea.
Analyses of dissolved organic matter by wet oxidation methods
have indicated a carbon content of 0.5 mg/litre more or less. Its
distribution in the North Atlantic (Duursma, 1960) shows some slight
indications of a relation with oxygen distribution ; for example, there
is a reduction in the region of the oxygen minimum layer. I n view of
the general similarity but less extreme variation, Riley et al. (1965)
suggested that the mean residence time is fairly long but not as long
as that of the water itself.
I n retrospect this conclusion seems too simple, and it should be
modified. Heterotrophic carbon utilization is directly related with
oxygen consumption, but this is not true of animal metabolism, which
is more likely to increase the dissolved organic content than to decrease
it. Some investigators have felt that mid-depth concentrations of
animals are very largely responsible for the development of the oxygen
minimum layer, and it is perhaps significant in this respect that the
decrease in dissolved organic matter is slight or lacking. For example,
Menzel and Ryther (1968a) found no mid-depth reduction in dissolved
organic carbon in the tropical South Atlantic where animal populations
are known to be unusually large (Jespersen, 1935).
As to the average age of the dissolved organic matter, the length
of time required to use 0.5 mg C/litre at a rate of 1-4 x
is 140-570 years. The larger value falls within the somewhat elastic
range of estimates that has been proposed as the age of the deep water
of the Atlantic Ocean. As a generalized statistic this range seems
reasonable ; however, the true situation is likely to be a rapid rate of
turnover of a small fraction of readily assimilable substances and a
much longer residence time for more refractory compounds.
Webb and Johannes (1967) and Johannes and Webb (1968) have
investigated the kinds and quantities of dissolved organic compounds
that are released by copepods and other invertebrates. The product
pg C.litre-
GORDON A. RILEY
value of <0.001 quoted by Hamilton et al. (1968) for deep water. The
daily uptake would be less than 10% of the carbon content of the
organisms (judging the latter by ATP analyses). This consumption
is small compared with metabolic rates that are commonly reported
for bacteria, but it is in accord with the low levels of bacterial activity
associated with long term laboratory experiments, as well as with the
experiments on heterotrophic utilization by natural populations.
Presumably both particulate and dissolved organic matter provide
carbon sources for these heterotrophs, and the latter is the larger
potential supply, although not all of it is likely to be assimilable.
Nevertheless, these rates have some bearing on the question of the
residence time of dissolved organic matter in the deep sea.
Analyses of dissolved organic matter by wet oxidation methods
have indicated a carbon content of 0.5 mg/litre more or less. Its
distribution in the North Atlantic (Duursma, 1960) shows some slight
indications of a relation with oxygen distribution ; for example, there
is a reduction in the region of the oxygen minimum layer. I n view of
the general similarity but less extreme variation, Riley et al. (1965)
suggested that the mean residence time is fairly long but not as long
as that of the water itself.
I n retrospect this conclusion seems too simple, and it should be
modified. Heterotrophic carbon utilization is directly related with
oxygen consumption, but this is not true of animal metabolism, which
is more likely to increase the dissolved organic content than to decrease
it. Some investigators have felt that mid-depth concentrations of
animals are very largely responsible for the development of the oxygen
minimum layer, and it is perhaps significant in this respect that the
decrease in dissolved organic matter is slight or lacking. For example,
Menzel and Ryther (1968a) found no mid-depth reduction in dissolved
organic carbon in the tropical South Atlantic where animal populations
are known to be unusually large (Jespersen, 1935).
As to the average age of the dissolved organic matter, the length
of time required to use 0.5 mg C/litre at a rate of 1-4 x
is 140-570 years. The larger value falls within the somewhat elastic
range of estimates that has been proposed as the age of the deep water
of the Atlantic Ocean. As a generalized statistic this range seems
reasonable ; however, the true situation is likely to be a rapid rate of
turnover of a small fraction of readily assimilable substances and a
much longer residence time for more refractory compounds.
Webb and Johannes (1967) and Johannes and Webb (1968) have
investigated the kinds and quantities of dissolved organic compounds
that are released by copepods and other invertebrates. The product
pg C.litre-