Destruction of Corals by Physical Stress
401
dollars (Johnson et al. 1990). Effective ones could be developed only after
finding an adequate solution to the problem of the origin of Acanthaster
outbreaks.
11.2 Destruction of Corals by Physical Stress
Among the factors of physical stress which damage corals, there could be
mentioned wave action, decreases in salinity, siltation, exposure, and
overheating during extra-low ebbs. Sometimes the action of these factors
reach a catastrophic level, causing mass mortality among corals. Most often
such catastrophic events are connected with the passing of tropical cyclones
over the reefs - typhoons or hurricanes - which are accompanied by heavy
rains causing the decreases in salinity and massive siltation of reefs. The
rates of wind velocity in the zone of passage of an average strong tropical
cyclone could be 100-250 km h -1. In the open sea such a wind creates waves
8-10 m high. Near the reef edge such a storm will raise waves 4-8 m high,
and in the lagoon up to 3 m (Highsmith et al. 1980). The waves induced by
such a cyclone smash all the ramose corals, especially on the winddward side
of the reef, where the depth of the zone of total destruction of corals may
reach 20-25 m. The massive and firm foliose corals are destroyed by
cyclones to a lesser degree (Stoddart 1985). Some of their large colonies on
being thrown out onto the flat remain these as so-called nigger heads. The
destruction of corals during the cyclone proceeds mostly under the action of
masses of rubble moving to and fro at high speed. They smash the ramose
corals and irreversibly damage the massive ones. The mass mortality of
corals during the passing of cyclones occurs also on the windward sides of
reefs. Corals perish there not only through large waves passing over the flat,
but also under the impact of heavy siltation by stirred-up bottom sediments
and by terrigenous suspended matter washed out by rains, which usually
accompany such cyclones. The corals covered by sediments die off in 3-4
days. The rains cause decreases in salinity, apart from the cyclones
themselves often causing extremely low ebbs.
The passage of an average-strength cyclone causes 50 to 80% mortality
among corals in the upper reef zones. Some corals survive in patches
situated in places shielded from the direct action of moving rubble masses.
These patches become then the centers of rehabiliation of damaged coral
communities (Graus et al. 1984). But extremely strong cyclones with winds
of 300 km h - r, as a rule, destroy corals on reefs almost totally. They induce
the movement of millions of tons of rubble. The reefs may even change their
configuration after the passage of this kind of cyclone. New keys and islands
appear on them. Such cyclones occur in reef zones one or two times in a
century (Maragos et al. 1973). After an extra-strong ccylone the reef benthic
401
dollars (Johnson et al. 1990). Effective ones could be developed only after
finding an adequate solution to the problem of the origin of Acanthaster
outbreaks.
11.2 Destruction of Corals by Physical Stress
Among the factors of physical stress which damage corals, there could be
mentioned wave action, decreases in salinity, siltation, exposure, and
overheating during extra-low ebbs. Sometimes the action of these factors
reach a catastrophic level, causing mass mortality among corals. Most often
such catastrophic events are connected with the passing of tropical cyclones
over the reefs - typhoons or hurricanes - which are accompanied by heavy
rains causing the decreases in salinity and massive siltation of reefs. The
rates of wind velocity in the zone of passage of an average strong tropical
cyclone could be 100-250 km h -1. In the open sea such a wind creates waves
8-10 m high. Near the reef edge such a storm will raise waves 4-8 m high,
and in the lagoon up to 3 m (Highsmith et al. 1980). The waves induced by
such a cyclone smash all the ramose corals, especially on the winddward side
of the reef, where the depth of the zone of total destruction of corals may
reach 20-25 m. The massive and firm foliose corals are destroyed by
cyclones to a lesser degree (Stoddart 1985). Some of their large colonies on
being thrown out onto the flat remain these as so-called nigger heads. The
destruction of corals during the cyclone proceeds mostly under the action of
masses of rubble moving to and fro at high speed. They smash the ramose
corals and irreversibly damage the massive ones. The mass mortality of
corals during the passing of cyclones occurs also on the windward sides of
reefs. Corals perish there not only through large waves passing over the flat,
but also under the impact of heavy siltation by stirred-up bottom sediments
and by terrigenous suspended matter washed out by rains, which usually
accompany such cyclones. The corals covered by sediments die off in 3-4
days. The rains cause decreases in salinity, apart from the cyclones
themselves often causing extremely low ebbs.
The passage of an average-strength cyclone causes 50 to 80% mortality
among corals in the upper reef zones. Some corals survive in patches
situated in places shielded from the direct action of moving rubble masses.
These patches become then the centers of rehabiliation of damaged coral
communities (Graus et al. 1984). But extremely strong cyclones with winds
of 300 km h - r, as a rule, destroy corals on reefs almost totally. They induce
the movement of millions of tons of rubble. The reefs may even change their
configuration after the passage of this kind of cyclone. New keys and islands
appear on them. Such cyclones occur in reef zones one or two times in a
century (Maragos et al. 1973). After an extra-strong ccylone the reef benthic