PARTICULATE ORGANIC MATTER IN SEA WATER
105
forms at low temperature as indicated in item (4) of the introductory
section, together with data from Leavitt (1938) on the quantity of zooplankton in deep water. The latter collections were made with a twometre stramin net. Recent work with finer mesh sizes indicates that
there is a significant fraction of small zooplankton that would not be
included in the present estimats. The true value for deep water consumption by animals probably is somewhat larger than the one given
here, and as indicated above, physical oceanographic computations may
also underrate total consumption. However, present information provides a basis for a general discussion of feeding problems in deep water.
One of the early attempts to express grazing rates of herbivorous
plankton in a simple form suitable for quantitative application was
that of Riley et al. (1949). The formula, restated in a slightly different
notation, was
where G is a grazing coefficient indicating ml/day of water swept clear
of particulate food by a quantity of zooplankton containing 1 mg C,
and R is the respiratory consumption in mg C/mg of zooplankton
carbon. This formula as expressed conformed with experimental
observations that grazing rates increased with temperature in much
the same way that respiratory rates do, and the constant in the equation
was more or less in the middle of the observed range of grazing coefficients. The equation was used with a fair degree of success in theoretical
computations of regional plankton abundance in various areas of the
western North Atlantic, including the Sargasso Sea.
Recent work has added a great deal of knowledge about grazing
and assimilation rates and respiratory requirements under various
conditions, but these investigations have not yet produced a definitive
statement of quantitative relationships, so that the formula given above
still serves as a rule of thumb in gauging the effectiveness of a given food
supply in supporting a crop of herbivorous zooplankton.
With regard to the surface layer of the Sargasso Sea, Menzel and
Ryther (1961) found that the respiratory requirement of the zooplankton in terms of carbon was about 12% of zooplankton dry weight so
that R = ca 0.06 mg C/day, and G = 1.4 litres. The general range of
particulate organic carbon as determined by cruise averages for various
seasons (Riley et al., 1965) was 46-168 pg C/litre. A reasonable annual
mean would be about 85 pg, leading to an estimated daily consumption
of 120 pg C/mg of zooplankton carbon. This is probably an adequate
food supply, on the average, for at least half of the particulate carbon
in the surface layer is likely to be assimilable. During the poorer part
of the year the supply would seem to be somewhat inadequate, but as
G = 235 R
105
forms at low temperature as indicated in item (4) of the introductory
section, together with data from Leavitt (1938) on the quantity of zooplankton in deep water. The latter collections were made with a twometre stramin net. Recent work with finer mesh sizes indicates that
there is a significant fraction of small zooplankton that would not be
included in the present estimats. The true value for deep water consumption by animals probably is somewhat larger than the one given
here, and as indicated above, physical oceanographic computations may
also underrate total consumption. However, present information provides a basis for a general discussion of feeding problems in deep water.
One of the early attempts to express grazing rates of herbivorous
plankton in a simple form suitable for quantitative application was
that of Riley et al. (1949). The formula, restated in a slightly different
notation, was
where G is a grazing coefficient indicating ml/day of water swept clear
of particulate food by a quantity of zooplankton containing 1 mg C,
and R is the respiratory consumption in mg C/mg of zooplankton
carbon. This formula as expressed conformed with experimental
observations that grazing rates increased with temperature in much
the same way that respiratory rates do, and the constant in the equation
was more or less in the middle of the observed range of grazing coefficients. The equation was used with a fair degree of success in theoretical
computations of regional plankton abundance in various areas of the
western North Atlantic, including the Sargasso Sea.
Recent work has added a great deal of knowledge about grazing
and assimilation rates and respiratory requirements under various
conditions, but these investigations have not yet produced a definitive
statement of quantitative relationships, so that the formula given above
still serves as a rule of thumb in gauging the effectiveness of a given food
supply in supporting a crop of herbivorous zooplankton.
With regard to the surface layer of the Sargasso Sea, Menzel and
Ryther (1961) found that the respiratory requirement of the zooplankton in terms of carbon was about 12% of zooplankton dry weight so
that R = ca 0.06 mg C/day, and G = 1.4 litres. The general range of
particulate organic carbon as determined by cruise averages for various
seasons (Riley et al., 1965) was 46-168 pg C/litre. A reasonable annual
mean would be about 85 pg, leading to an estimated daily consumption
of 120 pg C/mg of zooplankton carbon. This is probably an adequate
food supply, on the average, for at least half of the particulate carbon
in the surface layer is likely to be assimilable. During the poorer part
of the year the supply would seem to be somewhat inadequate, but as
G = 235 R