Factors Controlling Coral Community Structure
281
branches to the direction of stress. The ability of these ramose corals to
propagate vegetatively by pieces of their branches is also one of the factors
making for their abundance in stressed zones. The wave-stress also controls
the species composition of octocoral communities which inhabit the surfaces
of windward reef flats and upper reef slopes (Morton 1974). On reefs with a
week surf the zonal features in the composition of coral communities are
very slight (Loya 1972; Bouchon 1981). Instead of ramose corals, mostly the
massive Porites, faviids and also Montipora dominate (Grigg 1983; Latypov
1985; Potts et al. 1985; Cameron and Endean 1985). The planula larvae of
opportunistic corals seem also to be adapted to settling in stressed biotopes
(Connell 1973).
The influence of fluctuations of temperature and salinity on the structure
of coral communities was most spectacularly demonstrated on biotopes of
exposed reef flats in the Indo-Pacific area. These reefs reached sea level long
ago and their surfaces are now 0.5-1 m high over the level of sea at low
ebbs. Thus corals that inhabit these biotopes are periodically exposed during
low ebbs for 2-4 h, suffering at night from cold, in the day time from heat,
and in rainy weather from drops of salinity (Pichon 1964; Loya 1972; Ditlev
1978). Corals living on exposed reef biotopes can endure gradual changes in
salinity on a large scale: 20 to 40% (Kinsman 1964). But a sharp shift in it
kills them rather quickly (Goreau 1964b). The stress influences of the above
listed factors on corals living in exposed reef-flat biotopes are itetconnected
and have a cumulative damaging effect (Coles and Jokiel 1978). Thus the
coral associations in these biotopes exist close to the limit of sustainable
stress (Jokiel and Coles 1990). Even a slight general change in
environmental conditions, like some global changes in water temperature
caused by the El Nino current, or an increase in tide levels caused by
geostrophic events, as well as anomalous weather conditions accompanied
by long periods of rain, cause of large-scale mortality of corals in the
biotopes of exposed reef-flat areas.
On the exposed flat areas of the Red Sea and the Caribbean high-latitude
reefs, a basic factor influencing the composition of coral communities is the
extreme cooling down during low ebbs at night and during the periods of
breakthrough of cold waters in winter into the reef areas (Loya 1972;
Roberts et al. 1975; Scatterday 1977; Hudson 1981). These events play
havor even among the most opportunistic species of Acropora, Pocillopora,
Montipora, Millepora, Diploria, and Psammocora, which are usually leading
in the communities of exposed reef flats (Ditlev 1978).
Among the physical factors of stress inhibiting the growth of corals is also
their siltation in turbid waters in areas of intensive resuspension of
sediments by waves and currents, and in biotopes receiving the terrigenous,
suspended matter (Durand 1989). An intensive sedimentation of
resuspended sediments after storms may even kill corals (Marshall and Orr
1931). In this respect, the resuspension of sediments and the increase in
their terrigenous input caused by rains, which usually accompany the storms,
281
branches to the direction of stress. The ability of these ramose corals to
propagate vegetatively by pieces of their branches is also one of the factors
making for their abundance in stressed zones. The wave-stress also controls
the species composition of octocoral communities which inhabit the surfaces
of windward reef flats and upper reef slopes (Morton 1974). On reefs with a
week surf the zonal features in the composition of coral communities are
very slight (Loya 1972; Bouchon 1981). Instead of ramose corals, mostly the
massive Porites, faviids and also Montipora dominate (Grigg 1983; Latypov
1985; Potts et al. 1985; Cameron and Endean 1985). The planula larvae of
opportunistic corals seem also to be adapted to settling in stressed biotopes
(Connell 1973).
The influence of fluctuations of temperature and salinity on the structure
of coral communities was most spectacularly demonstrated on biotopes of
exposed reef flats in the Indo-Pacific area. These reefs reached sea level long
ago and their surfaces are now 0.5-1 m high over the level of sea at low
ebbs. Thus corals that inhabit these biotopes are periodically exposed during
low ebbs for 2-4 h, suffering at night from cold, in the day time from heat,
and in rainy weather from drops of salinity (Pichon 1964; Loya 1972; Ditlev
1978). Corals living on exposed reef biotopes can endure gradual changes in
salinity on a large scale: 20 to 40% (Kinsman 1964). But a sharp shift in it
kills them rather quickly (Goreau 1964b). The stress influences of the above
listed factors on corals living in exposed reef-flat biotopes are itetconnected
and have a cumulative damaging effect (Coles and Jokiel 1978). Thus the
coral associations in these biotopes exist close to the limit of sustainable
stress (Jokiel and Coles 1990). Even a slight general change in
environmental conditions, like some global changes in water temperature
caused by the El Nino current, or an increase in tide levels caused by
geostrophic events, as well as anomalous weather conditions accompanied
by long periods of rain, cause of large-scale mortality of corals in the
biotopes of exposed reef-flat areas.
On the exposed flat areas of the Red Sea and the Caribbean high-latitude
reefs, a basic factor influencing the composition of coral communities is the
extreme cooling down during low ebbs at night and during the periods of
breakthrough of cold waters in winter into the reef areas (Loya 1972;
Roberts et al. 1975; Scatterday 1977; Hudson 1981). These events play
havor even among the most opportunistic species of Acropora, Pocillopora,
Montipora, Millepora, Diploria, and Psammocora, which are usually leading
in the communities of exposed reef flats (Ditlev 1978).
Among the physical factors of stress inhibiting the growth of corals is also
their siltation in turbid waters in areas of intensive resuspension of
sediments by waves and currents, and in biotopes receiving the terrigenous,
suspended matter (Durand 1989). An intensive sedimentation of
resuspended sediments after storms may even kill corals (Marshall and Orr
1931). In this respect, the resuspension of sediments and the increase in
their terrigenous input caused by rains, which usually accompany the storms,