Proceedings 9th International Coral Reef Symposium, Bali, Indonesia 23-27 October 2000, Vol. 1.
Influence of disturbance on coral reef community structure in Belize M.D. McField1 ABSTRACT The Belize Barrier Reef Complex (BBRC) comprises the core region of the Mesoamerican Barrier Reef System, which has recently been recognized by WWF as a global priority for marine conservation. Coincidentally, this reef system has also recently experienced significant ecological disturbance. This study aims to quantify and describe changes in community structure and answer specific questions regarding regional differences and ecological phaseshifts. Twelve forereef sites throughout the BBRC were sampled in summer 1997 and 1999. In 1998 the combination of a major coral bleaching event and a catastrophic hurricane caused dramatic changes in reef community structure, including a 48% reduction in the live coral cover. Live coral cover decreased at all sites, with mean values dropping from 28% to 15% benthic cover. The southern barrier reef experienced the greatest losses (62%), followed by the northern barrier (55%), atolls (45%) and central barrier (36%). These response differences are thought to result from varying wave energies from the hurricane and differences in the resiliency of the reef communities. The coral species most affected were Agaricia tenuifolia and Acropora cervicornis, which were affected by both disturbances. However, the dominant massive Montastrea annularis was also highly affected, despite its general resilience to hurricane damage. Because there was not a phase-shift to macroalgal domination, the prognosis is favorable for the potential recovery of these reefs. However, if coral cover is to recover to pre-disturbance levels, sound management will be required to ensure that suitable water quality is preserved through environmentally sound coastal development and that adequate fish populations are maintained to control macroalgal growth. Although Connell's (1997) review of long-term reef studies with major disturbances found no documented examples of reef recovery from the Caribbean, it is possible that the BBRC could offer an opportunity to be the first.
Introduction The Belize Barrier Reef Complex (BBRC) includes the longest barrier reef in the Western Hemisphere, three off-shelf atolls, numerous fringing and patch reefs, unusual rhomboid shoals, and covers an area of approximately 22,800 km2 (Kramer et al. 2000). It is the core region of the Mesoamerican Barrier Reef System, which extends for over 1000 km from Yucatan, Mexico to the Bay Islands of Honduras. This reef system has been named a global priority conservation area by the World Wildlife Fund and will host a major World Bank regional conservation project beginning in 2001. Ironically, just as the international community began recognizing the importance of conserving this reef system, its ecological integrity was threatened by two major ecological disturbances; hurricane Mitch and the 1998 coral bleaching event Belizean reefs have been devastated by hurricanes in the past, including the 1961 catastrophic Hurricane Hattie which destroyed sections of the central barrier reef (Stoddart 1963, 1974). Less severe storms and hurricanes occur more frequently, with an approximate frequency of one per year (Gentry 1971), although there has been no documentation of their effects on reef communities. Overall, Belize's reefs were exposed to high wave energy and physical disturbance due to Hurricane Mitch in October 1998. Coral recruitment on the exposed Glovers forereef was reduced by 80% due to hurricane Mitch (Mumby 1999). Although hurricanes can act as a form of "intermediate disturbance" (see Connell (1978) and Rogers (1993)) to increase diversity, they can also be
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contributing factors to major ecological "phase-shifts" (Done 1992, Hughes 1994). The ecological significance of hurricanes may also be related to the their role in transporting fluvial pollutants, pathogens and larvae to distant reefs (Andréfouët et al. in press). Mass bleaching events, on the other hand, are relatively new phenomena in Belize. The first mass bleaching occurred in 1995, with an estimated partial mortality of 10% of coral colonies (McField 1999). The 1998 event appeared more severe, although no large-scale study was conducted during the event. One site in the southern inner lagoon suffered over 90% coral mortality (Aronson et al. 2000). Glovers Reef patch reefs experienced a 24% reduction in live coral cover between September and December 1998 and continued to lose coral, with a 42% reduction in live coral by September 1999 (McClanahan et al. 2001a). Recruit populations in the Glovers lagoon also suffered a 53% decline from September 1998 to September 1999, attributed primarily to bleaching (Huitric and McField, unpbl. data). The 1997/8 mass bleaching event affected reefs in every ocean and was considered the most intense on record (Wilkinson 2000). This paper aims to quantify and describe significant changes to forereef community structure which resulted from this combination of large scale ecological disturbances and relate the findings to a prognosis for recovery. Specific questions to be answered include: 1. Were there regional response differences within the BBRC? 2. Do community changes conform to expected "phase shifts" leading to macroalgal domination?
College of Marine Science, University of South Florida / World Wildlife Fund Mesoamerican Caribbean Reef Program, P.O. Box 512, Belize City, Belize, Central America. Email:
[email protected]
Methods
The twelve sampling locations analyzed in this study are distributed throughout the BBRC (Figure 1) and identified by GPS coordinates and site descriptions. Ten replicate 25 meter transects were haphazardly set at each reef site, as described in Aronson et al. (1994) and McField et al. (in press). Transects were oriented along individual reef spurs near the reef slope in a depth range of 12 to 19 meters. The 1997 data were collected with a Sony Hi-8 camcorder and 30 watt dual lights. The 1999 data were collected with a Sony TRV900 digital camcorder with no external lighting. Fifty video images per transect were analyzed with 10 random data points per image using PointCount for coral reefs software (Dustan Lab, University of Charleston). Data points were identified to species for scleractinian and milleporid, (stony corals) with unidentifiable specimens labeled “Scleractinia”. Other taxa, such as sponges and octocorals, were identified into functional groups. "Macroalgae" was defined by specimens having blades identifiable on the image. The “substrate” category contained smaller turf algae, encrusting coralline algae and bare rock. Points that fell on the transect equipment or could not be identified were removed from the analysis. The percentage cover of each organism or category was calculated from the total points per transect. Regional barrier reef divisions were based on the classifications of reef regions as defined by Burke (1979) and Wantland and Pusey (1971) and further supported by McField et al. (in press). Results During the study period the mean percent coral cover was reduced by 48%, dropping from 28% in 1997 to 15% in 1999. No sites had an increase in coral cover and the percent reductions ranged from 26% at South Water Caye to 85% at Nicholas Caye (Fig. 2a). A two-way ANOVA illustrates that these results are significant (P
