Combined Effects of Three Sequential Storms on the Huatulco Coral Reef Tract, Mexico
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BULLETIN OF MARINE SCIENCE, 69(1): 267–278, 2001 COMBINED EFFECTS OF THREE SEQUENTIAL STORMS ON THE HUATULCO CORAL REEF TRACT, MEXICO Diego Lirman, Peter W. Glynn, Andrew C. Baker and Gerardo E. Leyte Morales ABSTRACT Reefs of the Huatulco area, southern Mexico, were exposed to an unprecedented se- quence of three major storms (Olaf, Pauline, and Rick) over a 2-mo period (September– November 1997). The prior establishment of monitoring transects, as well as the timing of our surveys just 1 mo after the passage of Hurricane Rick, provided an unique oppor- tunity to document storm impacts on these recently described reef communities of the eastern Pacific. Considering the lack of prior hurricane damage to these reefs, the domi- nance of branching pocilloporid corals, and the intensity and high frequency of the 1997 storms that affected the area, it was hypothesized that storm-generated damage patterns would be significant and consistent among the reefs of Huatulco. However, the damage patterns documented were limited in severity and variable in spatial distribution. Of the six reefs surveyed prior to the first storm in July–August 1997, only three showed signifi- cant decreases in live coral cover, whereas the remaining reefs showed slight, non-sig- nificant increases in coral cover between surveys. The most common type of damage observed was the fragmentation of Pocillopora spp. colonies; at some locations, the den- sity of surviving Pocillopora spp. fragments exceeded 20 m–2. Fragmentation of the mas- sive coral Pavona gigantea (Verrill) was observed only at a single site. At several sites, large sections of reef framework (up to 245 cm in diameter) still exhibiting live Pocillopora spp. colonies on their upper surfaces were detached and transported away from their original locations. Even if the immediate damage observed was significantly less than predicted in light of the physical characteristics of the storms, the long-term effects of these storms will depend on the survivorship of detached colonies and fragments, the regeneration of damaged colonies, and the future impacts of bioerosion. The negative, and often catastrophic effects of El Niño–Southern Oscillation (ENSO) disturbances on the coral reefs of the world have been well documented. As a result of the 1982–1983 ENSO, there was significant coral mortality throughout the equatorial east- ern Pacific (Glynn, 1984, 1990, 2000). Similarly, the 1997–1998 ENSO, which was ranked as the strongest event in recorded history (Enfield, this issue), resulted in bleaching and coral mortality at local and regional levels across the tropical oceans (Wilkinson, 1998; Goreau et al., 2000). Although the main mechanisms of coral stress and mortality during ENSO events are elevated sea surface temperatures (SST) and changes in irradiance lev- els (Glynn and D’Croz, 1990; Gleason and Wellington, 1993; Brown, 1997), other ENSO– associated stressors such as storm damage, elevated nutrients, sedimentation, aerial ex- posure, and algal blooms are also known to cause coral mortality (Glynn, 2000). In this study, we document the effects of storm-generated physical disturbance on the reefs of Huatulco, Mexico. Altered storm paths during the 1997–1998 ENSO exposed the reefs of this area to an unprecedented sequence of physical disturbances where three major storms (Olaf, Pauline, and Rick) made landfall on the southern Mexican coast over a 2-mo period (September–November 1997). The prior establishment of transects on sev- eral of the affected reefs, as well as the timing of our surveys just one month after the passage of Hurricane Rick, provided an unique opportunity to document storm impacts 267 268 BULLETIN OF MARINE SCIENCE, VOL. 69, NO. 1, 2001 Figure 1. Tracks of the three storms that impacted the Huatulco area in the latter part of 1997. Tropical Storm Olaf (September 26–October 12; maximum sustained winds = 110 kph; maximum wind gusts = 140 kph), Hurricane Pauline (5–10 October; 215 kph; 260 kph), and Hurricane Rick (7–10 November; 140 kph; 170 kph). on these recently described reef communities of the eastern Pacific (Glynn and Leyte Morales, 1997). The effects of major storms on live coral cover, species diversity, and reef morphology are well documented (e.g., Stoddart, 1963; Glynn et al., 1964; Woodley et al., 1981; Done, 1992; Witman, 1992; Harmelin-Vivien, 1994). The physical characteristics of each storm as well as the timing and sequence of disturbances are key factors in describing damage patterns (Lirman and Fong, 1997a). The disturbance history of an area has also been shown to play a major role in determining damage and recovery patterns (Hughes, 1989). In general, reefs that have not been exposed to major physical disturbances for prolonged periods are believed to be more susceptible to storms as reef complexity and the abun- dance of less robust, fast-growing coral species with high susceptibility to physical