Bleaching-driven reef community shifts: impacts on carbonate budgets and reef sediment generation

Professor Chris Perry

Chair in Tropical Coastal Geoscience | Geography Department | University of Exeter, UK.

October 15th | 10:00 am  (CEST)


The ecological impacts of coral bleaching on reef communities are well documented, but resultant impacts upon the carbonate budgets of reefs and, especially, upon reef-derived sediment supply are poorly quantified. These are important knowledge gaps because reef budgets influence physical structural maintenance and reef growth potential, and because the biogenic sediments produced by reef taxa often represent the only source of sediment to sustain proximal shorelines and reef islands. This talk outlines the major impacts of the 2016 bleaching event on the carbonate budgets of reefs at sites in the southern Maldives, but then specifically discusses the resultant consequences for sediment generation by the two dominant sediment producers (parrotfish and Halimeda spp.). Recent data identifies two pulses of increased sediment generation in the 3 years since bleaching. The first occurred within approximately six months after bleaching as parrotfish biomass and resultant erosion rates increased, probably in response to enhanced food availability. The second pulse occurred 1 to 3 years post-bleaching, after further increases in parrotfish biomass and a major (approx. fourfold) increase in Halimeda spp. abundance. Total estimated sediment generation from these two producers increased from approximately 0.5 kg CaCO3 m−2 yr−1 (pre-bleaching; 2016) to approximately 3.7 kg CaCO3 m−2 yr−1 (post-bleaching; 2019), highlighting the strong links between reef ecology and sediment generation. However, the relevance of this sediment for shoreline maintenance probably diverges with each producer group, with parrotfish-derived sediment a more appropriate size fraction to potentially contribute to local island shorelines. Underlying these findings are interesting conceptual ideas about how rates of sediment production and supply from reefs may change post-disturbance, about the longevity of any pulse events, and about the potentially cyclical nature of reef-derived sediment supply.