Recovery trends of reef carbonate budgets at remote coral atolls 6 years post-bleaching (article)

Abstract

Coral bleaching events and resultant changes in benthic community composition and population size structure can diminish the important geo-ecological functions reefs provide, including habitat provision and carbonate production to support reef accretion. Net reef carbonate budgets, the balance between carbonate production and erosion processes, are thus important functional indicators of reef health. This study quantifies changes in coral community composition and colony size structures, and the resultant reef carbonate budget trajectories after the 2015/2016 bleaching event in the remote Chagos Archipelago, Indian Ocean. ReefBudget surveys were conducted at 12 sites across three atolls in 2015, 2018 and 2021, with calculations of biological carbonate production and erosion supported by locally obtained calcification and bioerosion rates. Carbonate budgets (in G = kg CaCO3 m-2 yr-1) shifted from net positive states in 2015 (mean±SD: 3.8±2.6 G) to net negative states in 2018 (-2.4±1.4 G) in response to bleaching-driven mass coral mortality. By 2021, all sites were on a trajectory of recovery, but net budgets differed significantly between atolls (-2.0±1.7 to 2.2±1.4 G). At Salomon atoll, the 3-fold faster recovery of carbonate production and return to positive reef budget states only six years post-bleaching was associated with the persistence of high structural complexity and the rapid recovery of fast growing tabular Acropora spp. Inter-atoll differences in colony size distributions furthermore illustrate that coral identity and size class are more important predictors of reef functions and post-disturbance recovery speed than coral cover alone.