Technique of Measuring Phytoplankton Primary Production
57
surements of relative photosynthesis rates in equal samples of water incubated
in situ at different depths for half a light day. In this case, the photosynthesis
rates in each sample will be determined by the integrated illumination conditions at a corresponding depth during the light day. The K t coefficients are then
calculated as the ratios of 14C-radioactivity photosynthetically assimilated by
phytoplankton in the sample incubated at a given depth to that in the sample
incubated at the surface. Having procured the K t coefficients estimated at
depth intervals down to the lower limit of the euphotic zone, the corresponding K t curves are then constructed (Fig. 2.14).
Field estimations of K t coefficients may be accomplished either by in situ
overboard incubations, or by employing shipboard simulated in situ incubations on the ship's deck (see Sect. 2.4.1), if for some reason long-term (half
light-day) stations are excluded from the ship's schedule.
The first technique of Kt-coefficient estimation is preferable because in
this case the samples are exposed to the naturally attenuated light changing
its spectral composition with depth, which is hardly reproducible in the deck
incubator. When using direct in situ exposures it is also possible to select the
optimal number of depths necessary for adequate estimation of the light
curves K t , while in the simulated in situ incubator, their range is restricted by
the number of cells in it and by the selection of optical filters; additionally, the
neutral filters which are most often used in them cannot simulate the quality
of light at lower horizons of the euphotic zone.
Estimations of Kt-coefficients at the stations according to the direct in situ
technique are performed as follows. Whenever possible, the experiment should
be done at stations which begin 1-2 h before sunrise and end after midday, to
accomplish complete half light-day incubation. At the beginning of such a
station the profile of chlorophyll fluorescence and the Secchi disk transparency
are instantly measured. If it is still too dark to measure the Secchi disk transparency, its approximate depth is used as measured at previous daytime stations. If the chlorophyll profile is not available, the temperature profile can be
used instead. Then two 4-1 samples of water are taken in water bottles: one
from the subsurface layer, and the second from the layer of deep chlorophyll
maximum (or from the upper part of the thermocline zone). The samples are
mixed in a glass jar or aquarium. This mixed sample is distributed into the
series of experimental bottles (12-16), their quantity depending on the number
of exposure depths planned for the in situ experiment. The bottles (only the
light ones) are attached in pairs with special clamps to the holders, to be later
fixed to the cable. Then in a darkened part of the laboratory, they are charged
pair by pair with equal portions of working 14C-carbonate solution. After injection of the isotope solution, they are closed with an air bubble 1-2 cm 3 and
mixed, their stoppers are fixed with the aid of plastic film and rubber bands
to prevent their accidental opening in the sea. After that, they are inserted into
a box to be protected from light. When all the light bottles are thus charged
with the isotope, the box is transferred to a winch for immersion into the water
body to perform the overboard in situ incubation. At dawn, before sunrise, the
sets of two light bottles are taken out of the dark box. One by one they are
57
surements of relative photosynthesis rates in equal samples of water incubated
in situ at different depths for half a light day. In this case, the photosynthesis
rates in each sample will be determined by the integrated illumination conditions at a corresponding depth during the light day. The K t coefficients are then
calculated as the ratios of 14C-radioactivity photosynthetically assimilated by
phytoplankton in the sample incubated at a given depth to that in the sample
incubated at the surface. Having procured the K t coefficients estimated at
depth intervals down to the lower limit of the euphotic zone, the corresponding K t curves are then constructed (Fig. 2.14).
Field estimations of K t coefficients may be accomplished either by in situ
overboard incubations, or by employing shipboard simulated in situ incubations on the ship's deck (see Sect. 2.4.1), if for some reason long-term (half
light-day) stations are excluded from the ship's schedule.
The first technique of Kt-coefficient estimation is preferable because in
this case the samples are exposed to the naturally attenuated light changing
its spectral composition with depth, which is hardly reproducible in the deck
incubator. When using direct in situ exposures it is also possible to select the
optimal number of depths necessary for adequate estimation of the light
curves K t , while in the simulated in situ incubator, their range is restricted by
the number of cells in it and by the selection of optical filters; additionally, the
neutral filters which are most often used in them cannot simulate the quality
of light at lower horizons of the euphotic zone.
Estimations of Kt-coefficients at the stations according to the direct in situ
technique are performed as follows. Whenever possible, the experiment should
be done at stations which begin 1-2 h before sunrise and end after midday, to
accomplish complete half light-day incubation. At the beginning of such a
station the profile of chlorophyll fluorescence and the Secchi disk transparency
are instantly measured. If it is still too dark to measure the Secchi disk transparency, its approximate depth is used as measured at previous daytime stations. If the chlorophyll profile is not available, the temperature profile can be
used instead. Then two 4-1 samples of water are taken in water bottles: one
from the subsurface layer, and the second from the layer of deep chlorophyll
maximum (or from the upper part of the thermocline zone). The samples are
mixed in a glass jar or aquarium. This mixed sample is distributed into the
series of experimental bottles (12-16), their quantity depending on the number
of exposure depths planned for the in situ experiment. The bottles (only the
light ones) are attached in pairs with special clamps to the holders, to be later
fixed to the cable. Then in a darkened part of the laboratory, they are charged
pair by pair with equal portions of working 14C-carbonate solution. After injection of the isotope solution, they are closed with an air bubble 1-2 cm 3 and
mixed, their stoppers are fixed with the aid of plastic film and rubber bands
to prevent their accidental opening in the sea. After that, they are inserted into
a box to be protected from light. When all the light bottles are thus charged
with the isotope, the box is transferred to a winch for immersion into the water
body to perform the overboard in situ incubation. At dawn, before sunrise, the
sets of two light bottles are taken out of the dark box. One by one they are