268
Use of Radioisotopes to Study Biogeochemical Cycling of Elements
Po - Pi' Two to three subsamples of 50-100 ml each are extracted from the
other bottle charged with the isotope to be filtered as described above for the
estimation of At. The same is done with the control sample. Then the value of
E is calculated as above. An example of experimental measurements of Ah P,
and E values is given in Fig. 5.18.
Recently, several alternative approaches have been develuped for experimental measuring in situ rates of phosphorus regeneration by planktonic
communities. They are based on the estimation of isotopic diluition (Harrison
1993), antibiotic treatment, blocking the reassimilation of regenerated dissolved phosphorous (Dodds et al. 1991), and also on its blocking by an addition of the excess of unlabeled phosphate (Hudson and Taylor 1996). All these
approaches include critical assumptions which appear unacceptable. The first
of them assumes that the label thus incorporated is not recycled during shortterm incubation, which is unfounded. The antibiotic treatment used in the
second technique often proves to be noneffective with natural microplankton.
A weak point of the third approach is too long incubation under enclosure
conditions (over 24h), during which the microplankton community, as a rule,
drastically changes. This technique relies on dissolved 33p fraction measured
during experiment as representing only P04-p. However, it should inevitably
also contain dissolved organic phosphorous, which is excreted or exudated, but
not simply regenerated.
5.7.5 Consumption of P04-P from the Water Column by the Elements
of Bottom Biotopes
Inorganic phosphate is consumed by numerous elements of benthic biotopes,
which include autotrophic plant components like benthic macrophytes, sea
grasses, periphytonic turfs, microphytobenthos of soft bottom sediments, and
is also consumed by animals possessing algal endosymbionts such as sponges,
hydroids, corals, and foraminiferans. A definite amount of P04-P is consumed
also by heterotrophic bacteria, which inhabit the periphyton and the surface
layer of bottom sediments (Pomeroy et al. 1974; Propp et al. 1983; Sorokin
1992). The rate of its consumption by the elements of bottom biotopes may
be measured only with the aid of radiolabeled phosphate, because in this case
the consumption flows of P04-P cannot be camouflaged by the reciprocal
excretion which flows from the bottom up to the water column. Therefore, the
numerous attempts to measure phosphate consumption and release by simply
recording the changes in ambient concentration over the benthic objects have
been unsuccessful: this concentration often remained relatively stable within
days of enclosure observations even over the benthic biotopes of coral reefs
(Propp et al. 1983), although the above-mentioned reciprocal flows of P04-P
can be very fast there: 1O-20mgPm- 1 day-l by diurnal fluctuations of POrP
ambient concentrations of only 0.5-1 mg m- I (Table 5.2).
Use of Radioisotopes to Study Biogeochemical Cycling of Elements
Po - Pi' Two to three subsamples of 50-100 ml each are extracted from the
other bottle charged with the isotope to be filtered as described above for the
estimation of At. The same is done with the control sample. Then the value of
E is calculated as above. An example of experimental measurements of Ah P,
and E values is given in Fig. 5.18.
Recently, several alternative approaches have been develuped for experimental measuring in situ rates of phosphorus regeneration by planktonic
communities. They are based on the estimation of isotopic diluition (Harrison
1993), antibiotic treatment, blocking the reassimilation of regenerated dissolved phosphorous (Dodds et al. 1991), and also on its blocking by an addition of the excess of unlabeled phosphate (Hudson and Taylor 1996). All these
approaches include critical assumptions which appear unacceptable. The first
of them assumes that the label thus incorporated is not recycled during shortterm incubation, which is unfounded. The antibiotic treatment used in the
second technique often proves to be noneffective with natural microplankton.
A weak point of the third approach is too long incubation under enclosure
conditions (over 24h), during which the microplankton community, as a rule,
drastically changes. This technique relies on dissolved 33p fraction measured
during experiment as representing only P04-p. However, it should inevitably
also contain dissolved organic phosphorous, which is excreted or exudated, but
not simply regenerated.
5.7.5 Consumption of P04-P from the Water Column by the Elements
of Bottom Biotopes
Inorganic phosphate is consumed by numerous elements of benthic biotopes,
which include autotrophic plant components like benthic macrophytes, sea
grasses, periphytonic turfs, microphytobenthos of soft bottom sediments, and
is also consumed by animals possessing algal endosymbionts such as sponges,
hydroids, corals, and foraminiferans. A definite amount of P04-P is consumed
also by heterotrophic bacteria, which inhabit the periphyton and the surface
layer of bottom sediments (Pomeroy et al. 1974; Propp et al. 1983; Sorokin
1992). The rate of its consumption by the elements of bottom biotopes may
be measured only with the aid of radiolabeled phosphate, because in this case
the consumption flows of P04-P cannot be camouflaged by the reciprocal
excretion which flows from the bottom up to the water column. Therefore, the
numerous attempts to measure phosphate consumption and release by simply
recording the changes in ambient concentration over the benthic objects have
been unsuccessful: this concentration often remained relatively stable within
days of enclosure observations even over the benthic biotopes of coral reefs
(Propp et al. 1983), although the above-mentioned reciprocal flows of P04-P
can be very fast there: 1O-20mgPm- 1 day-l by diurnal fluctuations of POrP
ambient concentrations of only 0.5-1 mg m- I (Table 5.2).