BULLETIN OF MARINE SCIENCE, 69(1): 133–149, 2001
CHANGES IN REEF COMMUNITY STRUCTURE AFTER FIFTEEN
YEARS OF NATURAL DISTURBANCES IN THE EASTERN PACIFIC
(COSTA RICA)
Héctor M. Guzmán and Jorge Cortés
ABSTRACT
Eastern Pacific coral reefs have been severely disturbed by natural events during the
past two decades. We have monitored changes in reef structure and reef recovery after
ENSO 1982–83 (starting in 1984), at sixteen permanent plots in four different habitats at
Caño Island, Costa Rica. Reefs were also severely affected by dinoflagellate blooms in
1985, and by warming events in 1987, 1990–95 and 1997–98. The 1982–83 event caused
approximately 100% coral mortality in shallow reef zones at Caño Island, particularly of
pocilloporid species. Coral recruitment may have coincided with putative larval pulses
during the various ENSO events or shortly after, as deduced by the presence of sexual
recruits during 1987–88 and widespread sexual recruitment in 1993–94. Mortality of
juvenile and adult colonies during the 1997–98 ENSO warming was low (5%), suggesting that populations of massive and branching corals may have been more tolerant of
elevated thermal stress than during previous events. Supporting this notion are the Reynolds
SST comparative plots for 1982–83 and 1997–98, which indicate similar warming trends
and temperature maxima at this locality. Reefs at Caño Island are recovering, with significant increases in the number of new sexual recruits. Although 1984 levels of coral
cover have not yet been attained island-wide, 70% cover occurs in reef areas on the north
side of the island. Other disturbances, such as phytoplankton blooms that affected
Pocillopora spp. in all habitats, may have retarded reef regeneration, complicating the
course of recovery after the 1982–83 ENSO warming disturbance.
Eastern Pacific coral reefs have been impacted by many pronounced natural disturbances during the last two decades. The most severe were the El Niño–Southern Oscillation (ENSO) warming events in 1982–83 (Glynn, 1984, 1992) and 1997–98 (this issue).
The latter, considered the most severe on record (McPhaden, 1999; Wilkinson et al., 1999),
caused coral bleaching in many regions of the world. Other documented natural disturbances have affected local areas of the eastern Pacific, for example, coral mortality due to
phytoplankton blooms in Panama and Costa Rica (Guzmán et al., 1990), and low tidal
exposures in Panama (Eakin and Glynn, 1996). Recovery from severe disturbances is
predicted to be low in the eastern Pacific due to lack of sexual recruitment (Glynn et al.,
1991, 1994, 1996), almost no recruitment (Cortés, 1997), and continuous predation by
corallivores (Glynn 1985a, Guzmán and Cortés, 1992). However, recent observations at
Caño Island, Costa Rica (Guzmán and Cortés, 1989a), indicate an increase in sexual
recruitment after 1994, before the 1997–98 ENSO coral bleaching event. High survival
rates of recruits and adult colonies of scleractinian corals followed the 1997–98 ENSO.
At Caño Island, coral populations were drastically reduced during the 1982–83 El Niño–
Southern Oscillation (ENSO), causing 50% mortality of coral cover overall (Guzmán et
al., 1987; Glynn et al., 1988). Even so, a high Pocillopora spp. and Porites lobata coral
cover (average ca 32%) remained in several areas, particularly in shallow water habitats.
In this paper, we describe the impacts of several El Niño events and other types of disturbances from 1984 to the present in the context of long-term coral population dynamics
and changes in community structure at Caño Island, Costa Rica.
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MATERIALS AND METHODS
Caño Island Biological Reserve (8∞43'N, 83∞52'W) is located off the south Pacific coast of Costa
Rica, approximately 15 km from the mainland Osa Peninsula (Fig. 1). The island is covered by
tropical rainforest and is 300 ha in area with a maximum altitude of 90 m. Coral reefs are well
developed on the north and east sides of the island. Reefs are built mainly by Pocillopora damicornis,
Pocillopora elegans and microatolls of P. lobata in shallow waters or on reef flats, and by massive
colonies of P. lobata from the reef-slope to the reef base (Guzmán and Cortés, 1989a). There are 15
coral species reported from Caño Island (Cortés and Murillo, 1985), but recent changes in the
taxonomy of some groups have increased the species count to 22 (see Cortés and Guzmán, 1998).
