74
Light and Dark Bottle
Oxygen Technique
The light and dark bottle oxygen technique was the
first method used to measure primary production in
an aquatic ecosystem (Gaarder and Gran 1927) and
is still commonly used (e.g., Aristegui et al. 1996).
This method compares the rate of change in oxygen
in bottles held in the light (which includes the net
effects of production and respiration) with those
held in the dark (respiration only) to calculate the
primary production rate by difference. Water samples containing ambient plankton populations are
placed into sets of incubation bottles or other containers, with some bottles clear and others covered
so as to be opaque. The bottles are then incubated
at in situ temperatures either in situ in the water
column (hung from a buoy or pier) or in an incubator (either outdoors using sunlight, or with artificiallights). The more sophisticated incubators are
both temperature controlled and correct the color
of the light to approximate the spectral composition
at depth (Lohrenz et al. 1992). At the end of the
incubation period, the oxygen concentration is
measured in both the light (clear) bottles and the
dark (opaque) bottles and compared with the oxygen concentration in the water column at the time
the bottles were filled. The change in oxygen in the
light bottle over time is the combined result of GPP
and respiration by all of the organisms in the bottle
(phytoplankton, but also heterotrophic bacteria and
zooplankton). The decrease in oxygen in the dark
bottle is a measure of this rate of respiration. Thus,
GPP can be estimated by subtracting the rate of
change of oxygen in the dark bottle from the rate
of change of oxygen in the light bottle.
GPP = A0 2 (light) - A02 (dark) (5.2)
where A0 2 (light) = [0 2 ] final - [0 2 ] initial in
the light bottles, and A02 (dark) = [0 2 ] final
- [0 2 ] initial in the dark bottles.
Usually the oxygen concentration increases in
the light bottle and decreases in the dark bottle. In
low-light or in organic-rich waters, the oxygen concentration in the light bottle can decrease if the rate
of respiration exceeds GPP. Still, in this case, the
rate of oxygen decrease in the light bottle will be
less than the rate of decrease in the dark bottle, so
that when the dark bottle result (decrease in O2) is
Robert W. Howarth and Anthony F. Michaels
subtracted from the light bottle change (smaller decrease in O2), GPP will be a positive value.
The comparisons with the initial oxygen concentration allow an estimate of the rate of planktonic
community respiration and net planktonic community production on the time-scale of the incubation. Note, however, that these rates measured in
the dark bottle are measures of planktonic respiration and production only. They are not measures of
whole-ecosystem respiration and whole-system net
ecosystem production, since benthic respiration
(see Chapter 6) and respiration by large animals
such as fish (which are not sampled and placed in
the incubation bottles) are not included. Since the
dark bottle measures total planktonic respiration,
including respiration by heterotrophic organisms
and not just respiration by phytoplankton, addition
of this rate to the rate of change of oxygen in the
light bottle yields an estimate of GPP, and not NPP.
There is no way to estimate NPP from the light and
dark bottle technique, unless there is some independent way to estimate the rate of respiration by
phytoplankton alone. As discussed below, phytoplankton respiration is very difficult to estimate. As
also discussed below, rates of respiration in the dark
may not equal rates of respiration in the light, which
introduces some error into the light and dark bottle
method of assessing GPP.
The light and dark bottle method yields an estimate of GPP in units of rate of change of oxygen
per volume of water. Generally, rates are converted
to units of carbon by assuming some stoichiometric
relationship between O2 produced and organic C
fixed: the photosynthetic quotient. The equation for
production, above, suggests a stoichiometry of one
mole O 2 produced per mole of organic C fixed.
However, phytoplankton also incorporate nitrogen
into their biomass during primary production and
can produce organic carbon that is more reduced
than represented by the generic representation of
organic matter as CH20. Consequently, in general,
more moles of O 2 are produced in GPP than the
number of moles of organic C that are fixed. Generally, the assumed stoichiometric ratios of OziC for
GPP range from 1.0 to 1.3 when phytoplankton are
growing on ammonium and from 1.4 to 1.8 when
algae grow on nitrate (Raine, 1983, Platt and Harrison, 1986). The higher ratio for nitrate assimilation represents the extra photosynthate required to
reduce nitrate to amine before incorporation, some
Light and Dark Bottle
Oxygen Technique
The light and dark bottle oxygen technique was the
first method used to measure primary production in
an aquatic ecosystem (Gaarder and Gran 1927) and
is still commonly used (e.g., Aristegui et al. 1996).
