A calcium carbonate budget of a Maldivian reef platform

Abstract

This thesis examines the production, breakdown and subsequent dispersal of reef-derived sediments on Vabbinfaru reef platform, North Malé Atoll, Republic of Maldives (4°18’N, 73°25’E). The findings generated provide quantitative data on the ecological, morphological, sedimentological and hydrodynamic processes on Vabbinfaru and highlight the dynamic linkages between each. A calcium carbonate budget and detrital sediment budget are constructed using direct measurements of gross carbonate production (primary and secondary), reef biological erosion (internal and external bioeroding agents) and sediment fluxes that are specific to Vabbinfaru Reef Platform. Gross carbonate production by biological reef communities was estimated in the field using a census-based approach. In situ rates of organism calcification and growth were measured from a number of growth experiments (alizarin staining, direct measurements) and from the deployment of artificial settlement substrates. Gross carbonate production of the entire live reef was estimated at 1,200,000 kg CaCO3 y-1 (16.8 kg CaCO3 m-2 y-1). Corals were the dominant producer of calcium carbonate (>98% in the live reef) which was a function of their high calcification rate and relative abundance on the reef. Secondary production by encrusting organisms contributed the remainder. Although secondary carbonate production did not significantly contribute to the reef-wide carbonate budget, encrusters are an important in terms of reef geomorphic development and sediment production for island maintenance. Reef biological erosion was estimated based on field measurements of the relative abundance and distribution of bioeroding functional groups (microborers, macroborers, urchins and parrotfish) coupled within organism specific bioerosion rates. Total bioerosion on Vabbinfaru was estimated at 950,000 kg CaCO3 y-1 (11.9 kg CaCO3 m-2 y-1). Two parrotfish species (C. strongylocephalus and C. sordidus) were the main bioeroding agent accounting for up to 99% of total bioerosion within the live reef. Net carbonate production was calculated as the balance of reef gross carbonate production less biological erosion, and was estimated at +290,000 kg CaCO3 y-1 for Vabbinfaru (production-dominated). Reef sediment texture and constituent composition were examined to compare the reef sediment reservoir with contemporary reef community structure. Reefal surface sediment was coarse (1.45ϕ) and coral-dominated (51%), with minor contributions of CCA (18%), Halimeda (16%) and molluscan (12%) skeletal grains. Sub-surface island sediments were coral-dominated (up to 82%) with secondary contributions by CCA reflecting contemporary live reef communities and the durability of grain specific from individual contributors. Sediment fluxes and off-reef export of reef-derived sediment was estimated from direct point measurements using an array of sedimentation traps. Platform reversals in hydrodynamic processes were the primary driver of variations in sediment transport pathways and the magnitude of sediment fluxes across the platform surface. Sediment was entrained and transported in the absence of large-scale storm events (up to 695 kg m-1 y-1). Off-reef export of sand and gravel was estimated at 113,000 kg y-1 and 13,000 kg y-1 respectively. Sediment fluxes and export of detrital sediment has geomorphic implications for the platform and the construction and maintenance of reef-associated landforms. A holistic detrital sediment budget was constructed by calculating the mass of new sediment produced (biological erosion + direct sediment production) less losses of reef sediments through export off-reef, incorporation into the reef edifice and through lagoon infill. Excess sedimentation retained on the platform is important as it is available for reef island construction and maintenance. This value for Vabbinfaru Reef Platform is was estimated at 250,000 kg y-1 (3.5 kg m-2 y-1). The calcium carbonate budget generated is of regional and global significance as findings present the first estimate of net carbonate production constructed for an Indian Ocean reef. A number of datasets used to estimate gross carbonate production and biological erosion are the first generated for Maldivian and central Indian Ocean reefs. Findings add to the limited number of budget studies available and datasets on specific budgetary components have expanded the narrow geographic range of reef environments examined. This budget also uses field-based measurements to produce a holistic reef budget that incorporates reef sedimentary processes allowing for the geomorphic implications of net carbonate production estimates on the Vabbinfaru platform surface to be examined.