Temperature data and ENSO indices were used to correlate field observations with estimated
geographic temperature anomalies. Monthly mean sea surface temperature (SST) anomalies from
1982 to 1999 were obtained from the Integrated Global Ocean Services System (IGOSS; http://
ingrid.ldgo.columbia.edu/SOURCES/.IGOSS/.nmc/.weekly/.dataset_documentation.html) (Fig. 2A).
Temperature measurements were blended from ship, buoy, and bias-corrected satellite data (see
Reynolds and Smith, 1994). Reynolds SSTs (∞C) are presented to compare temperature measure-
Figure 1. Caño Island and the location of the four reefs where permanent 20 m2 plots were established
in 1984 and monitored until 1999, southwest coast of Costa Rica. During the 1990–95 warming
event, coral colonies were counted at reefs 2–4 and the coral reef denoted by an asterisk. Inset
shows the locations of Caño Island and the Osa Peninsula.
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135
Figure 2. (A) Monthly sea surface temperature anomalies from January 1982 to April 1999 centered
at 8∞50'N, 83∞50'W; (B) the Japanese JMA El Niño–Southern Oscillation Index; and (C) the Southern
Oscillation Index (SOI) standardized for the same period. Vertical lines are monthly mean values
and curves are 5-mo moving averages.
ments of the 1982–83 and 1997–98 ENSO events (Fig. 3). The SST-based El Niño/La Niña prediction index created by the Japan Meteorological Agency (JMA) was also consulted (http://
www.coaps.fsu.edu/~legler/jma_index1.shtml) (Fig. 2B). This index is constructed from monthly
SST anomalies averaged for the area 4∞N to 4∞S and 150∞W to 90∞W and applies a 5-mo running
mean to smooth out possible intra-seasonal variations (see Meyers et al., 1999). The standardized
Southern Oscillation Index (SOI; http://www.nic.fb4.noaa.gov:pub/cac/cddb/indices) was obtained
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Figure 3. Mean weekly Reynolds SST records near Isla del Caño during the 1982-83 and 1997–98
ENSO events. Data grid (1 ¥ 1∞) centered at 8∞30'N, 83∞30'W.
from NOAA’s Climate Analysis Center (CAC) and is based on differences between Tahiti and Darwin sea level pressures (SLP) (Fig. 2C).
Sixteen permanent plots were established in January 1984 within representative reef habitats at
Caño Island (Guzmán and Cortés, 1989a). The selected habitats were: reef platform, between 0–3
m (Reef 1); upper reef slope, between >3–5 m (Reef 2); mid-slope, between >5–9 m (Reef 3); and
reef base, between >9–14 m (Reef 4) (Fig. 1). Repeated visual inspection was employed for monitoring 20 m2 plots/habitat (4 replicate plots of 5 m2 per habitat ¥ 4 habitats = 80 m2). The following
parameters were determined by using a 1 m2 quadrat divided into 100 cells of 100 cm2 each: live
coral cover, coral diversity, and the number of new colonies appearing between sampling intervals.
New colonies were identified as sexual or asexual recruits when possible. Asexual propagation in
massive coral colonies commonly resulted from partial mortality, and in branching colonies from
fragmentation. The reefs were surveyed in January and July 1984 (following the 1982–83 ENSO),
January and August 1985 (before and after massive dinoflagellate blooms), August 1987 (during
the 1987 ENSO), February 1989, June 1992 (during the 1990–95 ENSO), December 1996, May
1998 (near the end of the 1997 ENSO) and in February 1999. Additionally, the number of colonies
that bleached and/or died during the 1992 warming event were haphazardly counted at the four reef
sites (Fig. 1). Changes in percent cover and number of colonies between the first and last censuses
(January 1985 and February 1999) were evaluated by Repeated Measures Analysis of Variance
using Sigma Stat® statistical software. This design was necessary due to repeated sampling of the
same reef plots (Underwood, 1981). Abundance estimates of coral colonies in the study areas were
indicated as: R (rare), seldom encountered; U (uncommon), sometimes present; C (common), usually present; and A (abundant), large numbers present.
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RESULTS
OCEANOGRAPHIC CONDITIONS: 1982–1999.—Beginning in September 1982, SSTs near
Caño Island (Fig. 2A) showed a 15 mo anomaly above +0.5∞C, reaching a maximum of
+1.5∞C between May and June 1983. These data coincided with predictions of El Niño
activity by JMA and SOI indices (Fig. 2B,C). This event that devastated coral reefs in the
eastern Pacific is considered one of the most intense ENSOs of the century (Quinn et al.,
1987; Hansen, 1990). Coral mortality at Caño Island was about 50% overall (Guzmán et
al., 1987; Glynn et al.,1988). Since that time, four major natural disturbances have affected Caño Island: phytoplankton blooms (severe), apparently related to La Niña cooling in 1985 (Guzmán et al., 1990), and warming events in 1987 (moderate), 1990–95
(long-term) and 1997–98 (very strong).
