The 1998 Bleaching Event and Its Aftermath on a Coral Reef in Belize
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Marine Biology (2002) 141: 435–447 DOI 10.1007/s00227-002-0842-5 R.B. Aronson Æ W.F. Precht Æ M.A. Toscano K.H. Koltes The 1998 bleaching event and its aftermath on a coral reef in Belize Received: 14 November 2001 / Accepted: 13 March 2002 / Published online: 1 June 2002 Ó Springer-Verlag 2002 Abstract Widespread thermal anomalies in 1997–1998, early fall of 1998 were extraordinarily warm compared due primarily to regional effects of the El Nin˜ o–South- to other years. The lettuce coral, Agaricia tenuifolia, ern Oscillation and possibly augmented by global which was the dominant occupant of space on reef warming, caused severe coral bleaching worldwide. slopes in the central lagoon, was nearly eradicated at Corals in all habitats alongthe Belizean barrier reef Channel Cay between October 1998 and January 1999. bleached as a result of elevated sea temperatures in the Although the loss of Ag. tenuifolia opened extensive summer and fall of 1998, and in fore-reef habitats of the areas of carbonate substrate for colonization, coral outer barrier reef and offshore platforms they showed cover remained extremely low and coral recruitment was signs of recovery in 1999. In contrast, coral populations depressed through March 2001. High densities of the sea on reefs in the central shelf lagoon died off catastroph- urchin Echinometra viridis kept the cover of fleshy and ically. Based on an analysis of reef cores, this was the filamentous macroalgae to low levels, but the cover of an first bleaching-induced mass coral mortality in the cen- encrustingsponge, Chondrilla cf. nucula, increased. tral lagoon in at least the last 3,000 years. Satellite data Further increases in sponge cover will impede the for the Channel Cay reef complex, the most intensively recovery of Ag. tenuifolia and other coral species by studied of the lagoonal reefs, revealed a prolonged pe- decreasingthe availability of substrate for recruitment riod of elevated sea-surface temperatures (SSTs) in the and growth. If coral populations are depressed on a late summer and early fall of 1998. From 18 September long-term basis, the vertical accretion of skeletal car- to 1 October 1998, anomalies around this reef averaged bonates at Channel Cay will slow or cease over the +2.2°C, peakingat 4.0 °C above the local HotSpot comingdecades, a time duringwhich global-warming threshold. In situ temperature records from a nearby site scenarios predict accelerated sea-level rise. corroborated the observation that the late summer and Communicated by P.W. Sammarco, Chauvin Introduction R.B. Aronson (&) Hurricanes, disease outbreaks, bleaching, and various Dauphin Island Sea Lab, 101 Bienville Boulevard, disturbances and stresses due to human activities have Dauphin Island, AL 36528, USA killed corals throughout the Caribbean over the last E-mail: [email protected] 25 years (Ginsburg1994; Williams and Bunkley- R.B. Aronson Williams 2000; references in Aronson and Precht 2001). Department of Marine Sciences, At the same time, herbivorous fishes have been reduced University of South Alabama, Mobile, AL 36688, USA on some Caribbean reefs by human exploitation, and the W.F. Precht echinoid Diadema antillarum experienced >90% mor- PBS&J, 2001 Northwest 107th Avenue, tality from disease throughout the region in 1983–1984 Miami, FL 33172, USA (Hay 1984; Lessios 1988). Coral mortality has in general M.A. Toscano been followed by the proliferation of fleshy and filamen- National Oceanic and Atmospheric Administration, tous (non-coralline) macroalgae, because populations of NOAA/NESDIS/ORA/ORAD E/RA31, SSMC3 Room 3608, 1315 East-West Highway, herbivores have not been able to keep pace behaviorally Silver Spring, MD 20910, USA or numerically with algal growth in the large areas K.H. Koltes of space opened by the death of corals (Hughes 1994; Office of Insular Affairs, MS 4328, Steneck 1994; Szmant 1997; Aronson and Precht 2000, Department of the Interior, Washington DC 20240, USA 2001; McCook et al. 2001; Williams and Polunin 2001). 436 Widespread coral bleachingin response to anoma- widest, is the best-studied of the rhomboid shoals. lously high summer temperatures has become more Several investigators have cored this reef extensively frequent since the early 1980s (Glynn 1993; Goreau and (reviewed in Aronson and Precht 1997), and two of us Hayes 1994; Hoegh-Guldberg 1999; Williams and (R.B.A. and W.F.P.) have been conductingecological Bunkley-Williams 2000; Wellington et al. 2001a). The surveys there since 1986. Qualitative observations of role of high levels of incident solar radiation in these the ecology of Channel Cay date to the early 1970s bleachingevents is complex and not well understood (I.G. Macintyre, personal communication). (Dunne and Brown 2001; Fitt et al. 2001). Bleaching- The rhomboid shoals grew to sea level over the last induced mass mortalities of corals and other zooxan- 8,000–9,000 years, followingthe floodingof the central