Technique of Measuring Phytoplankton Primary Production
59
The alternative simulated in situ technique for the estimation of K tcoefficients is based on the use of the bath-incubator with a series of four to
six cells covered with a range of optical filters providing fractionated light from
100% down to 1-2% of PARS (see above, Sect. 2.3.3.1). The incubator must
be installed on deck in an open place without shade. Exposure should proceed
under natural illumination at a temperature close to that in situ. Incubation
should start either at dawn or at midday and proceed for half of the light day.
Other operations are as described above for direct in situ experiments.
As was mentioned above, and as can be seen from Fig. 2.14, the Kt-curves
are regular in character, being determined basically by one factor - underwater illumination (Talling 1960; Vollenweider 1974). Their forms can be easily
formalized (Rodhe 1958). In the temperate zone in summer during bright days,
as well as during the whole year in the tropics, the photosynthetic function of
phytoplankton is inhibited by excess of light (Steemann Nielsen 1952; Harris
1980). Thus the maxima on the light curves K t in this case are situated at some
depth below the surface, where the illumination is some 30 to 40% of PARS
(e.g., -15-20 000 Ix). On dull days in temperate basins, and most often in high
latitude basins, the maxima on K t curves are situated at the surface (Fig. 2.14).
Because of the predictability of the character of Kt-curves, they can be adopted
even from the literature without significant errors in use in a given water basin
at a given season and at a given water transparency. This way of acquiring K t -
curves for the Ks-Cps calculations is also quite acceptable, especially if the
primary production measurements are conducted repeatedly in the same kinds
of basins, which is often the case.
Among the weak points of these experimental techniques for K tcoefficient measurements using an equal water sample, a probable light adaptation of phytoplankton populations which inhabit deeper, less illuminated
layers of the euphotic zone can be mentioned (Sorokin 1964a; Falkovski 1980;
Lewis et al. 1984); but it is largely avoided by the use of mixed water samples
for those experiments which contain phytoplankton from variously illuminated habitats.
The Kp-coefficients characterize the dependence of the photosynthesis
rate on the vertical profile upon the distribution of the producing phytoplankton popUlation in the water column. They are estimated experimentally
at the stations by measuring the photosynthetic uptake of l4C-carbonate by
phytoplankton in samples taken on the vertical profile and incubated for 23 h under even temperate illumination. In this case, the 14C uptake rate in algae
will be a factor of the relative amount of photosynthetically active phytoplankton present in the samples. Practically, at a regular station, first the
profiles of chlorophyll fluorescence and water temperature should be obtained
with the aid of a Niel Brown or other STD probe. After comparing these
profiles with those from previous stations, at which the Kp-coefficients have
also been measured, it must be decided whether it is reasonable to repeat the
Kp measurements at a given station. If this decision is positive, Secchi disk or
photometric measurements of the depth of the euphotic zone are performed.
59
The alternative simulated in situ technique for the estimation of K tcoefficients is based on the use of the bath-incubator with a series of four to
six cells covered with a range of optical filters providing fractionated light from
100% down to 1-2% of PARS (see above, Sect. 2.3.3.1). The incubator must
be installed on deck in an open place without shade. Exposure should proceed
under natural illumination at a temperature close to that in situ. Incubation
should start either at dawn or at midday and proceed for half of the light day.
Other operations are as described above for direct in situ experiments.
As was mentioned above, and as can be seen from Fig. 2.14, the Kt-curves
are regular in character, being determined basically by one factor - underwater illumination (Talling 1960; Vollenweider 1974). Their forms can be easily
formalized (Rodhe 1958). In the temperate zone in summer during bright days,
as well as during the whole year in the tropics, the photosynthetic function of
phytoplankton is inhibited by excess of light (Steemann Nielsen 1952; Harris
1980). Thus the maxima on the light curves K t in this case are situated at some
depth below the surface, where the illumination is some 30 to 40% of PARS
(e.g., -15-20 000 Ix). On dull days in temperate basins, and most often in high
latitude basins, the maxima on K t curves are situated at the surface (Fig. 2.14).
Because of the predictability of the character of Kt-curves, they can be adopted
even from the literature without significant errors in use in a given water basin
at a given season and at a given water transparency. This way of acquiring K t -
curves for the Ks-Cps calculations is also quite acceptable, especially if the
primary production measurements are conducted repeatedly in the same kinds
of basins, which is often the case.
Among the weak points of these experimental techniques for K tcoefficient measurements using an equal water sample, a probable light adaptation of phytoplankton populations which inhabit deeper, less illuminated
layers of the euphotic zone can be mentioned (Sorokin 1964a; Falkovski 1980;
Lewis et al. 1984); but it is largely avoided by the use of mixed water samples
for those experiments which contain phytoplankton from variously illuminated habitats.
The Kp-coefficients characterize the dependence of the photosynthesis
rate on the vertical profile upon the distribution of the producing phytoplankton popUlation in the water column. They are estimated experimentally
at the stations by measuring the photosynthetic uptake of l4C-carbonate by
phytoplankton in samples taken on the vertical profile and incubated for 23 h under even temperate illumination. In this case, the 14C uptake rate in algae
will be a factor of the relative amount of photosynthetically active phytoplankton present in the samples. Practically, at a regular station, first the
profiles of chlorophyll fluorescence and water temperature should be obtained
with the aid of a Niel Brown or other STD probe. After comparing these
profiles with those from previous stations, at which the Kp-coefficients have
also been measured, it must be decided whether it is reasonable to repeat the
Kp measurements at a given station. If this decision is positive, Secchi disk or
photometric measurements of the depth of the euphotic zone are performed.