The phytoplankton blooms of 1985 are considered the most severe in Costa Rica since
the early 1970s (Guzmán et al., 1990). They occurred during intense upwelling in the Bay
of Panama and at a time of notable cooling in the eastern Pacific (Glynn, 1990). This
cooling episode caused severe damage to coral reefs in Costa Rica (see next section;
Guzmán et al., 1990). Before the occurrence of these blooms in the study area, temperature anomalies of -0.5∞C were detected around Caño Island between February and May
1985 (Fig. 2A). Sustained negative anomalies were also indicated by the JMA Index,
signifying La Niña activity (Fig. 2B). A pronounced surface water cooling was also detected by the JMA Index in 1988, indicating a possible La Niña event (Fig. 2B), however,
no unusually dense and widespread phytoplankton blooms were observed in the eastern
Pacific at that time.
A moderate El Niño occurred in 1987, associated with an unusual increase in SST
anomalies around the study area between June and the end of the year (Fig. 2A). The JMA
and SOI indices also indicated anomalies during this time (Fig. 2B,C). Bleaching and
coral mortality occurred during this period (see next section), which coincided with other
effects observed in the eastern Pacific (Reyes Bonilla, 1993; Glynn et al., 1996) and also
in the Caribbean region (Williams et al., 1987).
Several positive SST anomalies, of varied intensity, were recorded between 1989 and
1996. In 1990, an ENSO event of moderate and sustained intensity began, as indicated by
SST anomalies and ENSO indices (Fig. 2A–C). Positive anomalies persisted, some as
high as 0.7∞C from 1990 through 1995 (ENSO 1990–1995, see Trenberth and Hoar, 1996).
Coral bleaching occurred at Caño Island during this period (1992). However, bleaching
was not observed again until 1998, during the very strong 1997–98 ENSO event. This
warming disturbance was generally similar to the 1982–83 ENSO, but attained higher
and more frequent SST maxima (~31∞C) than in 1982–83 (30.5∞C) (Fig. 3). Also, the 2yr weekly mean SST was slightly higher during the 1997–98 event (29.2∞C, SD = 0.77, n
= 105) as compared to 1982–83 (29.0∞C, SD = 0.65, n = 104).
The 1997–98 ENSO is considered to be one of the two strongest El Niño events of the
century (McPhaden, 1999; Wilkinson et al., 1999; Enfield, this issue). At Caño Island, the
highest SSTs were observed between the middle and end of 1997, with an SST anomaly
of +1.6∞C, in agreement with the JMA Index (Fig. 2A,B). By May–June 1998, anomalies
greater than +1∞C were still observed and coral bleaching was intense, affecting all reefs
and species present (see next section).
CHANGES IN COMMUNITY STRUCTURE.—Table 1 summarizes the current (as of February
1999) relative abundances of 18 widespread eastern Pacific reef corals at Caño Island,
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Table 1. Current status (as of February 1999) of relative abundance estimates of zooxanthellate
scleractinian corals at Caño Island, Costa Rica. Qualitative abundance measures are based on
species contributions to reef frameworks and/or number of colonies in coral communities: R-rare,
U-uncommon, C-common and A-abundant.
Coral Species
Pocillopora damicornis (Linnaeus)
Pocillopora elegans Dana
Pocillopora eydouxi Milne-Edwards and Haime
Pocillopora capitata Verrill
Pocillopora meandrina Dana
Pocillopora inflata Glynn
Porites lobata Dana (Smooth Morph)
Porites lobata Dana (Knobby Morph)
Porites panamensis Verrill
Pavona clavus (Dana)
Pavona gigantea Verrill
Pavona varians Verrill
Pavona maldivensis (Gardiner)
Pavona frondifera (Lamarck)
Gardineroseris planulata (Dana)
Psammocora obtusangulata (Lamarck)
Psammocora superficialis Gardiner
Psammocora stellata (Verrill)
Category
A
A
U
R
U
R
A
A
A
A
A
A
C
U
C
U
C
A
based on several qualitative and haphazard surveys carried out since 1998. Nine coral
species are considered abundant, with numerous colonies present in most habitats. Only
two pocilloporid species (Pocillopora capitata, Pocillopora inflata) are rare. Large colonies of Pavona maldivensis (>1 m maximum dimension) are commonly found as encrusting sheets on the sides of rocks at various depths around the island. This species was
formerly identified as Pavona gigantea and has only recently been reported from Panama
(Holst and Guzmán, 1993) and Costa Rica (Cortés and Guzmán, 1998). Gardineroseris
planulata, considered to be an endangered species at several localities in the eastern Pacific (Glynn, 1997), is common. It builds small frameworks, consisting of 5–80 cm diameter colonies, on the eastern and northern sides of the island.
