32
Reef Lime Constructions
The zonal distributions of size fractions of sediments can be observed in
lagoons of atolls. Their shallows are covered with the crude sands mixed
with rubble material, sorted by the surf. The bottom of lagoon slopes is
occupied by medium coral sands which often consist of the abrased lime
plates of the calcareous alga Halimeda - the so-called Halimeda sands. The
bottom of the central deep part of the lagoon is covered by fine coral sands
of carbonate silts. A significant part of these sediments is transported from
flat to lagoon by fish (Emery 1956; Randall 1974). They feed during high
tide on the flat with periphyton, coralline algae and corals. While migrating
during the ebb-tide to the lagoon they excrete with feces large masses of the
swallowed carbonate material. The size of its annual transport was evaluated
to be about 2 tonnes ha -1, it being commensurate with that carried by surf
and currents (Bardach 1961).
The upper layer of coral sand, being well aerated and illuminated, is
inhabited by dense populations of micro algae , bacteria and microzoobenthon
(cf. Sects. 4.1, 5.6), this making the coral sands one of the most active
elements of the coral reef ecosystem, despite the fact that the absolute
content of organic carbon in them is relatively low: 0.15-1 % per dry weight.
The latter seems to be larger in reef areas with an abundant coral growth
(Sorokin 1986a; cf. Table 4.1). The corals shedding mucus obviously enrich
the nearby soft sediments with organic matter. Indeed, the content of
Table 1.8. Content of organic carbon in reef bottom sediments as percentage of dry
weight; data by Thomassin et al. (1976), Sorokin (1978b, 1991), Hubbard et al. (1981b),
Vaugelas (1981), Vaugelas and Nairn (1981), Propp et al. (1983), Thomassin and Cauwet
(1985)
Location
Biotope
Coral sands among
thickets of macrophytes
and seagrasses
Aqaba, northern Red
0.14-0.34
Sea.
Coast of central
0.7-1.2
Vietnam
Moorea I., French
0.17
Polynesia
Kaneohe Bay, Hawaii 0.27
Velangilala atoll, Lau, 0.17
Fiji
Saint Croix, Virgin
0.4-0.6
Islands
Barrier reef of Tulear, 0.11-0.16
Madagascar
Phantom bank, Timor 0.24-0.55
Sea
Heron Island, GBR
0.32
Coral sands
in lagoons
0.09-0.26
0.12-0.35
0.17
0.37
0.47
0.4-0.6
0.16-0.23
Silts or silted coral
sands with admixture
of terrigenous material
0.11-0.16
0.7-1.0
0.45
0.29
0.2-0.4
Reef Lime Constructions
The zonal distributions of size fractions of sediments can be observed in
lagoons of atolls. Their shallows are covered with the crude sands mixed
with rubble material, sorted by the surf. The bottom of lagoon slopes is
occupied by medium coral sands which often consist of the abrased lime
plates of the calcareous alga Halimeda - the so-called Halimeda sands. The
bottom of the central deep part of the lagoon is covered by fine coral sands
of carbonate silts. A significant part of these sediments is transported from
flat to lagoon by fish (Emery 1956; Randall 1974). They feed during high
tide on the flat with periphyton, coralline algae and corals. While migrating
during the ebb-tide to the lagoon they excrete with feces large masses of the
swallowed carbonate material. The size of its annual transport was evaluated
to be about 2 tonnes ha -1, it being commensurate with that carried by surf
and currents (Bardach 1961).
The upper layer of coral sand, being well aerated and illuminated, is
inhabited by dense populations of micro algae , bacteria and microzoobenthon
(cf. Sects. 4.1, 5.6), this making the coral sands one of the most active
elements of the coral reef ecosystem, despite the fact that the absolute
content of organic carbon in them is relatively low: 0.15-1 % per dry weight.
The latter seems to be larger in reef areas with an abundant coral growth
(Sorokin 1986a; cf. Table 4.1). The corals shedding mucus obviously enrich
the nearby soft sediments with organic matter. Indeed, the content of
Table 1.8. Content of organic carbon in reef bottom sediments as percentage of dry
weight; data by Thomassin et al. (1976), Sorokin (1978b, 1991), Hubbard et al. (1981b),
Vaugelas (1981), Vaugelas and Nairn (1981), Propp et al. (1983), Thomassin and Cauwet
(1985)
Location
Biotope
Coral sands among
thickets of macrophytes
and seagrasses
Aqaba, northern Red
0.14-0.34
Sea.
Coast of central
0.7-1.2
Vietnam
Moorea I., French
0.17
Polynesia
Kaneohe Bay, Hawaii 0.27
Velangilala atoll, Lau, 0.17
Fiji
Saint Croix, Virgin
0.4-0.6
Islands
Barrier reef of Tulear, 0.11-0.16
Madagascar
Phantom bank, Timor 0.24-0.55
Sea
Heron Island, GBR
0.32
Coral sands
in lagoons
0.09-0.26
0.12-0.35
0.17
0.37
0.47
0.4-0.6
0.16-0.23
Silts or silted coral
sands with admixture
of terrigenous material
0.11-0.16
0.7-1.0
0.45
0.29
0.2-0.4