Biological Destruction on the Great Barrier Reef

Abstract

This thesis is an experimentally based study of the processes of biological destruction on dead coral substrates on the Great Barrier Reef. By placing recently killed coral samples in reef environments it has been possible to compare the impact of bioerosion processes on hard coral skeletons between different environments and reefs. The experiments have shown that once a dead coral substrate becomes available it is subjected to recruitment by a diverse assemblage of endolithic borers that excavate the interior of the substrate, while its surface is eroded by the grazers that are adapted to scraping carbonate substrates to remove epilithic and endolithic algal growth.

Experiments on Lizard Island reef have identified grazing as the major destruction agent on coral substrates over the initial 4 years of exposure. Major differences between environments in the amount of grazing are controlled by the distribution of the highly mobile herbivorous fish community that is dominated by scarids and acanthurids. Subtidal reef slopes and lagoon environments of water depths less than 20 m are subjected to higher rates of grazing erosion than shallow reef flat environments. These differences in grazing are a result of tidal cycles limiting the access of grazing fish to reef flats to high tides only. The variability of grazing activity by bioeroding fish is highly complex due to ecological and behavioral factors. However, over time periods important to the geological development of reef environments, these factors would be less important than the major physical features of the reef that determine scarids and acanthurids populations between environments.

The Lizard Island experiments indicate that extensive borer populations require at least 4 years to develop and that consecutive 2-year sampling periods show significant differences in the successful recruitment of borers. Early cryptofauna to colonise dead coral substrates are small fabriciniid polychaetes. These small polychaetes are followed by larger cirratulids, spionids, eunicids and sabellines. Sipunculans, bivalve molluscs and sponges only become important in the experiments after 4 years. This importance of time to the extent of borer excavation in experimental substrates suggests that reef surfaces will be extensively bored if they survive the destructive effects of grazing. Recruitment differences within and between environments and season will influence the structure of the borer community. However, the long-term erosional impact of endolithic borers on reef framework will be closely linked to the rate of surface destruction by grazers.

The geological impact of the relationship between grazing and boring is further revealed in experiments in the southern Great Barrier Reef. These experiments were designed to assess the differences in grazer and borer communities for 2 years on reefs that represent adolescent, mature and senile stages of evolution at sea level. These reefs were Llewellyn, One Tree, and Wreck. Experimental samples placed in reef slope, flat, and lagoon environments show that destruction caused by grazing fish is reduced as lagoon environments are restricted by sedimentary infill that expands reef flat conditions. In addition, grazing by gastropods becomes increasingly important in these reef flat conditions. However, the rates of erosion by these grazing molluscs in senile environments on the protected surfaces of substrates are considerably less than the rates of destruction caused by fish in subtidal adolescent reef environments. With reduced destruction, accretion by encrusters is important on substrates. This accretion further protects surfaces, preserving the volume of the substrate and allowing the establishment of borer communities. Through the turnover of these borers and the addition of encrustation, substrates may be preserved, but they are likely to be converted from original coral to substrates composed of bored encrustation. Extrapolating the patterns of the accretion - erosion balance on experiments to the long term impact on reef surfaces predicts that framework in senile reef environments will be subjected to many cycles of boring and encrustation, producing a highly altered reef rock. Dead coral framework in adolescent environments is subjected to rapid destruction by grazers such that borer communities may remain in early successional stages. The conversion of framework to sediment through this destruction is an important contribution to the infilling of the lagoons and to the way that growth frameworks are ultimately incorporated into the foundation of the reef.

In addition to these experiments in subtidal environments, an experiment was designed to measure rates of surface destruction on windward intertidal reef crest surfaces at One Tree Reef. Using an instrument that measures differences in the elevation of reef surfaces through time it was possible to establish that rates of destruction increase toward the reef margin and that the highest rates recorded approach the rates of construction that have been estimated by others for the environment. Biological processes, including grazing, are interpreted to be responsible for this destruction since longer exposure to subtidal conditions increases erosion and the activity of grazers and borers. The intertidal pavement that is submerged for longer periods during tidal cycles recorded rates of erosion more than twice those recorded on the supertidal cemented rubble platform. The rates indicate the magnitude of accretion that has been necessary to maintain this environment in intertidal conditions.

Overall, the study provides a different view of short-term biological processes that effect the long-term geological development of reefs. The changing relationship of grazing, boring and accretion on frameworks as a reef evolves provides a geological record that is a potential measure of the physical and ecological change that has occurred in the past, and is occurring on reefs today. Show more