CORAL COVER.—After the 1982–83 ENSO, live coral cover at the Caño Island Biological Reserve in January 1984 was about 40% (Pocillopora spp.) on the shallow reef flats
(0–5 m). Relatively low Pocillopora spp. cover (about 2–8%) occurred at the reef base
(>9–14 m) (Fig. 4). The massive coral, P. lobata, growing as microatolls and encrusting
colonies, covered about 2% of the substrate at approximately 0–3 m, increasing to about
35% on the mid-reef slope (>5–9 m). P. lobata cover decreased to about 12% at the reef
base (>9–14 m), where large massive colonies occurred. Pavona spp. and Psammocora
spp. were rare in shallow habitats. In deeper habitats, however, Pavona clavus contributed
up to 4% of the substrate. After January 1984, Pavona spp. cover, mainly P. clavus, was
reduced by 50% due to predation by Acanthaster planci.
During June–August 1985, severe and recurrent phytoplankton blooms affected the
eastern Pacific for about 40 d, causing coral death of up to 100% on shallow reef areas in
Costa Rica and 13% in Panama (Guzmán et al., 1990). P. lobata cover declined slightly
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Figure 4. Variations in mean percent cover m–2 (and SE) over a 15 yr period (1984–99) for the four
most abundant reef-building coral taxa at Caño Island. Species identities are Pocillopora damicornis,
P. elegans, Pavona clavus, P. gigantea, P. varians, Psammocora superficialis and P. stellata. Only
Pocillopora spp. and Porites lobata were present at all four depths. The survey in August 1985 was
conducted at the end of dinoflagellate blooms and surveys in June 1992 and May 1998 were performed
during ENSO bleaching events.
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Figure 5. Variations in total number of colonies over a 15 yr period (1984–1999) per 20 m2 for the
four most abundant coral taxa at Caño Island. Explanatory notes as in Figure 4.
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141
(<5%) in all habitats down to 14 m, while Pavona spp. (P. clavus) showed a slight increase
in cover at >9–14 m (Fig. 4).
The brief bleaching event in 1987 affected mostly P. lobata at >5–9 m, resulting in a
loss of 70% cover (Fig. 4). The live cover of most other corals at all depths remained
relatively stable or increased, as did species of Psammocora. The reef community changed
relatively little between 1987 and 1992. No coral mortality was observed and no net
change in coral cover occurred during this event.
In 1996, a small increase in coral cover was observed in shallow environments
(Pocillopora spp. and P. lobata) and also in deeper (>9–14 m) habitats (P. clavus and
Psammocora spp.). In mid-depth habitats, no notable change was observed in these coral
taxa (Fig. 4).
All coral colonies (100%) bleached at all reef sites during the early months of 1998.
This extensive bleaching event coincided with a notable increase in SSTs of ~30∞ to
30.5∞C (Fig. 3). Even though SSTs were greatly elevated, slightly higher than during a
comparable period in 1983, mortality was only about 2% in Pocillopora spp. (P. damicornis
and P. elegans) at >3–5 m and >9–14 m, about 5% in P. lobata (>5–9 m) and 1% in
Psammocora spp. (P. superficialis and P. stellata) at >9–14 m (Fig. 4).
NUMBERS OF COLONIES.—At Caño Island, 100% of the Pocillopora spp. colonies died on
the reef flat and shallow forereef (down to 5 m), due to the phytoplankton blooms of 1985,
while those at >9–14 m remained unchanged (Fig. 4). Guzmán et al. (1990) attributed this
coral mortality mainly to smothering by mucus and oxygen reduction due to high phytoplankton population densities. This was reflected in the plots of colony numbers (Fig.
5).
Between August 1985 and August 1987, the number of P. lobata colonies increased due
to sexual recruitment in shallow water (>3–5 m) and to partial mortality of surviving
colonies at depths of >5–14 m (Fig. 5). However, after 1987 P. lobata was repeatedly
damaged by the damselfish Stegastes acapulcoensis, producing a pattern of live and dead
patches on large colonies, and thereby increasing colony numbers, especially at >5–9 m.
