Early trajectories of benthic coral reef communities following the 2015/16 coral bleaching event at remote Aldabra Atoll, Seychelles

Anna Koester,Nancy Bunbury,Christian Wild,Valentina Migani,Amanda Ford,Cheryl Sanchez

doi:10.1038/s41598-020-74077-x

Anna Koester, Nancy Bunbury + Show 4 more

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https://doi.org/10.1038/s41598-020-74077-x

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Abstract

Documenting post-bleaching trajectories of coral reef communities is crucial to understand their resilience to climate change. We investigated reef community changes following the 2015/16 bleaching event at Aldabra Atoll, where direct human impact is minimal. We combined benthic data collected pre- (2014) and post-bleaching (2016–2019) at 12 sites across three locations (lagoon, 2 m depth; seaward west and east, 5 and 15 m depth) with water temperature measurements. While seaward reefs experienced relative hard coral reductions of 51–62%, lagoonal coral loss was lower (− 34%), probably due to three-fold higher daily water temperature variability there. Between 2016 and 2019, hard coral cover did not change on deep reefs which remained dominated by turf algae and Halimeda, but absolute cover on shallow reefs increased annually by 1.3% (east), 2.3% (west) and 3.0% (lagoon), reaching, respectively, 54%, 68% and 93% of the pre-bleaching cover in 2019. Full recovery at the shallow seaward locations may take at least five more years, but remains uncertain for the deeper reefs. The expected increase in frequency and severity of coral bleaching events is likely to make even rapid recovery as observed in Aldabra’s lagoon too slow to prevent long-term reef degradation, even at remote sites.

Highlights

Climate change-induced coral bleaching events are increasing in frequency and severity, threatening the persistence of coral reef ecosystems worldwide
Following the findings of Cerutti et al.[33] that bleaching induced coral mortality was lower at Aldabra’s lagoon than at the seaward reef after the 2015/16 coral bleaching (35% vs. 55% loss), we examine early post-bleaching reef trajectories at Aldabra Atoll in the context of spatial variations in bleaching impact by: (1) assessing changes of benthic communities across locations between 2014 and 2016 and quantifying daily water temperature variability to explore whether this links to spatial differences in bleaching impact[12], and (2) evaluating the post-bleaching trajectories of the benthic communities at these locations between 2016 and 2019
Soft coral cover declined by 91–92% at all locations (Fig. 1b,g), but absolute losses were considerably lower inside the lagoon as soft coral cover there was already < 2% in 2014

Summary

Introduction

Climate change-induced coral bleaching events are increasing in frequency and severity, threatening the persistence of coral reef ecosystems worldwide. Global warming reduced the time frames between bleaching events from 27 years in the early 1980s to 5.9 years in 2 0161 and recovery windows are predicted to shorten even further as severe bleaching events are expected to occur annually on 90% of the world’s coral reefs by 2 0552 In this context, assessing post-disturbance reef trajectories is crucial to understand which conditions favour reef recovery[3,4]. Limits coral growth and coral larvae settlement[24], while coral recruitment and survival can be enhanced by herbivores that control algal turf and fleshy macroalgae and promote crustose coralline algae (CCA) growth[25,26,27] These natural drivers of recovery may be disrupted by direct human stressors such as overfishing of herbivores and/or nutrient enrichment, favouring algal proliferation and impeding or preventing reef recovery. Coral bleaching events have caused high coral mortality at Aldabra in 1998/1999 (38% and 66% on the seaward reef at 10 m and 20 m water depth, respectively32) and in 2015/2016 (35% in the lagoon at 2 m water depth; 54% at 5 m and 55% at 15 m water depth on the seaward reef[33])

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