Destruction of Corals by Physical Stress
405
this area. Only casual accidents of mortality happened during that time on
some reef shallows, caused by extra-low ebbs, so that their cause had been
evident.
The phenomenon of bleaching of corals stressed by high temperature,
oxygen deficiency, or by the presence of toxic pollutants manifests itself in
the expulsion of zooxanthellae by their polyps. High temperature stimulates
the activity of lysosomes in the cells of zooxanthellae, triggering the process
of theirselfdigestion in polyps (Sucharsono and Brown 1989). The bleached
corals then most often soon die. The percentage of mortality among
bleached corals is especially high in corals sensitive to stress, like
Pocillopora, and some species of Acropora and Agaricia. Over 60-90% of
their bleached colonies subsequently die (Harriott 1985; Fisk and Done
1985; Cook et al. 1990). But some corals, like Montastrea and Millepora,
may survive bleaching and later on may recover. There are also corals that
do not bleach et all, like Porites or Psammocora. But because the first of
their above-nentioned groups dominates in upper reef zones, the mass
bleaching in coral communities leads to the death of up to a half of the coral
populations in the upper 10-15 m zone.
The bleaching of corals was first recorded on Guam in 1962, on reefs of
Florida in 1978, on Okinawa and on the Christmas Island in 1980 (Goreau
1964b; Jaap 1979; Yamazato 1981; Egana and Di Salvo 1982). These authors
believe that the main cause of these events could be found in anthropogenic
influence. In the first case it was the washing out of the soil, a consequence
of military construction activity, and in two other cases pollution. The global
destabilization of reef ecosystems took place in 1982-1983 and was
accompanied by mass bleaching and mortality of scleractinian corals (Glynn,
1982; Williams and Williams 1990). Most resarchers see its cause in a
general warming of the world ocean. It was brought about at that time by
the strongest warming during the century of EI Nino (Glynu 1985). The
surface water temperature rose in the tropical eastern Pacific by 4-6 °C, and
in other tropical regions by 2-3 0c. In the western and the central Pacific the
sea level during the EI Nino period sank by 50-70cm. It resulted in long
exposure of vast shallow areas during the low ebbs, moreover it happened in
conditions of the above-mentioned general surface warming of oceanic
waters (Glynn 1984; Oliver 1985; Fisk and Done 1985; Glynn et al. 1988).
From the viewpoint of the first approach, corals had he because adapted
to the fluctuations of temperature. But actually this is not so. It appears that
the usual summer temperatures of 29-31°C in the tropics is the upper
temperature limit for their normal growth. They can endure only short-term
rises up to 34°C. A long-term warming of the water up to 32-33°C cause
such stress that they start to expel their zooxanthellae and to excrete excess
amounts of mucus. The mucus stimulates microbial growth in water and on
the surface of corals weakened by the tmperature stress Blached corals lose
gradually their tissues (Szamant and Gassman 1989). Finally, it leads to their
death Ducklow and Mitchell 1979b; Lessios et al. 1984). At the time of
405
this area. Only casual accidents of mortality happened during that time on
some reef shallows, caused by extra-low ebbs, so that their cause had been
evident.
The phenomenon of bleaching of corals stressed by high temperature,
oxygen deficiency, or by the presence of toxic pollutants manifests itself in
the expulsion of zooxanthellae by their polyps. High temperature stimulates
the activity of lysosomes in the cells of zooxanthellae, triggering the process
of theirselfdigestion in polyps (Sucharsono and Brown 1989). The bleached
corals then most often soon die. The percentage of mortality among
bleached corals is especially high in corals sensitive to stress, like
Pocillopora, and some species of Acropora and Agaricia. Over 60-90% of
their bleached colonies subsequently die (Harriott 1985; Fisk and Done
1985; Cook et al. 1990). But some corals, like Montastrea and Millepora,
may survive bleaching and later on may recover. There are also corals that
do not bleach et all, like Porites or Psammocora. But because the first of
their above-nentioned groups dominates in upper reef zones, the mass
bleaching in coral communities leads to the death of up to a half of the coral
populations in the upper 10-15 m zone.
The bleaching of corals was first recorded on Guam in 1962, on reefs of
Florida in 1978, on Okinawa and on the Christmas Island in 1980 (Goreau
1964b; Jaap 1979; Yamazato 1981; Egana and Di Salvo 1982). These authors
believe that the main cause of these events could be found in anthropogenic
influence. In the first case it was the washing out of the soil, a consequence
of military construction activity, and in two other cases pollution. The global
destabilization of reef ecosystems took place in 1982-1983 and was
accompanied by mass bleaching and mortality of scleractinian corals (Glynn,
1982; Williams and Williams 1990). Most resarchers see its cause in a
general warming of the world ocean. It was brought about at that time by
the strongest warming during the century of EI Nino (Glynu 1985). The
surface water temperature rose in the tropical eastern Pacific by 4-6 °C, and
in other tropical regions by 2-3 0c. In the western and the central Pacific the
sea level during the EI Nino period sank by 50-70cm. It resulted in long
exposure of vast shallow areas during the low ebbs, moreover it happened in
conditions of the above-mentioned general surface warming of oceanic
waters (Glynn 1984; Oliver 1985; Fisk and Done 1985; Glynn et al. 1988).
From the viewpoint of the first approach, corals had he because adapted
to the fluctuations of temperature. But actually this is not so. It appears that
the usual summer temperatures of 29-31°C in the tropics is the upper
temperature limit for their normal growth. They can endure only short-term
rises up to 34°C. A long-term warming of the water up to 32-33°C cause
such stress that they start to expel their zooxanthellae and to excrete excess
amounts of mucus. The mucus stimulates microbial growth in water and on
the surface of corals weakened by the tmperature stress Blached corals lose
gradually their tissues (Szamant and Gassman 1989). Finally, it leads to their
death Ducklow and Mitchell 1979b; Lessios et al. 1984). At the time of