During this period, the number of colonies of all other species also increased. A pulse of
sexual recruitment of Pocillopora spp. (dominantly P. elegans) was observed with a larger
increase in colony numbers at >9–14 m than at >3–9 m. Pocillopora spp. (notably P.
damicornis) showed a slight increase in colony abundance at very shallow depths (0–3 m)
only. Similarly, an increase in the number of colonies of Psammocora spp. at >9–14 m
was a result of sexual recruitment (Fig. 5). P. clavus numbers increased at >9–14 m due to
partial mortality, and a few sexual recruits of Pavona varians colonized substrates at >5–
9 m.
During the 1990–1995 warming event, 34–92% of the colonies in seven coral taxa
were bleached (Table 2). Pavona gigantea and P. varians were most affected, with 91.6
and 81.8% of their colonies bleached, respectively. Small colonies of P. lobata, resulting
from partial mortality, were affected at >5–9 m as were sexual recruits of Psammocora
spp. and P. elegans in deeper waters (Fig. 5).
During the 1996 survey, many similar-sized sexual recruits of both P. damicornis and P.
elegans (6–8 cm in diameter) were observed on all shallow reef substrates as well as in
deeper habitats. Numerous sexual recruits of Pocillopora spp. (mainly P. elegans) and
Psammocora spp. were present at >9–14 m (Fig. 5). Judging by the size of these recruits
(<10 cm in diameter) and their growth rates (Guzmán and Cortés, 1989b), it is possible
that a recruitment pulse occurred between 1993 and 1994. The abundances of sexual
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Table 2. Percent number of colonies bleached (total number of colonies assessed in parentheses)
in seven coral taxa during the 1992 warming event at four reefs, Caño Island, Costa Rica. Total
percent summary column shows the overall mean ± 1 SE, and the numbers of colonies (in
parentheses) affected at all four sites. NP denotes species not present. See Figure 1 for location
of reef sites. Reef* denotes a coral reef located at the NW corner of the island.
Coral Species
Porites lobata
Psammocora spp.
Pocillopora spp.
Gardineroseris planulata
Pavona varians
Pavona clavus
Pavona gigantea
Reef*
60.0 (90)
45.6 (57)
19.3 (57)
91.3 (23)
90.0 (20)
90.5 (21)
91.6 (24)
Reef 2
41.1 (158)
38.5 (39)
39.8 (173)
62.5 (8)
NP
38.9 (59)
NP
Reef 3
44.2 (138)
35.0 (60)
39.6 (134)
76.9 (13)
73.7 (19)
34.2 (38)
NP
Reef 4
25.5 (51)
NP
39.1 (69)
NP
NP
13.5 (37)
NP
Total percent
42.7 ± 6.1 (437)
39.7 ± 2.0 (156)
34.4 ± 4.3 (433)
76.9 ± 6.8 (44)
81.8 ± 5.7 (39)
44.3 ± 14.2 (155)
91.6 ± 0.0 (24)
recruits of P. lobata also showed slight increases at intermediate (>3–5 m) and deep (>9–
14 m) habitats. The great increase in colony abundance of P. lobata at >5–9 m was due to
partial mortality caused by damselfish. Although an increase in cover was observed in
Psammocora spp., the number of colonies was greatly reduced (<25) at deeper sites after
1996.
During the first months of 1998, the number of colonies increased slightly in all species except for Psammocora spp. and P. varians, which declined through the last survey
(Fig. 5).
STATISTICAL SYNTHESIS.—Changes in coral community structure over the course of this
study (all species in each depth range, and all habitats pooled) are summarized in Figure
6. Coral cover was reduced by 95% in shallow reef areas (0–5 m) by late 1985. No
significant recovery in total coral cover was evident between January 1984 (first survey
after ENSO 1982–83) and February 1999 (most recent survey, after ENSO 1997–98) in
shallow habitats (0–3 m, >3–5 m) (one-way repeated measures ANOVA; F = 67.8, P =
0.004 and F = 276.1, P < 0.001, respectively). However, there was a significant increase
during this same period and at the same depth ranges in the number of new colonies due
to sexual recruitment (F = 98.2, P < 0.01; F = 9.5, P = 0.05, respectively). No significant
changes in coral cover occurred in the lower reef slope (>5–9 m) (F = 4.4, P = 0.12),
however, there was a significant increase in the number of asexual recruits due to partial
mortality of the dominant species, P. lobata (F =169.2, P < 0.01). In terms of live cover,
deep (>9–14 m) reef zones remained relatively stable between 1984 and 1999 (F = 0.08,
P = 0.786), even though there were significant changes in the number of colonies (F =
38.6, P < 0.01). When corals present at all depths are pooled, coral cover in 1999 was
significantly lower than in 1984, demonstrating that recovery had not attained 1984 levels (F = 24.2, P < 0.001). However, recruitment is occurring since significantly high
numbers of sexual recruits (F = 20.4, P < 0.001) have been identified within the 20 m2
plots (excluding Reef 3): 45 (Pocillopora spp.), 15 (Pavona spp.) and 50 (Psammocora
spp.) colonies.