thellate reef organisms have occurred several times and sector of the Belizean shelf (Precht 1993; Burke 1994; at a number of localities in the Indo-Pacific, in at least one Aronson et al. 1998; Macintyre et al. 2000). The maxi- case leadingto the local elimination of two species (Oliver mum measured vertical accretion rate for Channel Cay, 1985; Glynn 1988; Glynn and de Weerdt 1991; Brown and 8 m/1,000 years, is high compared to other Caribbean Suharsono 1990; Brown 1997; Wilkinson 2000; Glynn reefs (Macintyre et al. 1977; Westphall 1986). Because et al. 2001; Riegl 2002). In contrast, bleaching episodes the rhomboid shoals are situated in a low-energy envi- on reefs in the western Atlantic–Caribbean region have ronment, there is little to no submarine cementation (see until now been followed by recovery of most of the Purser and Schroeder 1986; Macintyre and Marshall affected coral colonies (Lasker et al. 1984; Porter et al. 1988). The Holocene deposits that underlie the living 1989; Williams and Bunkley-Williams 1990; Langet al. communities consist primarily of interlockingskeletons 1992; McField 1999). In 1997–1998 the highest sea- of the staghorn coral Acropora cervicornis packed in fine surface temperatures ever recorded, related to the sediment (Aronson and Precht 1997). Debris fans at the El Nin˜ o-Southern Oscillation (ENSO) and possibly bases of the outer flanks (22–30 m water depth) suggest enhanced by global warming (Hansen et al. 1999; Mann occasional storm disturbance; however, Hurricane Greta et al. 1999; Karl et al. 2000; Lough 2000; Enfield 2001), in September 1978, the last major storm in Belize prior were associated with severe coral bleachingand subse- to 1998, had no discernible long-term effect on the living quent mortality in many areas of the world (Wilkinson community at Channel Cay (Westphall 1986). et al. 1999; Goreau et al. 2000; Wilkinson 2000; Glynn Before the late 1980s, the communities inhabitingthe et al. 2001; Wellington et al. 2001a). outer flanks of Channel Cay and the other rhomboid On reefs in the central sector of the Belizean shelf shoals were dominated by Ac. cervicornis (>70% live lagoon, positive thermal anomalies during the La Nin˜ a cover of Ac. cervicornis in some places) from 3–15 m phase of the ENSO cycle in 1998 resulted in the most depth (Aronson and Precht 1997). Agaricia tenuifolia extensive bleaching-related mass mortality of sclerac- and other species of lettuce coral of the family Agaric- tinian corals recorded in the Caribbean to date, with iidae were subdominant components in that depth nearly 100% of the coral colonies completely killed by range, and they dominated the benthos below 15 m. early 1999 (Aronson et al. 2000). Paleoecological records Duringthe 1980s, white-band disease (WBD) nearly from cores extracted from the Belizean reefs suggest that eliminated the Ac. cervicornis populations in the shelf this mass mortality was unprecedented in at least the last lagoon, as well as on the outer barrier reef and in the 3,000 years (Aronson et al. 2000, 2002). As with the lagoon at Glovers Reef, an atoll-like carbonate platform earlier trends to increased coral mortality elsewhere in seaward of the barrier reef (McClanahan and Muthiga the Caribbean, the collapse of coral populations on 1998; Aronson and Precht 2001). Ac. cervicornis colonies lagoonal reefs in Belize in 1998–1999 opened extensive killed by WBD collapsed rapidly, due to the weakening areas of substrate for colonization. Unlike the situation effects of bioerosion. on other Caribbean reefs, however, herbivores contin- In the lagoon at Glovers Reef, fleshy and filamentous ued to control macroalgal cover. In this paper we doc- macroalgae colonized patch reefs formerly occupied by ument the thermal conditions in 1998 that led to large stands of Ac. cervicornis (this also happened in fore- bleachingon a well-studied reef in the Belizean shelf reef habitats at Glovers Reef and alongthe outer barrier lagoon, the Channel Cay reef complex. We explore reef; McClanahan 1999; McClanahan et al. 1999). community dynamics duringand after the 1998–1999 Although regular echinoids, Echinometra viridis, were mass coral mortality, and we discuss the prospects for abundant on patch reefs in the lagoon at Glovers Reef, recovery of affected coral populations and the implica- their foraging was severely constrained by predatory tions for continued reef accretion. fishes (McClanahan 1999). Since herbivorous fishes – parrotfish (Scaridae) and surgeonfish (Acanthuridae) – Study area were not abundant enough to control algal growth, macroalgae came to dominate the patch-reef habitat in the The central sector of the shelf lagoon of the Belizean absence of the echinoid D. antillarum. barrier reef system is characterized by numerous atoll- The predators of E. viridis that McClanahan (1999) like, diamond-shaped reefs known as rhomboid shoals. identified at Glovers Reef – triggerfish (Balistidae), the The Channel Cay reef complex (16°38¢N, 88°10¢W; jolthead porgy Calamus bajonado (Sparidae), and the Fig.