280
Communities of Corals in Reef Ecosystems
and Icilligorgia. But some symbiotic species of gorgonians well-adapted to a
low light, could be met with sometimes down to a depth of 50-60 m (Kinzie
1973). Most species of symbiotic reef gorgonians in the western Atlantic
have big, firm, flexible and fast-growing colonies, which well sustain
siltation, turbid water and surf (Opresko 1973). These features enable them
to compete successfully with corals and other sessile fauna and plants and so
to play an important role in bottom biotopes of western Atlantic reefs, most
of which are situated on the continental shelf or near high islands, and are
subjected to intensive terrigenous siltation.
7.3 Factors Controlling Coral Community Structure
The main factors which control the structure of coral communities on the
reef are the physical and biological stresses, on the one hand, and the
differentiated reaction of various coral species to disturbing action by their
different potential ability to compete for the hard substrate, on the other
(Bak and Powell 1988; Claerebandt 1988). Under the heading biological
stress we list here the grazing of corals by fish and other animals, their
bioerosion by boring fauna, and their diseases caused by microorganisms
(Connell 1973, 1978; Loya 1976c; Glynn 1976). The physical stress as a
controlling factor for coral communities is more important in shallow
turbulent zones of reef, while in calm leeward or deeper zones the biological
stress begins to prevail (Sheppard 1981b).
Among the factors of physical stress hydrodynamics, resuspension of
sediments, changes in salinity and temperature, excess of light, and exposure
seem to be the most important (Bradbury and Young 1981). In shallow reef
zones and on windward reef flats the structure of coral communities is
influenced largely by wave stress and tidal currents (Roberts 1974; Geister
1977; Jokiel 1978; Sheppard 1980; Dollar 1982). Intensive hydrodynamics in
itself is a positive factor in the life of sessile fauna. It provides permanent
supply of particulated food and oxygen, driving out suspended material,
which arrives from the littoral zone, thus preventing its siltation (Hubbard
1974; Wainwright and Koehl 1976). But in the surf zones of windward flats
and in the channels cut across them, waves and tidal currents could be so
strong that they may damage or uproot coral colonies. Therefore, coral
communities in zones of active hydrodynamics are dominated by species or
by their ecomorphs which well sustain hydrodynamic stress. These are most
often the ramose acroporids, which may form ecomorphs corresponding to
the level of this stress (Kuhlmann 1975), and thus survive well in this zone.
The same features are peculiar also to some other ramose corals, like
Pocillopora, Stylophora, and Millepora which successfully survive in
turbulent zones. Their ability to oppose stress depends not only upon the
mechanical durability of colonies, but also upon precise orientation of their
Communities of Corals in Reef Ecosystems
and Icilligorgia. But some symbiotic species of gorgonians well-adapted to a
low light, could be met with sometimes down to a depth of 50-60 m (Kinzie
1973). Most species of symbiotic reef gorgonians in the western Atlantic
have big, firm, flexible and fast-growing colonies, which well sustain
siltation, turbid water and surf (Opresko 1973). These features enable them
to compete successfully with corals and other sessile fauna and plants and so
to play an important role in bottom biotopes of western Atlantic reefs, most
of which are situated on the continental shelf or near high islands, and are
subjected to intensive terrigenous siltation.
7.3 Factors Controlling Coral Community Structure
The main factors which control the structure of coral communities on the
reef are the physical and biological stresses, on the one hand, and the
differentiated reaction of various coral species to disturbing action by their
different potential ability to compete for the hard substrate, on the other
(Bak and Powell 1988; Claerebandt 1988). Under the heading biological
stress we list here the grazing of corals by fish and other animals, their
bioerosion by boring fauna, and their diseases caused by microorganisms
(Connell 1973, 1978; Loya 1976c; Glynn 1976). The physical stress as a
controlling factor for coral communities is more important in shallow
turbulent zones of reef, while in calm leeward or deeper zones the biological
stress begins to prevail (Sheppard 1981b).
Among the factors of physical stress hydrodynamics, resuspension of
sediments, changes in salinity and temperature, excess of light, and exposure
seem to be the most important (Bradbury and Young 1981). In shallow reef
zones and on windward reef flats the structure of coral communities is
influenced largely by wave stress and tidal currents (Roberts 1974; Geister
1977; Jokiel 1978; Sheppard 1980; Dollar 1982). Intensive hydrodynamics in
itself is a positive factor in the life of sessile fauna. It provides permanent
supply of particulated food and oxygen, driving out suspended material,
which arrives from the littoral zone, thus preventing its siltation (Hubbard
1974; Wainwright and Koehl 1976). But in the surf zones of windward flats
and in the channels cut across them, waves and tidal currents could be so
strong that they may damage or uproot coral colonies. Therefore, coral
communities in zones of active hydrodynamics are dominated by species or
by their ecomorphs which well sustain hydrodynamic stress. These are most
often the ramose acroporids, which may form ecomorphs corresponding to
the level of this stress (Kuhlmann 1975), and thus survive well in this zone.
The same features are peculiar also to some other ramose corals, like
Pocillopora, Stylophora, and Millepora which successfully survive in
turbulent zones. Their ability to oppose stress depends not only upon the
mechanical durability of colonies, but also upon precise orientation of their