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P.W.Glynn
four categories of natural coral reef disturbances noted in UNEP/IUCN
{1988) are coral bleaching, hurricane/storm damage, coral diseases and
mass Diadema mortalities. In eastern Pacific coral reef ecosystems, the
importance of these disturbances varies greatly.
Coral bleaching and mortality that accompanied the 1982-1983 El NinoSouthern Oscillation (ENSO) event were the most devastating, widespread
and sustained thus far documented in the eastern Pacific region. Coral
mortality resulting from this ENSO was geographically variable. Where
SST anomalies reached 1-2 oc (Cafio Island, Costa Rica, and Gulf of Chiriqui, Panama), mean reef coral mortalities ranged between 50 and 75 o/o; at
localities with 3 and 4 oc anomalies (Gulf of Panama, Panama, and the
Galapagos Islands, Ecuador) mean coral mortality was 85 and 97 o/o, respectively (Glynn 1990). Galapagos coral communities and coral reefs were
devastated, and low rates of coral recruitment and intense bioerosion in
subsequent years have virtually eliminated coral reef biotopes from this
region. Controlled microcosm tolerance experiments have substantiated
the field results observed during the ENSO disturbance, demonstrating
that prolonged high sea temperatures were the major causal factor (Glynn
and D'Croz 1990). A comparison of coral mortality during three recent
ENSO events also demonstrates a high degree of variability. In 1982-1983
the equatorial eastern Pacific (EEP), i.e., mainland Costa Rica, Panama,
Colombia and Ecuador, including offshore Cocos Island and the Galapagos
Islands, was severely affected. However, only moderate coral bleaching and
no detectable mortality were observed in parts (Galapagos Islands) of the
EEP during the 1987 ENSO, but bleaching and mortality (about 10 o/o)
occurred at the southern tip of Baja California, Mexico, an area apparently
not affected in 1982-1983 (Reyes Bonilla 1993; Podesta and Glynn 1997).
While still under study, the 1997-1998 ENSO had stronger effects in Mexico than in 1987, and caused coral mortality throughout the EEP, but apparently not so strong as in 1982-1983. Several secondary disturbances
following the ENSO, e. g. strong upwelling pulses, phytoplankton blooms,
extreme midday low water exposures, intense bioerosion and high predator concentrations, have continued to cause coral mortality and the erosion of coral reef frameworks.
Disturbances by storm damage are infrequent (hurricanes normally
form off the Pacific coast and then move toward the west), and coral diseases and mass sea urchin mortalities are unknown. Some disturbances not
included in the UNEP/IUCN listing can have severe local effects. On a
decadal time scale, tectonic events and volcanism affect only small areas.
One such event in 1954, resulting from a volcanic eruption and the uplift of
a shallow shelf, exposed several km 2 of coral communities in the Galapagos
Islands (Colgan 1990). While several species of corallivores occur in the
P.W.Glynn
four categories of natural coral reef disturbances noted in UNEP/IUCN
{1988) are coral bleaching, hurricane/storm damage, coral diseases and
mass Diadema mortalities. In eastern Pacific coral reef ecosystems, the
importance of these disturbances varies greatly.
Coral bleaching and mortality that accompanied the 1982-1983 El NinoSouthern Oscillation (ENSO) event were the most devastating, widespread
and sustained thus far documented in the eastern Pacific region. Coral
mortality resulting from this ENSO was geographically variable. Where
SST anomalies reached 1-2 oc (Cafio Island, Costa Rica, and Gulf of Chiriqui, Panama), mean reef coral mortalities ranged between 50 and 75 o/o; at
localities with 3 and 4 oc anomalies (Gulf of Panama, Panama, and the
Galapagos Islands, Ecuador) mean coral mortality was 85 and 97 o/o, respectively (Glynn 1990). Galapagos coral communities and coral reefs were
devastated, and low rates of coral recruitment and intense bioerosion in
subsequent years have virtually eliminated coral reef biotopes from this
region. Controlled microcosm tolerance experiments have substantiated
the field results observed during the ENSO disturbance, demonstrating
that prolonged high sea temperatures were the major causal factor (Glynn
and D'Croz 1990). A comparison of coral mortality during three recent
ENSO events also demonstrates a high degree of variability. In 1982-1983
the equatorial eastern Pacific (EEP), i.e., mainland Costa Rica, Panama,
Colombia and Ecuador, including offshore Cocos Island and the Galapagos
Islands, was severely affected. However, only moderate coral bleaching and
no detectable mortality were observed in parts (Galapagos Islands) of the
EEP during the 1987 ENSO, but bleaching and mortality (about 10 o/o)
occurred at the southern tip of Baja California, Mexico, an area apparently
not affected in 1982-1983 (Reyes Bonilla 1993; Podesta and Glynn 1997).
While still under study, the 1997-1998 ENSO had stronger effects in Mexico than in 1987, and caused coral mortality throughout the EEP, but apparently not so strong as in 1982-1983. Several secondary disturbances
following the ENSO, e. g. strong upwelling pulses, phytoplankton blooms,
extreme midday low water exposures, intense bioerosion and high predator concentrations, have continued to cause coral mortality and the erosion of coral reef frameworks.
Disturbances by storm damage are infrequent (hurricanes normally
form off the Pacific coast and then move toward the west), and coral diseases and mass sea urchin mortalities are unknown. Some disturbances not
included in the UNEP/IUCN listing can have severe local effects. On a
decadal time scale, tectonic events and volcanism affect only small areas.
One such event in 1954, resulting from a volcanic eruption and the uplift of
a shallow shelf, exposed several km 2 of coral communities in the Galapagos
Islands (Colgan 1990). While several species of corallivores occur in the