Succession and community structure of reef flat algae at Heron Island, Great Barrier Reef, Australia

There is little published information on the distribution, abundance, seasonality and ecological roles of benthic algae on the Great Barrier Reef, although they are of fundamental importance in the ecology of coral reef communities. This study sought to provide information on algal community dynamics in two contrasting reef-flat zones: the live coral and algal turf-dominated outer flat, and the fleshy macroalgaldominated inner flat, at Heron Reef, Great Barrier Reef, Queensland, Australia. Algal community structure on dead coral limestone was studied on the inner and outer reef flat over a three-year period (from May 1986 to February 1989), and algal transplant and grazer exclusion experiments were carried out to determine the role of grazers in producing and maintaining community structure. Changes during ecological succession provide useful information about structuring processes in natural communities. Algal succession was studied using artificial substratum (concrete), because it provided a flat surface which offered logistical advantages over naturally contoured coral substratum, and because previous studies had found that artificial substrata adequately mimicked natural surfaces in marine successional studies. Succession on natural substratum was also studied for comparison, using coral clumps which had died after a bleaching episode in February 1987. The effects of season on algal succession, were investigated using four series of concrete settlement blocks, started in February, May, August and November 1986. The effect of habitat complexity on succession was examined by comparing succession on an exposed concrete surface with succession on a protected concrete surface provided with crevices. Each successional study lasted two years. Cover data for all algal species present on each sample were obtained using a sampling grid of sixty points, and these data were analysed using the non-parametric multivariate analysis program, PRIMER. A total of 105 algal taxa were identified in this study, with 101 of these recorded in the natural reef flat algal community over the three-year period. Fleshy macroalgae were the dominant group on the inner flat (56% cover), and filamentous turf algae iv were dominant on the outer flat (47% cover). The importance of water temperature and grazing intensity in producing the differences in distribution of these two groups of algae on the reef-flat were considered. Temperature was rejected as a factor, because there was virtually no temperature difference between the two sites over a two-year period. Grazing was found to be an important factor, based on a much higher rate of damage to algal transplants on the outer flat than on the inner flat; and on the development of higher algal biomass on caged than on uncaged concrete settlement blocks. Between 58 and 69 algal taxa were recorded in each of the four successional studies on exposed concrete, 78 taxa were recorded on protected concrete, and 85 taxa were recorded on bleached coral. Algal successional trajectories were similar on concrete and on bleached coral, but the rate varied substantially. Succession was fastest on bleached coral, slowest on exposed concrete, and intermediate on protected concrete. On all substrata, at both sites, early dominance by green and brown filamentous algae decreased over time, while the abundance of other forms increased. Dominance in terms of cover by early successional algae (in particular Ectocarpus) was prolonged on concrete in comparison to bleached coral, and this effect was stronger on exposed concrete than on protected concrete. It is hypothesised that inhibition of algal community development on concrete because of a relatively dense growth of early successional algae was due to one or both of two factors: 1. Reduced algal recruitment due to a lower level of substratum heterogeneity; and 2. Reduced grazing by the dominant herbivores in the system, parrotfish (Family Scaridae), because of the hardness of concrete. Succession was faster on the outer flat than on the inner flat. On bleached coral, succession to a community similar to the natural background algal community took between three and nine months on the outer flat, and between one and two years on the inner flat. There was no regular seasonal pattern of change in the reef flat algal community over a three-year period, but during early succession, algal community structure was v strongly influenced by season of initial exposure of the concrete settlement blocks. However, this variability did not persist, and by two years all seasonal series of blocks had similar communities. Six species of algae were fertile within 10 days of establishment, including the common early successional algae Ectocarpus, Enteromorpha and Polysiphonia, illustrating the potential of these species for rapid exploitation of available space. The results of this study provide much needed baseline data on algal community dynamics on the southern Great Barrier Reef, and on succession and community development following a bleaching episode. Given the rapid and dense colonisation of bare substratum by algae, and the very low rate of recruitment by corals, it seems that corals will be at a disadvantage in maintaining their abundance in this reef flat environment if coral bleaching becomes a regular event