Reef development on a remote coral atoll before and after coral bleaching: A geospatial assessment

Abstract

Coral reefs are consolidated calcium carbonate (CaCO3) platforms built over hundreds of thousands of years by calcifying organisms including corals, encrusting algae and invertebrates. The geomorphic integrity of coral reefs depends on the carbonate budget, which quantifies the net gain or loss of calcium carbonate, given additive and removal processes. Here, we construct three carbonate budgets for Cocos (Keeling) atoll in the East Indian Ocean from seafloor records of carbonate producers before, during and after a coral bleaching event (2015–2017). A satellite image is used to upscale 367 in-situ observations across the complete atoll (are 225 km2). Growth of calcifying encrusters is monitored on settlement plates from 2015 to 2016, while bioerosion is quantified from parrotfish surveys and macroborer analysis of rubble. The highest rates of production were on the outer atoll forereef and in the northern lagoon (1.5 and 1.61 kg m−2 yr−1 respectively). The forereef produced an order of magnitude more calcium carbonate (approximately 13,114 t per year in 2015) than the other zones due to a high coverage of live branching coral (ca 50%) combined with a large forereef area (123 km2). The greatest rates of removal occurred on the shallow forereef (−1.67 kg m−2 yr−1) driven largely by Bolbometopon muricatum parrotfish bioerosion, resulting in a net loss of carbonate in 2017 (−0.15 kg m−2 yr−1). Accounting for parrotfish bioerosion, the net atoll budget was estimated to be 5976 t of calcium carbonate in 2015, which reduced by 46% over the coral bleaching period to 3200 t in 2017. Compared to other Indian Ocean reefs, this is a relatively minor impact of coral bleaching on the carbonate budget at Cocos (Keeling) atoll.