Bleaching drives collapse in reef carbonate budgets and reef growth potential on southern Maldives reefs

Abstract

Sea-surface temperature (SST) warming events, which are projected to increase in frequency and intensity with climate change, represent major threats to coral reefs. How these events impact reef carbonate budgets, and thus the capacity of reefs to sustain vertical growth under rising sea levels, remains poorly quantified. Here we quantify the magnitude of changes that followed the ENSO-induced SST warming that affected the Indian Ocean region in mid-2016. Resultant coral bleaching caused an average 75% reduction in coral cover (present mean 6.2%). Most critically we report major declines in shallow fore-reef carbonate budgets, these shifting from strongly net positive (mean 5.92 G, where G = kg CaCO₃ m^-2 yr^-1) to strongly net negative (mean -2.96 G). These changes have driven major reductions in reef growth potential, which have declined from an average 4.2 to -0.4 mm yr^-1. Thus these shallow fore-reef habitats are now in a phase of net erosion. Based on past bleaching recovery trajectories, and predicted increases in bleaching frequency, we predict a prolonged period of suppressed budget and reef growth states. This will limit reef capacity to track IPCC projections of sea-level rise, thus limiting the natural breakwater capacity of these reefs and threatening reef island stability.

Figure 1. Study location and SST anomaly data.

(A) Location of Gaafu Dhaalu atoll (inset map) in the southern Maldives, and the location of the study sites in the southern part of the atoll (white boxed area). Inset map (traced using Adobe Illustrator Version CS5) and satellite image from Google Earth Imagery (Map data: Google, Landsat 2016); (B) Time-series data showing satellite-derived sea-surface temperature (SST) data for the Maldives (01/09/15 to 31/09/16). Daily data were extracted and replotted from the NOAA Coral Reef Watch site (http://coralreefwatch.noaa.gov/vs/gauges/maldives.php), accessed 03/10/16; (C,D) Maps showing satellite derived SST anomaly data for the Indian Ocean; (C) 1^st March 2016, (D) 1^st May 2016. Plots derived from NOAA Coral Reef Watch. 2016, updated daily. NOAA Coral Reef Watch Daily Global 5-km Satellite Virtual Station Time Series Data for Indian Ocean, Mar. 1, 2016 and May 1, 2016. College Park, Maryland, USA: NOAA Coral Reef Watch. Data set accessed 2016-10-03 at http://coralreefwatch.noaa.gov/vs/index.php.

Figure 2. Comparisons between key ecological, structural and carbonate budget metrics across reef sites in Gaafu Dhaalu atoll, Maldives between January 2016 and September 2016.

Box (median and 50% quartile) and whisker (95% quantile) plots showing differences in; (A) coral cover, (B) rugosity, (C) net budget, (D) coral production, and (E) parrotfish erosion. MAH – Mahutigala, KAN – Kandahalagala, KOD – Kodehutigalaa, KAD – Kadumaigala, KAF – Kafigahlaa.

Figure 3. Proportional contributions to coral carbonate production rates (kg CaCO₃ m⁻² yr⁻¹) across reef sites in Gaafu Dhaalu atoll, Maldives between January 2016 and September 2016.

Box (median and 50% quantile) and whisker (95% quartile) plots showing differences in; (A) Acropora (all branched and tabular species), (B) Massive and sub-massive taxa (including Porites), (C) Non-Acropora branched taxa, including Pocillopora spp., and (D) Encrusting taxa. MAH – Mahutigala, KAN – Kandahalagala, KOD – Kodehutigalaa, KAD – Kadumaigala, KAF – Kafigahlaa.

Figure 4. Coral reef accretion rates.

(A) Changes in rates of reef accretion (mean mm yr⁻¹ +/−1 standard deviation) between atoll interior reef sites in Gaafu Dhaalu atoll, Maldives between January 2016 and September 2016. All sites are at 2 m depth on the fore-reef slopes. Comparative data from Acropora-dominated sites around the Chagos Archipelago, central Indian Ocean (all 9 m depth) (in 23), and from the Caribbean (in 12), and recalculated based on a conservative assumption of 20% of framework being removed by physical processes. (B–D) The linear regression and 95% confidence intervals for the relationships between: (B) Coral cover and net carbonate production (G); (C) Coral cover and reef accretion potential (mm yr⁻¹); (D) Acropora spp. cover and reef accretion potential (mm yr⁻¹); and (E) Acropora spp. cover pre-bleaching (Jan 2016) and the magnitude of the subsequent reduction in the net carbonate budget (G).