Rapid transition in the structure of a coral reef community: the effects of coral bleaching and physical disturbance

Abstract

Coral reef communities are in a state of change throughout their geographical range. Factors contributing to this change include bleaching (the loss of algal symbionts), storm damage, disease, and increasing abundance of macroalgae. An additional factor for Caribbean reefs is the aftereffects of the epizootic that reduced the abundance of the herbivorous sea urchin, Diadema antillarum. Although coral reef communities have undergone phase shifts, there are few studies that document the details of such transitions. We report the results of a 40-month study that documents changes in a Caribbean reef community affected by bleaching, hurricane damage, and an increasing abundance of macroalgae. The study site was in a relatively pristine area of the reef surrounding the island of San Salvador in the Bahamas. Ten transects were sampled every 3-9 months from November 1994 to February 1998. During this period, the corals experienced a massive bleaching event resulting in a significant decline in coral abundance. Algae, especially macroalgae, increased in abundance until they effectively dominated the substrate. The direct impact of Hurricane Lili in October 1996 did not alter the developing community structure and may have facilitated increasing algal abundance. The results of this study document the rapid transition of this reef community from one in which corals and algae were codominant to a community dominated by macroalgae. The relatively brief time period required for this transition illustrates the dynamic nature of reef communities.

Figure 1

Map of the Caribbean showing the location of the island of San Salvador. (Inset) The location of the transect sites. The north and south sites were 40 m apart. The coordinates refer to the grid system used to ensure random placement of the transects.

Figure 2

Mean percentage of combined hard corals and algae categories expressed as a function of total transect length over time. The vertical lines represent 1 SE.

Figure 3

The impact of Hurricane Lili, which struck San Salvador in October 1996. Major transect categories (mean percentage ± 1 SE) are organized into unoccupied substrate, organisms with an erect growth form, and encrusting/low profile species. The two sampling dates before and after the hurricane are plotted for comparison.