ZMT - Leibniz Zentrum für Marine Tropenforschung

OCTOBER

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)

Abstract

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.

NOVEMBER

Projections of coral cover and habitat change on turbid reefs under future sea-level rise

Dr. Kyle Morgan | AXA Research Fellow, Asian School of the Environment, Nanyang Technological University, Singapore.

November 16th | 09:00 am  (CEST)

Global sea-level rise (SLR) is projected to increase water depths above coral reefs. Although the impacts of climate disturbance events on coral cover and three-dimensional complexity are well documented, knowledge of how higher sea levels will influence future reef habitat extent and bioconstruction is limited. Here, we use 31 reef cores, coupled with detailed benthic ecological data, from turbid reefs on the central Great Barrier Reef, Australia, to model broad-scale changes in reef habitat following adjustments to reef geomorphology under different SLR scenarios. Model outputs show that modest increases in relative water depth above reefs (Representative Concentration Pathway (RCP) 4.5) over the next 100 years will increase the spatial extent of habitats with low coral cover and generic diversity. More severe SLR (RCP8.5) will completely submerge reef flats and move reef slope coral communities below the euphotic depth, despite the high vertical accretion rates that characterize these reefs. Our findings suggest adverse future trajectories associated with high emission climate scenarios which could threaten turbid reefs globally and their capacity to act as coral refugia from climate change.