210
Exercise 14
extreme, under "pea soup" conditions of very high algal biomass, an hour of
incubation could result in supersaturated conditions of dissolved oxygen within the
light bottles. Then, upon opening the bottles for chemical fixation, bubbles of oxygen
likely would be lost and the productivity underestimated. Under conditions of
moderate algal productivity, an incubation time of 2 to 4 h should be adequate. The
incubation is terminated by immediate chemical fixation of dissolved oxygen as the
bottles are retrieved.
All traces of iodine, acid, and nutrients must be removed from the bottles prior to
reuse. Bottles and stoppers must be cleaned thoroughly with acid, scrubbing, and
copious rinsing with redistilled water.
Calculations
During the incubation of the samples of the phytoplankton, the initial concentration
of dissolved oxygen ( = initial bottle, I B) at given depth would be expected to decrease
to a lower concentration (= dark bottle value, DB) in the opaque bottles from
respiration. Conversely, the initial concentration (lB) would change (usually it would
increase) to another concentration ( = light bottle value, LB) in the transparent bottles
as a result of the difference between photosynthetic production and respiratory
consumption of oxygen.
When the influences on oxygen concentrations, other than algal photosynthesis
and respiration, are small and can be neglected (see discussion below),
IB - DB = respiratory activity per unit volume per time interval
LB - I B = net photosynthetic activity per unit volume per time interval
(LB - IB) + (lB - DB) = gross photosynthetic activity
The initial bottle values cancel each other in this equation, and it is possible to
estimate gross photosynthetic activity directly from: LB - DB. The method then
estimates: Gross photosynthesis = net O 2 evolved + O 2 used in respiration.
Gross photosynthesis refers to the gross true synthesis of organic matter resulting
from exposure to light. Net photosynthesis refers to the net formation of organic
matter after losses from respiration, extracellular release of soluble organic matter,
and other losses (e.g., death) that occur from the metabolic activities of algae
simultaneously with the photosynthetic processes.
It may be desirable to express the changes in oxygen concentration in terms of
carbon, since carbon is both the initial material and the end product of synthesis
and of respiration. The photosynthetic quotient (PQ) and respiratory quotient (RQ)
are dimensionless numbers indicating the relative amounts of oxygen and carbon
involved in the processes of photosynthesis and respiration:
+ ~02 molecules of oxygen liberated during photosynthesis
PQ = -- - = - ---- - --- ------ - ----- - -- ~C02
molecules of CO2 assimilated
+ ~C02 molecules of CO2 liberated during respiration
RQ=- - = - -------- - - - - - - - -
- - - - -
- ~02
molecules of oxygen consumed
The PQ and RQ values vary greatly among different algae, their chemical composition,
and environmental conditions [cf., discussion of Strickland (1960)]. With "normal"
algal populations exposed to moderate light intensities, a PQ of 1.2 and an RQ of
1.0 are typical. To convert from mass of oxygen to mass of carbon, the values of
Exercise 14
extreme, under "pea soup" conditions of very high algal biomass, an hour of
incubation could result in supersaturated conditions of dissolved oxygen within the
light bottles. Then, upon opening the bottles for chemical fixation, bubbles of oxygen
likely would be lost and the productivity underestimated. Under conditions of
moderate algal productivity, an incubation time of 2 to 4 h should be adequate. The
incubation is terminated by immediate chemical fixation of dissolved oxygen as the
bottles are retrieved.
All traces of iodine, acid, and nutrients must be removed from the bottles prior to
reuse. Bottles and stoppers must be cleaned thoroughly with acid, scrubbing, and
copious rinsing with redistilled water.
Calculations
During the incubation of the samples of the phytoplankton, the initial concentration
of dissolved oxygen ( = initial bottle, I B) at given depth would be expected to decrease
to a lower concentration (= dark bottle value, DB) in the opaque bottles from
respiration. Conversely, the initial concentration (lB) would change (usually it would
increase) to another concentration ( = light bottle value, LB) in the transparent bottles
as a result of the difference between photosynthetic production and respiratory
consumption of oxygen.
When the influences on oxygen concentrations, other than algal photosynthesis
and respiration, are small and can be neglected (see discussion below),
IB - DB = respiratory activity per unit volume per time interval
LB - I B = net photosynthetic activity per unit volume per time interval
(LB - IB) + (lB - DB) = gross photosynthetic activity
The initial bottle values cancel each other in this equation, and it is possible to
estimate gross photosynthetic activity directly from: LB - DB. The method then
estimates: Gross photosynthesis = net O 2 evolved + O 2 used in respiration.
Gross photosynthesis refers to the gross true synthesis of organic matter resulting
from exposure to light. Net photosynthesis refers to the net formation of organic
matter after losses from respiration, extracellular release of soluble organic matter,
and other losses (e.g., death) that occur from the metabolic activities of algae
simultaneously with the photosynthetic processes.
It may be desirable to express the changes in oxygen concentration in terms of
carbon, since carbon is both the initial material and the end product of synthesis
and of respiration. The photosynthetic quotient (PQ) and respiratory quotient (RQ)
are dimensionless numbers indicating the relative amounts of oxygen and carbon
involved in the processes of photosynthesis and respiration:
+ ~02 molecules of oxygen liberated during photosynthesis
PQ = -- - = - ---- - --- ------ - ----- - -- ~C02
molecules of CO2 assimilated
+ ~C02 molecules of CO2 liberated during respiration
RQ=- - = - -------- - - - - - - - -
- - - - -
- ~02
molecules of oxygen consumed
The PQ and RQ values vary greatly among different algae, their chemical composition,
and environmental conditions [cf., discussion of Strickland (1960)]. With "normal"
algal populations exposed to moderate light intensities, a PQ of 1.2 and an RQ of
1.0 are typical. To convert from mass of oxygen to mass of carbon, the values of