124
J . E. 0 . RAYMONT
advocates the use of fuming hydrochloric acid to reduce errors arising
from this source. Since “C02 fixation” can occur in darkness and can
occur with non-photosynthetic organisms, without any real gain in
carbon (Steemann Nielsen, 1960), Steemann Nielsen (1964a) suggests
the use of a “dark” bottle; the changes in I4C occurring in the “dark”
bottle can then be used as a correction factor for the normal “light”
bottle. Perhaps the chief criticism which may be levelled against the
14C technique is that some of the carbon assimilated during the course
of the experiment may be used for respiration purposes. The quantity
is not known and therefore it is not entirely clear whether the I4C technique measures gross photosynthesis, net photosynthesis, or some
measure of photosynthetic activity between these two extreme values.
Some workers such as Ryther believe that the value approxiniates to
the net photosynthetic rate; Steemann Nielsen believes that with
moderate production rates, respiration amounts to about 10% of photosynthesis, so that 14C experiment estimates about 90% of gross production. There is little doubt, however, that the difference between net
and gross production varies with the time of year at high latitudes, and
also geographically. Thus in oligotrophic tropical waters, net production
may be less than 60% of gross production, whereas in high latitudes,
during the height of summer, it may be a much higher proportion.
If rates of primary production are to be compared, they must be
related to a standard unit of concentration of phytoplankton present in
the sea. The measurement of the standing crop of plankton in the sea is
a very difficult problem (vide infra). Usually the amount of chlorophyll,
as the active photosynthetic pigment, has been used as an approximation of the standing crop of phytoplankton, but this is not constant,
varying from species to species and being dependent on the state of
nutrition of the individual cell. The chlorophyll content may also change
with time of day, light intensity, and with other factors. Chlorophyll
and degradation products of this pigment may also occur free in the
water and the length of life of such material is still uncertain; the
amount of chlorophyll found by the normal filtration and extraction
processes may thus be misleading. The precise method of extraction
may also affect the amount of chlorophyll; for instance, grinding of the
cells during extraction may lead to an increase in the amount of pigment extracted. Clear1y”the concentration of phytoplankton expressed
as the amount of organic carbon or of dry organic matter would be 8
much more accurate measurement, but no rapid and accurate method
is known for determining either of these quantities. Chlorophyll, therefore, is widely used as an index of the standing crop of phytoplankton.
Any technique which estimates photosynthetic activity may not
necessarily be measuring accurately the increase of phytoplankton
J . E. 0 . RAYMONT
advocates the use of fuming hydrochloric acid to reduce errors arising
from this source. Since “C02 fixation” can occur in darkness and can
occur with non-photosynthetic organisms, without any real gain in
carbon (Steemann Nielsen, 1960), Steemann Nielsen (1964a) suggests
the use of a “dark” bottle; the changes in I4C occurring in the “dark”
bottle can then be used as a correction factor for the normal “light”
bottle. Perhaps the chief criticism which may be levelled against the
14C technique is that some of the carbon assimilated during the course
of the experiment may be used for respiration purposes. The quantity
is not known and therefore it is not entirely clear whether the I4C technique measures gross photosynthesis, net photosynthesis, or some
measure of photosynthetic activity between these two extreme values.
Some workers such as Ryther believe that the value approxiniates to
the net photosynthetic rate; Steemann Nielsen believes that with
moderate production rates, respiration amounts to about 10% of photosynthesis, so that 14C experiment estimates about 90% of gross production. There is little doubt, however, that the difference between net
and gross production varies with the time of year at high latitudes, and
also geographically. Thus in oligotrophic tropical waters, net production
may be less than 60% of gross production, whereas in high latitudes,
during the height of summer, it may be a much higher proportion.
If rates of primary production are to be compared, they must be
related to a standard unit of concentration of phytoplankton present in
the sea. The measurement of the standing crop of plankton in the sea is
a very difficult problem (vide infra). Usually the amount of chlorophyll,
as the active photosynthetic pigment, has been used as an approximation of the standing crop of phytoplankton, but this is not constant,
varying from species to species and being dependent on the state of
nutrition of the individual cell. The chlorophyll content may also change
with time of day, light intensity, and with other factors. Chlorophyll
and degradation products of this pigment may also occur free in the
water and the length of life of such material is still uncertain; the
amount of chlorophyll found by the normal filtration and extraction
processes may thus be misleading. The precise method of extraction
may also affect the amount of chlorophyll; for instance, grinding of the
cells during extraction may lead to an increase in the amount of pigment extracted. Clear1y”the concentration of phytoplankton expressed
as the amount of organic carbon or of dry organic matter would be 8
much more accurate measurement, but no rapid and accurate method
is known for determining either of these quantities. Chlorophyll, therefore, is widely used as an index of the standing crop of phytoplankton.
Any technique which estimates photosynthetic activity may not
necessarily be measuring accurately the increase of phytoplankton