THE PRODUCTION O F MARINE PLANKTON
187
may be used by filter feeding animals, there is in relative terms a far
greater amount of dissolved organic material (of. Duursma, 1961). Putter
(1909, 1925) suggested that this dissolved malker might be used as food
by marine animals, but direct experiment indicates that even if metazoans can absorb such substances, the gain in energy w i l l be negligible.
But small protozoans might utilize dissolved organic material effectively.
Apart from relatively large planktonic protozoans such as Radiolaria
and Foraminifera, there may be a considerable population of very small
protozoans, particularly ciliates, in the seas. If such ciliates could feed
on the dissolved matter, they in turn could serve as the base of a food
chain leading to metazoan bathypelagic plankton.
Heterotrophic bacteria are certainly able to use dissolved organic
material, and in forming particulate living substance, they could serve
as food for Protozoa, or in turn, for Metazoa. Elven though Metazoa may
not filter bacteria directly, a food chain through larger Protozoa could
build up a useful food supply in deep water. The problem, however,
largely turns on the density of heterotrophio bacteria in deep-water
layers. Plating techniques and direct counts can give markedly variable
assessments of the bacterial population. Moreover it is almost impossible
to separate bacteria quantitatively by filtralion; a considerable proportion of the bacteria will pass even very fine filters and thus comparisons of the amount of particulate organic matter left on the filter
and of the so-called “dissolved” organic maliter passing through the
filter cannot give a correct assessment of the bacterial population. To a
large extent bacterial substance will be included in the “dissolved”
organic fraction. Earlier observations summarized by Zobell (1946)
suggest that deep water has relatively small populations of bacteria.
Kriss (1963), however, indicated that even in deep-water layers there
may be considerable populations of heterotrophic bacteria, especially
in tropical waters, which Kriss considered richer in organic material.
The recent investigations of Sorokin (1 964b) throw doubt on these rich
populations of heterotrophic bacteria in deep water. Even a limited
heterotrophic bacterial population, in providing particulate material
for higher trophic levels, would be of great advantage in deep water.
Parsons and Strickland (1962) have attempted an assessment, and have
assumed a value of 0.1 mg. per m3 as the standing crop of microheterotrophs. Assuming a growth rate similar to the phytoplankton, they
obtain a production equivalent to about 0.5 - 1% of that due to
photosynthetic organisms. But bacteria can exist through the whole
column of water, and even though their denriity is much reduced at
deeper levels, their contribution to the production of particulate living
matter may not be inconsiderable.
Another possible source of particulate food for the zooplankton in the
187
may be used by filter feeding animals, there is in relative terms a far
greater amount of dissolved organic material (of. Duursma, 1961). Putter
(1909, 1925) suggested that this dissolved malker might be used as food
by marine animals, but direct experiment indicates that even if metazoans can absorb such substances, the gain in energy w i l l be negligible.
But small protozoans might utilize dissolved organic material effectively.
Apart from relatively large planktonic protozoans such as Radiolaria
and Foraminifera, there may be a considerable population of very small
protozoans, particularly ciliates, in the seas. If such ciliates could feed
on the dissolved matter, they in turn could serve as the base of a food
chain leading to metazoan bathypelagic plankton.
Heterotrophic bacteria are certainly able to use dissolved organic
material, and in forming particulate living substance, they could serve
as food for Protozoa, or in turn, for Metazoa. Elven though Metazoa may
not filter bacteria directly, a food chain through larger Protozoa could
build up a useful food supply in deep water. The problem, however,
largely turns on the density of heterotrophio bacteria in deep-water
layers. Plating techniques and direct counts can give markedly variable
assessments of the bacterial population. Moreover it is almost impossible
to separate bacteria quantitatively by filtralion; a considerable proportion of the bacteria will pass even very fine filters and thus comparisons of the amount of particulate organic matter left on the filter
and of the so-called “dissolved” organic maliter passing through the
filter cannot give a correct assessment of the bacterial population. To a
large extent bacterial substance will be included in the “dissolved”
organic fraction. Earlier observations summarized by Zobell (1946)
suggest that deep water has relatively small populations of bacteria.
Kriss (1963), however, indicated that even in deep-water layers there
may be considerable populations of heterotrophic bacteria, especially
in tropical waters, which Kriss considered richer in organic material.
The recent investigations of Sorokin (1 964b) throw doubt on these rich
populations of heterotrophic bacteria in deep water. Even a limited
heterotrophic bacterial population, in providing particulate material
for higher trophic levels, would be of great advantage in deep water.
Parsons and Strickland (1962) have attempted an assessment, and have
assumed a value of 0.1 mg. per m3 as the standing crop of microheterotrophs. Assuming a growth rate similar to the phytoplankton, they
obtain a production equivalent to about 0.5 - 1% of that due to
photosynthetic organisms. But bacteria can exist through the whole
column of water, and even though their denriity is much reduced at
deeper levels, their contribution to the production of particulate living
matter may not be inconsiderable.
Another possible source of particulate food for the zooplankton in the