Effects of Acanthaster predation on bioerosion: design and preliminary results

The downstream effects of COTS outbreaks on non-coral organisms have rarely been investigated. The present work is concerned with the effects of outbreaks on the process of biological degradation of the reef framework. On reefs that have experienced severe outbreaks, the overall reworking of calcium carbonate by biological agents is expected to be greater than on reefs with high live coral cover. To establish whether the rates of bioerosion per unit area of substrate are enhanced on a reef following an outbreak, and what are the causes and effects of degradation of the standing coral skeletons left behind by COTS predation, two research programs have been undertaken. An experimental program consists of simulating COTS predation by killing coral colonies in situ in restricted areas on the reef crest. Three species of Acropora, usually highly preyed upon by Acanthaster, have been used. To date, results show that Acropora hyacinthus undergoes rapid skeletal degradation by external destructive agents, while skeletons of A. gemmzfera and A. cuneata remain for longer periods exposed to the action of excavating organisms. The second research program consists of sampling dead substrates occurring on large Porites bommies, on both COT-affected and unaffected reefs. The year of death of these substrates can be estimated, and rates of bioerosion relative to periods of over 30 years could be calculated. Preliminary results show that the method is a powerful tool for establishing both large scale and disturbance-induced variations in the process of internal bioerosion.