Nutrients
57
autotrophic com mum tIes of reef flats was recorded in analogous studies
accomplished later on other reefs (Hanson and Gundersen 1976; Kimmerer
and Walsh 1981; Hatcher and Hatcher 1981; Crossland and Barnes 1983;
Johannes et al. 1983b). In most cases a weak release of inorganic N and of
organic Nand P, as well as a slight uptake of inorganic Poccurred
(Atkinson 1983, 1987a; Atkinson and Smith 1987;). But in the diurnal
balance even the changes of P04-P content were found to be near zero.
Moreover, in the biotopes of lagoonal patch reefs dominated by living corals
the net release of phosphorus also most often was recorded with a diurnal
balance (Andrews and Muller 1983). Thus all attempts not only to quantify,
but even establish the direction of the flows of nutrients between the water
masses passing over the reef and its bottom communities appeared to be in
general unsuccessful (Atkinson 1983; Hatcher 1985). But at least the results
thus obtained confirmed statements about the importance of N2-fixation as
one of the main sources of nitrogen supply in coral reef communities (Webb
et al. 1975).
Additional attempts to gain insight into the problem-of nutrient flows
were made in experiments with plastic transparent enclosures secured to the
bottom and covered with sand or rubble. In these the changes of nutrients in
enclosed volumes of water were also measured in diurnal cycles (Henderson
1981; Propp 1981; Propp et al. 1983; Harrison 1983). The results were again
indefinite. In most cases during the day and also at night the concentration
of inorganic Nand P in water inside an enclosure did not change at all or it
slightly increased, and not only in the concentration of N but even of
inorganic P, instead of their expected consumption by autotrophic
communities of periphyton and microphytobenthos.
The faults in the above approaches seem to be quite obvious. The bottom
communities of reef flat as well as communities of rubble and sand comprise
together the autotrophic and the heterotrophic components. So they should
simultaneously consume nutrients and release them. Therefore, the abovelisted measurements of the change in their ambient concentration in oceanic
waters passing over the reef or in the in situ enclosures reflect only the
resulting equilibrium values of these two reciprocal flows, i.e., between the
uptake of nutrients by the autotrophic part of the community or the cellular
systems of symbiotic animals, and their release by the heterotrophic ones.
But these measurements cannot reflect the size of these flows per se. These
flows, the same as is the case with corals, could be measured only by
employing the isotopic traces techniques. For example, with the aid of
labeled P04-P it is possible to measure the real value of the uptake flow
between water and bottom, because this uptake will not be masked by the
release of nonlabeled phosphate from the bottom. If we will know the
uptake flow rate Ac and the net change of P04-P concentration (±At), we
may calculate the release flow rate as: Ae = Ac + At. The corresponding
studies we have carried out at the Heron Island reefs with the use of
radioisotope 32p as the label. Experiments with the periphytonic
57
autotrophic com mum tIes of reef flats was recorded in analogous studies
accomplished later on other reefs (Hanson and Gundersen 1976; Kimmerer
and Walsh 1981; Hatcher and Hatcher 1981; Crossland and Barnes 1983;
Johannes et al. 1983b). In most cases a weak release of inorganic N and of
organic Nand P, as well as a slight uptake of inorganic Poccurred
(Atkinson 1983, 1987a; Atkinson and Smith 1987;). But in the diurnal
balance even the changes of P04-P content were found to be near zero.
Moreover, in the biotopes of lagoonal patch reefs dominated by living corals
the net release of phosphorus also most often was recorded with a diurnal
balance (Andrews and Muller 1983). Thus all attempts not only to quantify,
but even establish the direction of the flows of nutrients between the water
masses passing over the reef and its bottom communities appeared to be in
general unsuccessful (Atkinson 1983; Hatcher 1985). But at least the results
thus obtained confirmed statements about the importance of N2-fixation as
one of the main sources of nitrogen supply in coral reef communities (Webb
et al. 1975).
Additional attempts to gain insight into the problem-of nutrient flows
were made in experiments with plastic transparent enclosures secured to the
bottom and covered with sand or rubble. In these the changes of nutrients in
enclosed volumes of water were also measured in diurnal cycles (Henderson
1981; Propp 1981; Propp et al. 1983; Harrison 1983). The results were again
indefinite. In most cases during the day and also at night the concentration
of inorganic Nand P in water inside an enclosure did not change at all or it
slightly increased, and not only in the concentration of N but even of
inorganic P, instead of their expected consumption by autotrophic
communities of periphyton and microphytobenthos.
The faults in the above approaches seem to be quite obvious. The bottom
communities of reef flat as well as communities of rubble and sand comprise
together the autotrophic and the heterotrophic components. So they should
simultaneously consume nutrients and release them. Therefore, the abovelisted measurements of the change in their ambient concentration in oceanic
waters passing over the reef or in the in situ enclosures reflect only the
resulting equilibrium values of these two reciprocal flows, i.e., between the
uptake of nutrients by the autotrophic part of the community or the cellular
systems of symbiotic animals, and their release by the heterotrophic ones.
But these measurements cannot reflect the size of these flows per se. These
flows, the same as is the case with corals, could be measured only by
employing the isotopic traces techniques. For example, with the aid of
labeled P04-P it is possible to measure the real value of the uptake flow
between water and bottom, because this uptake will not be masked by the
release of nonlabeled phosphate from the bottom. If we will know the
uptake flow rate Ac and the net change of P04-P concentration (±At), we
may calculate the release flow rate as: Ae = Ac + At. The corresponding
studies we have carried out at the Heron Island reefs with the use of
radioisotope 32p as the label. Experiments with the periphytonic