DISCUSSION
CORAL REEF RECOVERY.—Reef recovery after severe disturbances is affected by complex physical and biological processes, e.g., the regenerative capability of surviving colonies, the presence of healthy nearby source populations, larval dispersal capabilities and
GUZMÁN AND CORTÉS: CHANGES IN REEF STRUCTURE IN PACIFIC COSTA RICA
143
reproductive ecologies (e.g. sexuality, dispersal mode, seasonality) (Pearson, 1981; Colgan,
1987; Richmond and Hunter, 1990; Roberts, 1997; Hughes et al., 1999). The term ‘recovery’ can be defined in several ways. For example, recovery of original coral cover, species
abundances or diversity can be distinguished from recovery of the reef framework structure itself, especially if erosion is substantial. Therefore, recovery of reef frameworks
may take from decades to centuries to occur (Pearson, 1981; Colgan, 1990; Guzmán and
Cortés, 1992; Reaka-Kudla et al., 1996, but see Eakin, 1996). Biogeographic differences
are also important in reef recovery. For example, some reefs in the Indian Ocean have
attained 50% of their original cover only 5 yrs after the 1982–83 ENSO (Brown and
Suharsono, 1990), while others on the Great Barrier Reef have demonstrated no signs of
recovery after a decade (Done,1992).
Recovery of live coral cover on eastern Pacific reefs after the 1982–83 ENSO has been
limited at best (see review by Guzmán and Cortés, 1993). This has been due to: (1) the
extreme conditions characteristic of eastern Pacific coral reef environments (Glynn, 1977;
Guzmán and Cortés, 1993; Cortés, 1997); (2) the high coral mortality experienced in this
region after the 1982–83 ENSO (50–100%) (Glynn 1984, 1985b; Guzmán et al., 1987;
Glynn et al., 1988); (3) the intense herbivory by concentrations of sea urchins that have
caused high rates of reef bioerosion (Glynn, 1988, 1990; Colgan, 1990; Eakin, 1996;
Reaka-Kudla et al., 1996); (4) the low abundances of sexual recruits of reef-building
coral species, even though sexually mature colonies exist nearby on most reefs (Glynn et
al., 1991, 1994, 1996); and (5) possibly the diminished potential of recruitment success
due to the low abundance of crustose coralline red algae. The abundance of crustose
coralline red algae has been shown to be associated with enhanced settlement in a Caribbean coral species (Morse, 1990). Eastern Pacific reefs have been reported to lack an
abundance of coralline algal cover (Glynn and Ault, 2000; but see Guzmán and Cortés,
1989a).
Not all of these factors are important to coral recruitment on Caño Island reefs. Although the reefs at Caño Island and at other locations in the eastern equatorial Pacific
were exposed to several types of natural disturbances during the past 15 yrs (disturbance
frequency, every 2–5 yrs), evidence presented here indicates that coral populations in
southern Costa Rica were not affected so severely as elsewhere. First, coral mortality was
greater in Panama (Gulf of Chiriquí) and Ecuador (Galápagos Islands and mainland Ecuador) than in Costa Rica after the two strongest ENSO events of the century (Glynn et
al., this issue). Coral mortality at Caño Island was about 52% after the 1982–83 ENSO
(Guzmán et al., 1987) and only about 5% after the 1997–98 event (Fig. 6). A more gradual
warming occurred around Caño Island during 1982–83, which was of lower intensity
than at other reef sites in the equatorial eastern Pacific, such as the Galápagos Islands
(Glynn et al., 1988; this study). This is also supported by proxy stable isotopic analyses
(d18O), which indicate relatively low SSTs during skeletogenesis at Caño in 1982–83
(Carriquiry et al.,1994). Second, population densities of the sea urchins Diadema
mexicanum and Eucidaris thouarsii are low at Caño Island as compared with other reefal
regions (Guzmán, 1988; Guzmán and Cortés, 1993), therefore, bioerosion of the reef
structure from this source is not great. Third, sexual recruitment is occurring at Caño
Island, which has harbored relatively stable, deep water (9–14 m) coral populations over
the last 15 yrs. These deep water corals are a likely source of new recruits to shallow reef
zones. Fourth, SSTs are more stable off southwestern Costa Rica than at other EEP locales, and the reproductive seasonality of corals is longer (Glynn et al., 1994, 1996, 2000).
