EXERCISE 14
Primary Productivity
of Phytoplankton
Although appreciable quantItIes of organic
matter synthesized by terrestrial plants within
the drainage basin can be transported to freshwater ecosystems in either dissolved or particulate forms (allochthonous primary productivity), much of the organic matter of lakes is
produced within the lake by phytoplanktonic
algae, by littoral macrophytic vegetation, and by
sessile algae (autochthonous primary productivity). In situ rates of photosynthesis by
phytoplankton are the subject of this exercise;
those of other plant forms will be treated
separately in Exercise 22.
It is a distinct advantage to measure rates of
metabolism directly in situ since extrapolation
of laboratory results to natural conditions is
usually difficult. When the rates of synthesis of
organic matter and changes in primary production can be measured over time, efforts can
be directed to the experimental evaluation of
causal mechanisms regulating the synthesis,
utilization, and loss of the organic matter.
The complex biochemical reactions of photosynthesis can be summarized by the general
redox reaction:
6C02 + 12H20 ~ C6H 1206 + 60 2 + 6H 2 0
pigment
receptor
Techniques for measuring rates of photosynthesis
are based on the stoichiometry of this reaction,
e.g., rates of oxygen production, rates of utilization of CO2 or water, or changes in the concentration of organic matter. The objective is to
modify the natural community as little as possible during assays of in situ rates of photosynthesis. Variations in the metabolic state of the
phytoplankton can be large; measurements of
primary productivity may reflect the rates of
certain species rather accurately, but for other
species rates are estimated poorly. Nonetheless,
with reasonable precautions and guarded
interpretations, the methods provide useful
estimates of in situ rates of phytoplanktonic
photosynthesis.
The approach used most commonly is to
make measurements on samples of a natural
community that have been isolated in some type
of container. Changes in oxygen evolution or
CO2 uptake then are measured after the samples
have been incubated either in situ or under conditions of light and temperature simulated for
the depth from which the samples were collected.
Although in situ incubation is most desirable, it
is not always practical when studying large lakes
where simultaneous measurements at distant
stations are necessary. Shipboard incubations
and interpretations have been treated elsewhere
[e.g., Strickland, (1960), Saunders et al. (1962),
Fee (1969, 1971, 1973), Parsons et al. (1984)] and
will not be discussed here.
Alternatively, measurements of in situ photosynthesis can be estimated indirectly in the
natural environment from changes in environmental parameters affected by photosynthesis,
such as changes in CO 2 and oxygen concentra207
Primary Productivity
of Phytoplankton
Although appreciable quantItIes of organic
matter synthesized by terrestrial plants within
the drainage basin can be transported to freshwater ecosystems in either dissolved or particulate forms (allochthonous primary productivity), much of the organic matter of lakes is
produced within the lake by phytoplanktonic
algae, by littoral macrophytic vegetation, and by
sessile algae (autochthonous primary productivity). In situ rates of photosynthesis by
phytoplankton are the subject of this exercise;
those of other plant forms will be treated
separately in Exercise 22.
It is a distinct advantage to measure rates of
metabolism directly in situ since extrapolation
of laboratory results to natural conditions is
usually difficult. When the rates of synthesis of
organic matter and changes in primary production can be measured over time, efforts can
be directed to the experimental evaluation of
causal mechanisms regulating the synthesis,
utilization, and loss of the organic matter.
The complex biochemical reactions of photosynthesis can be summarized by the general
redox reaction:
6C02 + 12H20 ~ C6H 1206 + 60 2 + 6H 2 0
pigment
receptor
Techniques for measuring rates of photosynthesis
are based on the stoichiometry of this reaction,
e.g., rates of oxygen production, rates of utilization of CO2 or water, or changes in the concentration of organic matter. The objective is to
modify the natural community as little as possible during assays of in situ rates of photosynthesis. Variations in the metabolic state of the
phytoplankton can be large; measurements of
primary productivity may reflect the rates of
certain species rather accurately, but for other
species rates are estimated poorly. Nonetheless,
with reasonable precautions and guarded
interpretations, the methods provide useful
estimates of in situ rates of phytoplanktonic
photosynthesis.
The approach used most commonly is to
make measurements on samples of a natural
community that have been isolated in some type
of container. Changes in oxygen evolution or
CO2 uptake then are measured after the samples
have been incubated either in situ or under conditions of light and temperature simulated for
the depth from which the samples were collected.
Although in situ incubation is most desirable, it
is not always practical when studying large lakes
where simultaneous measurements at distant
stations are necessary. Shipboard incubations
and interpretations have been treated elsewhere
[e.g., Strickland, (1960), Saunders et al. (1962),
Fee (1969, 1971, 1973), Parsons et al. (1984)] and
will not be discussed here.
Alternatively, measurements of in situ photosynthesis can be estimated indirectly in the
natural environment from changes in environmental parameters affected by photosynthesis,
such as changes in CO 2 and oxygen concentra207