Abstract:
The aim of this thesis was to construct a calcium carbonate budget of a lagoonal reef platform in the south Maldives, approximately 12 months after the 2015-2016 global mass coral bleaching event. In situ census data from 12,000 observations of benthic ecology spanning 1,200 m of the platform surface were combined with organism-specific calcification rates to quantify gross carbonate production (kg CaCO3 m-2 y-1 or ‘G’). Census data of parrotfish and urchin populations were used to calculate carbonate loss from bioerosion. Finally, resultant net carbonate budgets were generated for survey sites (n =40), statistically derived reef habitats (n = 5) and the whole reef platform. Constructive processes generated 581,858 kg CaCO3 y-1 of calcium carbonate (+1.91 G), with habitat specific rates ranging from +0.62 G to +5.31 G. Thermally resilient Porites corals were identified as the dominant carbonate producer in four of five habitats, accounting for up to 96.1% (4.88 G) of production. Results also identify the importance of green calcifying macroalgae Halimeda, responsible for 10.5% of platform production (0.20 G). Bioerosion totalled 252,900 kg CaCO3 y-1 (0.83 kg m-2 y-1), with habitat-specific rates ranging from 0.07 to 1.42 kg m-2 y-1. Parrotfish were the dominant bioeroding taxa across all reef framework habitats (up to 1.21 kg m-2 y-1). In contrast to recent research on select fore-reef slope habitats in the region (2 m depth), reef-surface results from this study indicate that the platform maintained a positive net carbonate budget (+1.08 G) despite the bleaching event, generating +328,958 kg CaCO3 y-1 in total. Findings from this research highlight the important role thermally resilient coral species can play in maintaining a net positive carbonate budget following a coral bleaching event. However, while such assemblages form an important backstop during acute disturbance events, the shift toward monospecific coral carbonate production resulted in low positive budget states across most habitats. Estimates of reef accretionary potential derived from ecological survey data indicate that the rate of future sea-level rise would substantially exceed the rate of vertical reef accretion if the reef remained as is (i.e. no recovery occurred), making the likelihood and speed of reef recovery crucial to future reef geomorphic performance.