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Figure 6. Variations in community structure over a 15 yr period (1984–99) for all reef building coral
taxa combined, Caño Island. Symbols are sums of mean values of all taxa per m2 of percent coral
cover (A) and number of coral colonies (B) at the various sampled depths. Error bars are too large
to be shown. Widths of the shaded areas approximate the lengths of disturbances. The 1990–1995
warming period is according to Trenberth and Hoar (1996).
And finally, more crustose coralline algal cover is present at Caño Island than at other
eastern Pacific reef locales (Glynn and Wellington, 1983; Guzmán and Cortés, 1989a),
thus perhaps facilitating larval settlement (Morse, 1990). Crustose coralline algal cover
sampled on the reef flat at Caño Island commonly ranged from 80–95%, indicative of
relatively high cover for the eastern Pacific.
In spite of relatively high coral mortality (~52% overall) in 1982–83, Caño Island reef
flats still supported a high cover of mainly pocilloporid corals, between 30–40% at 0–5 m
depth (Fig. 4). High coral cover on the reef flat at Caño Island in 1984, at the beginning of
this study, suggested previous successful recruitment. Recovery of P. damicornis and P.
elegans, the major reef-building corals at Caño, differed among habitats. Deep denuded
GUZMÁN AND CORTÉS: CHANGES IN REEF STRUCTURE IN PACIFIC COSTA RICA
145
substrates were recolonized by P. elegans (late 1987), while P. damicornis was not observed in its former shallow water habitat until early1996. The sizes of pocilloporid colonies observed in 1996 suggested a 1993–1994 recruitment pulse, based on an extrapolation from the growth rates of these species at Caño Island (Guzmán and Cortés, 1989b).
Sexually derived recruits are now present in the reef flat habitat. The sexual recruitment
of almost all species was observed in mid- to deep habitats (>3–14 m), beginning in late
1987. P. lobata was an exception with its abundance increasing at >5–9 m due to partial
mortality by damselfish (Figs. 4,5). At other depths, P. lobata colony numbers remained
stable.
Two possible sources of larvae are proposed: (1) from local populations or (2) from
more distant populations within the EEP. The potential for Caño Island reefs to self-seed
is supported by reproductive studies, which have demonstrated the presence of reproductively active colonies of the major reef-building species (Glynn et al., 1991, 1994, 1996).
However, even though extensive patches of sexually mature populations exist throughout
the EEP, with sex ratios approximately 1:1, little or no sexual recruitment has been observed at most monitored sites at Caño Island, in Panama or the Galápagos Islands. Therefore, conditions may favor local and regional populations to self-seed or recruit from
greater distances, however, other pre- or post-settlement parameters may result in recruitment failure. At Caño Island, corals were exposed to the environmental disturbances
(ENSO) at the same frequency as the rest of the EEP. Perhaps sub-lethal effects within the
EEP, due to stressful warming, may have temporarily impaired the reproductive capacity
on distant reefs (sensu Szmant and Gassman, 1990). This has been experimentally demonstrated for P. damicornis in Panama (Glynn and D’Croz, 1990). Corals have been shown
to be sexually active at Caño more often than at other EEP regions, however, recruitment
in most cases is still far lower than that reported for central and western Pacific regions
(Harrison and Wallace, 1990, and references therein).
Another possible source of larvae is from distant regions, e.g., the central Pacific (Richmond, 1990; Glynn, 1997). This hypothesis is supported by the sporadic occurrence of
organisms such as fishes, mollusks and sea urchins within the EEP from other Pacific
regions (see Guzmán and Cortés, 1993; Lessios et al., 1996; Cortés, 1997; Glynn, 1997).
Recruitment may coincide with putative larval pulses from central oceanic islands of the
Pacific (Line Islands or Cocos Island). The Equatorial Countercurrent flow is accelerated
during ENSO events, during which the transit time for larvae is reduced by one-half or
more (Richmond, 1990; Glynn, 1997; Glynn and Ault, 2000). Coral recruitment was observed at Caño Island during the ENSO years (1987, 1992) or shortly thereafter. To determine the source of coral larvae, whether from local, regional or trans-Pacific areas, population genetic studies could offer insight into this issue. We are currently working on the
origin of coral sexual recruits within shallow- and deep-reef habitats at Caño Island. Our
preliminary studies in Panama demonstrate that gene flow occurs between local populations of P. damicornis.