This method compares the rate of change in oxygen
in bottles held in the light (which includes the net
effects of production and respiration) with those
held in the dark (respiration only) to calculate the
primary production rate by difference. Water samples containing ambient plankton populations are
placed into sets of incubation bottles or other containers, with some bottles clear and others covered
so as to be opaque. The bottles are then incubated
at in situ temperatures either in situ in the water
column (hung from a buoy or pier) or in an incubator (either outdoors using sunlight, or with artificiallights). The more sophisticated incubators are
both temperature controlled and correct the color
of the light to approximate the spectral composition
at depth (Lohrenz et al. 1992). At the end of the
incubation period, the oxygen concentration is
measured in both the light (clear) bottles and the
dark (opaque) bottles and compared with the oxygen concentration in the water column at the time
the bottles were filled. The change in oxygen in the
light bottle over time is the combined result of GPP
and respiration by all of the organisms in the bottle
(phytoplankton, but also heterotrophic bacteria and
zooplankton). The decrease in oxygen in the dark
bottle is a measure of this rate of respiration. Thus,
GPP can be estimated by subtracting the rate of
change of oxygen in the dark bottle from the rate
of change of oxygen in the light bottle.
GPP = A0 2 (light) - A02 (dark) (5.2)
where A0 2 (light) = [0 2 ] final - [0 2 ] initial in
the light bottles, and A02 (dark) = [0 2 ] final
- [0 2 ] initial in the dark bottles.
Usually the oxygen concentration increases in
the light bottle and decreases in the dark bottle. In
low-light or in organic-rich waters, the oxygen concentration in the light bottle can decrease if the rate
of respiration exceeds GPP. Still, in this case, the
rate of oxygen decrease in the light bottle will be
less than the rate of decrease in the dark bottle, so
that when the dark bottle result (decrease in O2) is
Robert W. Howarth and Anthony F. Michaels
subtracted from the light bottle change (smaller decrease in O2), GPP will be a positive value.
The comparisons with the initial oxygen concentration allow an estimate of the rate of planktonic
community respiration and net planktonic community production on the time-scale of the incubation. Note, however, that these rates measured in
the dark bottle are measures of planktonic respiration and production only. They are not measures of
whole-ecosystem respiration and whole-system net
ecosystem production, since benthic respiration
(see Chapter 6) and respiration by large animals
such as fish (which are not sampled and placed in
the incubation bottles) are not included. Since the
dark bottle measures total planktonic respiration,
including respiration by heterotrophic organisms
and not just respiration by phytoplankton, addition
of this rate to the rate of change of oxygen in the
light bottle yields an estimate of GPP, and not NPP.
There is no way to estimate NPP from the light and
dark bottle technique, unless there is some independent way to estimate the rate of respiration by
phytoplankton alone. As discussed below, phytoplankton respiration is very difficult to estimate. As
also discussed below, rates of respiration in the dark
may not equal rates of respiration in the light, which
introduces some error into the light and dark bottle
method of assessing GPP.
The light and dark bottle method yields an estimate of GPP in units of rate of change of oxygen
per volume of water. Generally, rates are converted
to units of carbon by assuming some stoichiometric
relationship between O2 produced and organic C
fixed: the photosynthetic quotient. The equation for
production, above, suggests a stoichiometry of one
mole O 2 produced per mole of organic C fixed.
However, phytoplankton also incorporate nitrogen
into their biomass during primary production and
can produce organic carbon that is more reduced
than represented by the generic representation of
organic matter as CH20. Consequently, in general,
more moles of O 2 are produced in GPP than the
number of moles of organic C that are fixed. Generally, the assumed stoichiometric ratios of OziC for
GPP range from 1.0 to 1.3 when phytoplankton are
growing on ammonium and from 1.4 to 1.8 when
algae grow on nitrate (Raine, 1983, Platt and Harrison, 1986). The higher ratio for nitrate assimilation represents the extra photosynthate required to
reduce nitrate to amine before incorporation, some