56
Reef Environments
waters even at very low ambient content. It varied within 0.3 to
3.4JlgPI- l day-l; which resulted in a turnover time of2 to 11 days (Sorokin
1990a). The rates of total P04-P consumption by microplankton (At) in
general were higher in spring (October), when the water was warmer, than
in winter. Seasonal changes in nutrients cycling were also detected by
Hatcher (1985). The At-values were not significantly dependent upon the
level of primary production of phytoplankton. It was often high even with
very low primary production. This happened because a larger portion of
P04-P was consumed by bacteria, but not by the phytoplankton. The share
of bacterioplankton in At in coral reef waters exceeds usually 50% and
varies within 50 to 80% (Table 2.2). Thus the bacterioplankton appears to
be one of the most important and active agents of nutrients cycling in water.
Its function could be seen in the consumption of inorganic P and N from
oceanic water with the use of energy of the organic matter depleted by
nutrients, excreted by bottom animals (corals) and by macrophytes. The
misrobial biomass thus produced is used by filtering the fauna, which
includes the nutrients contained in it, into the nutrient pool of the reef
ecosystem (cf. scheme; Fig 2.12).
In conditions of relative stability of ambient P04-P concentrations in
waters over the reef, and by their relatively low net exchange rates (see
below), the At values should be in an equilibrium with the reciprocal values
of the rates of regeneration of P04-P from organic forms during the
processes of heterotrophic decomposition. Calculations of the latter, using
the data on the rate of organic matter destruction (total plankton respiration
measured as BOD at in situ to) and on the ratio C: P = 60 (by weight), gave
values of possible rates of P04-P regeneration in water columns as 13JlgPI- l day-l at rates of destruction of 50-150JlgCl- l day-1 (ct. Sect.
2.3). So the calculated rates of regeneration are within the same range as
rates of P04-P consumption. This proves a tight equilibration of a main
limiting nutrient in reef systems - phosphorus. As for the cycling nitrogen,
the processes of N2-fixation provide a permanent excess of its organic and
inorganic compounds which are released into and brought out by passing
oceanic waters.
The processes of exchange of nutrients between water and bottom
sediments were studied by a team working at the Enivetok atoll (Johannes
et al. 1972; Pilson and Betzer 1973; Webb et al. 1975). The team intended to
solve this problem by measuring the changes in their ambient concentrations
in water along the standard cross-sections through windward reef flats in
areas where corals and macrophytes are dominant. It was supposed that in
the oceanic waters, passing over the reef along the transsect, the content of
inorganic nutrients should decrease as a consequence of their consumption
by autotrophic communities of benthic plants and coral. But these
observations revealed a picture of indefinite fluctuations in them, which
were difficult to interpret. The same indefinite character of change in the
concentration of inorganic nutrients in oceanic water passing over benthic
Reef Environments
waters even at very low ambient content. It varied within 0.3 to
3.4JlgPI- l day-l; which resulted in a turnover time of2 to 11 days (Sorokin
1990a). The rates of total P04-P consumption by microplankton (At) in
general were higher in spring (October), when the water was warmer, than
in winter. Seasonal changes in nutrients cycling were also detected by
Hatcher (1985). The At-values were not significantly dependent upon the
level of primary production of phytoplankton. It was often high even with
very low primary production. This happened because a larger portion of
P04-P was consumed by bacteria, but not by the phytoplankton. The share
of bacterioplankton in At in coral reef waters exceeds usually 50% and
varies within 50 to 80% (Table 2.2). Thus the bacterioplankton appears to
be one of the most important and active agents of nutrients cycling in water.
Its function could be seen in the consumption of inorganic P and N from
oceanic water with the use of energy of the organic matter depleted by
nutrients, excreted by bottom animals (corals) and by macrophytes. The
misrobial biomass thus produced is used by filtering the fauna, which
includes the nutrients contained in it, into the nutrient pool of the reef
ecosystem (cf. scheme; Fig 2.12).
In conditions of relative stability of ambient P04-P concentrations in
waters over the reef, and by their relatively low net exchange rates (see
below), the At values should be in an equilibrium with the reciprocal values
of the rates of regeneration of P04-P from organic forms during the
processes of heterotrophic decomposition. Calculations of the latter, using
the data on the rate of organic matter destruction (total plankton respiration
measured as BOD at in situ to) and on the ratio C: P = 60 (by weight), gave
values of possible rates of P04-P regeneration in water columns as 13JlgPI- l day-l at rates of destruction of 50-150JlgCl- l day-1 (ct. Sect.
2.3). So the calculated rates of regeneration are within the same range as
rates of P04-P consumption. This proves a tight equilibration of a main
limiting nutrient in reef systems - phosphorus. As for the cycling nitrogen,
the processes of N2-fixation provide a permanent excess of its organic and
inorganic compounds which are released into and brought out by passing
oceanic waters.
The processes of exchange of nutrients between water and bottom
sediments were studied by a team working at the Enivetok atoll (Johannes
et al. 1972; Pilson and Betzer 1973; Webb et al. 1975). The team intended to
solve this problem by measuring the changes in their ambient concentrations
in water along the standard cross-sections through windward reef flats in
areas where corals and macrophytes are dominant. It was supposed that in
the oceanic waters, passing over the reef along the transsect, the content of
inorganic nutrients should decrease as a consequence of their consumption
by autotrophic communities of benthic plants and coral. But these
observations revealed a picture of indefinite fluctuations in them, which
were difficult to interpret. The same indefinite character of change in the
concentration of inorganic nutrients in oceanic water passing over benthic