LONG-TERM CHANGES IN REEF COMMUNITY STRUCTURE.—The reefs at Caño Island have
not been notably affected in species composition or diversity by several severe impacts
experienced during the last 15 yrs. Although recruits of P. varians and Psammocora spp.
were not observed until 1985 in the study plots, both species were always present on the
reefs. They seem to be opportunistic and are the first to colonize denuded areas, as also
observed in Colombia (Guzmán and López, 1991) and at Cocos Island, Costa Rica (Guzmán
and Cortés, 1992). While populations of some species have fluctuated greatly in abun-
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dance (e.g., G. planulata), no species has gone extinct at Caño Island during the study
period. In our opinion, no zooxanthellate coral extinctions have occurred in the eastern
Pacific as a result of ENSO disturbances as suggested by other workers (Glynn, 2000;
Glynn and Ault, 2000). Some coral species with extremely small populations, e.g.,
Siderastrea glynni (Budd and Guzmán, 1994), might be threatened by ENSO activity or
other disturbances.
Significant temporal and spatial variations were observed in the abundance of recovering coral species at Caño Island. However, absence from the monitoring plots does not
mean that certain species are locally endangered since colonies may be present in other
reef habitats (see Table 2).
Cortés et al. (1994) have found that coral species abundances change over time, but
generally the same suite of species occupy given habitats over millennial-scale periods.
Over a time scale of several years, species relative abundances have fluctuated widely in
our study plots, but in general the same coral species tend to recolonize the same reef
areas. Indeed, we have observed that the reefs at Caño Island continue to be built primarily by the same massive and branching corals, but now seem to be more tolerant or better
adapted to marked, positive thermal excursions. Even though higher SST values occurred
during the 1997–98 ENSO compared with the 1982–83 event (Fig. 3), adult and juvenile
coral mortality was significantly lower more recently, supporting the hypothesis of local
selection of more resistant genotypes (sensu Buddemeier and Fautin, 1993). In the future,
however, the composition and structure of eastern tropical Pacific coral reefs may change
if predictions of increasing intensity and frequency of ENSO disturbances are realized
(Timmermann et al., 1999).
In summary, recovery to the previous overall level of coral cover (~32%) on Caño
Island reefs just after the 1982–83 ENSO event has not yet been attained. Recovery of
coral cover to 1984 levels, in our study plots, is also not yet statistically significant. However, reefs on the north side of Caño Island, not represented in our sampling plots, presently contain approximately 70% coral cover. Therefore, we propose that the process of
recovery is occurring and is only noticeable in some areas. Natural disturbances, such as
phytoplankton blooms that affected Pocillopora spp. in all habitats (Guzmán et al., 1990),
has interfered with reef regeneration, thus contributing to multiple disturbance impacts
(Glynn, 1990; Guzmán, 1991; Cortés, 1997).
ACKNOWLEDGMENTS
We thank the Costa Rican National Conservation Area System for logistical support and permission to work at the Caño Island Biological Reserve, and all resident park rangers for their kindness
and constant support during all these years. Thanks are due C. Jiménez, A. León, E. Ruiz and O.
Breedy for their help in the field; and R. Steller for providing transportation to the island. We thank
P. W. Glynn, S. B. Colley, and two anonymous reviewers for their comments and improvements to
the manuscript. This study was partially sponsored by grants from the Smithsonian Tropical Research Institute to H. M. Guzmán, from the Universidad de Costa Rica and CONICIT (Project 90326-BID) to J. Cortés, and from the National Science Foundation to P. W. Glynn (OCE-8415615,
8716726 and 9018392).
GUZMÁN AND CORTÉS: CHANGES IN REEF STRUCTURE IN PACIFIC COSTA RICA
147
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ADDRESSES: (H.M.G.) Smithsonian Tropical Research Institute, P.O. Box 2072, Balboa, Ancón, Republic of Panama. (J.C.) Centro de Investigación en Ciencias del Mar y Limnología (CIMAR), and Escuela
de Biología, Universidad de Costa Rica, San Pedro, San José 2060, Costa Rica. CORRESPONDING AUTHOR: (H.M.G.) Smithsonian Tropical Research Institute, Unit 0948, APO AA 34002-0948. E-mail:
<guzmanh@naos.si.edu>.