dam- age proliferate (Porter et al., 1981; Rogers, 1993; Aronson and Precht, 1995). For ex- ample, the devastating effects of Hurricane Allen on Jamaican reefs in August 1980 have been attributed to the high abundance of fragile branching species such as Acropora cervicornis (Lamarck) that developed during a 36-yr period without major storm impacts in that area (Woodley et al., 1981; Woodley, 1992). Similarly, a sequence of strong storms over a short time period can have compounding effects on reef communities. In Florida, three major storms impacted reefs over a 2-yr period, and damage patterns documented after Hurricane Andrew (August 1992), were exacerbated during two subsequent storms (Lirman and Fong, 1997a). In this case, the additional damage was attributed in part to the remobilization of coral fragments during subsequent storms (Lirman and Fong, 1997a,b). LIRMAN ET AL.: HUATULCO REEF STORM DAMAGE 269 METHODS The 1997 tropical storm season in the eastern Pacific was very active, with a total of 17 named tropical storms and hurricanes. More notable for the region, however, was the unprecedented se- quence of major storms that made landfall along the coast of Mexico in the latter part of the season (Fig. 1). Hurricane Nora (16–25 September; maximum sustained winds = 205 kph; maximum wind gusts = 260 kph), which affected the Baja California region, and Tropical Storm Olaf (26 Septem- ber–12 October; 110 kph; 140 kph), Hurricane Pauline (5–10 October; 215 kph; 260 kph), and Hurricane Rick (7–10 November; 140 kph; 170 kph), which affected the southern Mexican states of Oaxaca and Guerrero, resulted in severe flooding and coastal destruction [Fig. 1; Mexican Na- tional Weather Service (http://smn.can.gob.mx); U.S. National Hurricane Center (http:// nhc.noaa.gov)]. Land observations during the storms indicated variable wind and swell direction for each storm (Leyte Morales, pers. obs.). Swells from Tropical Storm Pauline and Hurricane Olaf impacted the Huatulco coastline from a SE direction. In contrast, swells from Hurricane Rick origi- nated from a NW direction. The reefs of the Huatulco area were described in detail previously by Glynn and Leyte Morales (1997). To determine the severity of hurricane damage, reefs in the Huatulco area were surveyed on 12–22 December 1997, only 1 mo after Hurricane Rick (Fig. 2). Changes in percent cover of live coral were evaluated on six reefs (Cacaluta, Chachacual, Entrega, Dos Hermanas, San Agustín, Montosa) that were initially surveyed in July–August 1997, 2 mo prior to the landfall of Tropical Storm Olaf. Survey transects were established perpendicular to the main axis of each reef along a depth contour (from the upper to the bottom edge of each reef), and 1 m2-quadrats were flipped on- Figure 2. Map of the Huatulco area with the location of reefs surveyed on December 1997. 270 BULLETIN OF MARINE SCIENCE, VOL. 69, NO. 1, 2001 end along the transects to estimate live coral cover. Quadrats were divided into 100 subdivisions and the number of subdivisions occupied by live coral cover was visually estimated. The number of quadrats surveyed along each belt varied according to the width of each reef. Data from all quadrats were combined for each reef. In addition, the abundance of coral fragments and detached coral colonies were assessed on six reefs where such damage patterns were observed (Riscalillo, Mixteca, Órgano, Rincón Sabroso, Montosa, Maguey). The abundance of live Pocillopora fragments was quantified within transects placed along the depth contour of these reefs. Furthermore, to quantify the transport of detached colonies and fragments, transects were surveyed along the bottom edge on sandy substrata on three reefs (Órgano, Rincón Sabroso, Maguey). The abundance of live Pocillopora colonies and frag- ments were quantified within 1 m2 quadrats along these transects. Although storm damage was most evident on the branching pocilloporid corals that predominate on the reefs sampled, damage to the massive coral Pavona gigantea was noted on Riscalillo reef. Here, surveys were carried out to quantify the abundance and size of attached and detached P. gigantea colonies, as well as the percent live and dead tissue on both attached and dislodged colo- nies. The abundance of P. gigantea was quantified within three parallel transects located along the depth contour of Riscalillo Reef, as well as within a transect located along the bottom edge of the reef on sandy substratum. The maximum diameter of each P. gigantea colony was measured with a flexible tape and the percent live and dead tissue area were visually estimated. The data were examined for conformity to the assumptions of each statistical test prior to analy- sis. Normality was tested with the Shapiro-Wilk test and homoscedasticity was tested with the Bartlett’s test. When violations of the normality assumptions were encountered for percent cover estimates, and arcsine transformations (arcsine ÷x) were not successful, non-parametric statistical methods were employed for comparisons instead (Sokal and Rohlf, 1981).