THE PRODUCTION O F MARINE PLANKTON
121
is added to the amount of oxygen produced in a “light” bottle, the
total amount of oxygen is equivalent to (P - R,). As R, is normally
very small in short-term experiments, this total amount of oxygen change
is practically a measure of gross photosynthesis. If longer term experiments are employed R, may become considerably larger and the estimate of photosynthesis then approaches a net value again.
One disadvantage of the oxygen bottle method is that the temperatures in the “dark” bottle series may be somewhat different, respiration
proceeding at different rates in the “light” and “dark” bottles. Furthermore, fat may be synthesized to some extent rather than carbohydrates,
particularly by diatom cultures, which will af Fect the amount of oxygen
produced. Growth, especially of marine bactwia, tends to be increased
by surface, and therefore there may be an almormally high density of
bacteria in the enclosed phytoplankton cultures. In any event, probably
no population of phytoplankton can live perfectly normally enclosed in
a relatively small volume of virtually stationary sea water. But, above
all, the most serious disadvantage of the oxygen bottle experiment lies
in its comparative insensitivity, especially with low concentrations of
phytoplankton as in oligotrophic areas of the ;sea; under such conditions
the amount of oxygen produced over a moderately short-term experiment may be quite small (cf. Currie, 1959, 1962). The technique is also
unsuitable for productivity experiments in very rich, highly polluted,
inshore waters, especially with high bacterial populations.
Other methods have been tried for estimating the rate of primary
production in the marine environment. Changes in the quantities of
nutrients such as nitrate and phosphate, which are essential for the
upbuilding of cell tissue, have been used as a measure of plant growth.
One of the main difficulties is that unless a steady state in water movements can be assumed, any large scale lateral exchange of water
through the area may introduce major errora in calculations. Vertical
exchange of water may also be important. In a deep oceanic area the
extent of vertical mixing may be assessed so that any nutrient brought
into the photosynthetic zone from deeper water may be included in the
calculations. In shallow water it may be possible to integrate the whole
changes in nutrient from surface to bottom, but the leaching of nutrients
from the bottom deposits can be significant. Perhaps the greatest difficulty with using nutrient changes as a measure of production, however,
turns on the problem of regeneration. The regeneration of inorganic
nutrient materials from organic matter (or mineralization) can be rapid
under certain conditions. Investmigations such as those of Harris (1959)
suggest that at least a portion of the phosphorus and some of the nitrogen, particularly as ammonia, may be fairly rapidly recycled. Ketchum
(1962) suggests that phosphorus may be cycled some 6 or 7 times a year
121
is added to the amount of oxygen produced in a “light” bottle, the
total amount of oxygen is equivalent to (P - R,). As R, is normally
very small in short-term experiments, this total amount of oxygen change
is practically a measure of gross photosynthesis. If longer term experiments are employed R, may become considerably larger and the estimate of photosynthesis then approaches a net value again.
One disadvantage of the oxygen bottle method is that the temperatures in the “dark” bottle series may be somewhat different, respiration
proceeding at different rates in the “light” and “dark” bottles. Furthermore, fat may be synthesized to some extent rather than carbohydrates,
particularly by diatom cultures, which will af Fect the amount of oxygen
produced. Growth, especially of marine bactwia, tends to be increased
by surface, and therefore there may be an almormally high density of
bacteria in the enclosed phytoplankton cultures. In any event, probably
no population of phytoplankton can live perfectly normally enclosed in
a relatively small volume of virtually stationary sea water. But, above
all, the most serious disadvantage of the oxygen bottle experiment lies
in its comparative insensitivity, especially with low concentrations of
phytoplankton as in oligotrophic areas of the ;sea; under such conditions
the amount of oxygen produced over a moderately short-term experiment may be quite small (cf. Currie, 1959, 1962). The technique is also
unsuitable for productivity experiments in very rich, highly polluted,
inshore waters, especially with high bacterial populations.
Other methods have been tried for estimating the rate of primary
production in the marine environment. Changes in the quantities of
nutrients such as nitrate and phosphate, which are essential for the
upbuilding of cell tissue, have been used as a measure of plant growth.
One of the main difficulties is that unless a steady state in water movements can be assumed, any large scale lateral exchange of water
through the area may introduce major errora in calculations. Vertical
exchange of water may also be important. In a deep oceanic area the
extent of vertical mixing may be assessed so that any nutrient brought
into the photosynthetic zone from deeper water may be included in the
calculations. In shallow water it may be possible to integrate the whole
changes in nutrient from surface to bottom, but the leaching of nutrients
from the bottom deposits can be significant. Perhaps the greatest difficulty with using nutrient changes as a measure of production, however,
turns on the problem of regeneration. The regeneration of inorganic
nutrient materials from organic matter (or mineralization) can be rapid
under certain conditions. Investmigations such as those of Harris (1959)
suggest that at least a portion of the phosphorus and some of the nitrogen, particularly as ammonia, may be fairly rapidly recycled. Ketchum
(1962) suggests that phosphorus may be cycled some 6 or 7 times a year