Internal bioerosion of in situ living and dead corals on the Great Barrier Reef

This thesis reports on a series of sampling and experimental studies aimed at quantifying the extent of internal bioerosion by four major groups of boring organisms in in situ coral substrates. Additional objectives of the study were to investigate the patterns of variability of internal bioerosion and overall skeletal degradation across common coral substrates and a number of spatial scales within a single habitat. The response of internal bioeroders to availability of dead coral substrate, such as following an Acanthaster planci outbreak, was also addressed. Internal bioerosion and external degradation were studied in living and dead corals of three species of Acropora (A.hyacinthus, A.gemmifera and A.cuneata) using a manipulative field experiment over a 21 month period. Coral colonies were killed in situ to simulate local episode of coral predation. In addition, living colonies of the same species were sampled and inte[rn]al bioerosion was quantified. The extent of internal bioerosion in living colonies was generally small, but differed strongly among the three coral species. A.cuneata was excavated up to ten times more than the other two species. The major source of difference among the species was the presence of the live-coral boring bivalve, Lithophaga lessepsiana, in colonies of A.cuneata. The majority of the remaining variation across the three species was explained, to different extent for each boring group, by inter-specific differences in the amount of exposed skeleton. This was found to be species-specific and, in the case of A.hyacinthus, to differ also between the two locations. Variation in the extent of excavation was high at small spatial scales for all coral species and all boring taxa. Sponges (Cliothosa hancocki in particular) were by far the most destructive borers in living colonies of the three species, together with the bivalve L.lessepsiana, which bored exclusively in living portions of A.cuneata. Worms, mostly polychaetes, although numerically more abundant, accounted for little of the total bioerosion in living corals. Colonies of the three species of Acropora were killed and left in situ for 21 months. Changes in colony size and rugosity over the period of exposure were examined using photographic monitoring. Both colony size and rugosity decreased significantly over time following death. The decrease in both size and rugosity varied considerably among the three coral species with very rapid skeletal degradation occurring soon after death in plate colonies of A.hyacinthus. The rates of external degradation in A.hyacinthus were not costant through time and were higher soon after death. In contrast, A.cuneata experienced no significant variation in size, while A.gemmifera decreased in size at a relatively uniform rate. In A.hyacinthus, the position of the colonies with regard to location, resulted in different rates of external degradation but this pattern decreased with time. At the study sites, external erosion of colonies of A.hyacinthus killed and left standing on the reef surface contributed a total of 0.41 to 1.19 kg of CaCO₃ per m² per year (when assuming a 100% cover of A.hyacinthus). Following the 21 month period of exposure to borers, the colonies were collected and analysed for the extent and nature of internal bioerosion. This differed greatly among the three coral species. The difference in the extent of bioerosion by worms was entirely explained by the extent of colony surface area available for settlement in the three coral species. On the contrary, availability of surface area did not explain the considerable difference in the extent of bioerosion by sponges in the three species. It is suggested that newly available substrates on the reef surface are readily excavated by boring sponges which are already present in adjacent substrates. Furthermore, it appears that the occurrence of large bivalve borings in colonies of A.cuneata might have determined a larger extent of excavation by sponges in this species. Bioerosion by bivalves was larger at South East exposed sites but no effect of location was detected for any other individual groups of borers. A.cuneata had a greater extent of total bioerosion by all taxa at the front sites, which is explained by the cumulative effect of sponges and bivalves. As was the case in the living colonies, the majority of the variability encountered was due to variation at small spatial scales (within-site variation for all groups and within colony variation for worms and bivalves in A.hyacinthus and A.gemrnifera). However, a small proportion of variability was explained at the spatial scale of site (hundreds of meters) for both worms and sponges. Total bioerosion was not correlated to any of the colony parameters considered. However, when bioerosion was considered for each group separately, there was a high correlation between colony surface area and bioerosion by worms, while bioerosion by sponges and bivalves correlated with colony volume. The comparison between bioerosion in living and dead coral colonies of the same species over a period of 21 months following death showed that the volume removed by internal bioeroders from dead colonies per unit of colony surface area, was significantly higher than the volume of skeleton excavated from living colonies. The average amount of CaCO₃ reworked by internal bioeroders per m² per year varied from a minimum of 0.23 g x m⁻² x y⁻¹ for barnacles to a maximum of 766.1 g x m⁻² x y⁻¹ for sponges. The reworking by borers of CaCO₃ in dead coral colonies varied up to 4-5 times among the coral species. This suggests that coral community composition and the dynamics of mortality of individual species may affect the overall reef bioerosion. Rates of excavation also varied between locations, with the SE more exposed sites experiencing higher rates of CaCO₃ loss via excavation by sponges and bivalves. In all coral species considered, sponges were the group that excavated the most of the CaCO₃ per unit of surface area and time. In order to investigate the effect of massive coral mortality on rates of internal bioerosion, a sampling study was conducted at two reefs which had been severely affected by Acanthaster planci, and at two reefs which had not experienced outbreaks in the last 20 years. Internal bioerosion was quantified for all reefs from dead Porites substrates. These were dated for the year of death of the living tissue using fluorescent bands occurring in the skeleton of these corals. The time elapsed since death was assumed to be the time of exposure to boring organisms. Bioerosion rates in Porites were not linear over time, but decreased with time after death of the substrate. Rates of bioerosion among reefs were not different although there were differences among sites within reefs. The major borers were worms, mostly sipunculans at all reefs, although sponges were locally abundant (i.e. Green Island). Sponges, when present, tended to be far more destructive than the other groups. No effect of outbreaks of Acanthaster planci was detected. The volume excavated per unit of time did not appear to either increase nor decrease as a result of severe episodes of coral mass mortality. This suggests that following an outbreak the production of carbonate by-products via excavation of dead corals may increase proportionally to the increase in dead coral cover. The majority of variation in rates of bioerosion was due to the duration of exposure of the substrate to borers. Bioerosion rates were highly variable at small spatial scales also (centimeters to meters), suggesting that processes occurring at small spatial scales, such as recruitment of borers, are most important in determining spatial patterns of internal bioerosion in Porites substrates. The instantaneous rates of bioerosion in dead Porites ranged between 8±4 to 626±170 g CaCO₃ m⁻² y⁻¹.