Decadal erosion of coral assemblages by multiple disturbances in the Palm Islands, central Great Barrier Reef

Gergely Torda,Tessa Hill,Yui Sato,David G Bourne,Georgina Torras Jorda,Vimoksalehi Lukoschek,Aurelie Moya,Peter Cross,Bette L Willis,Katie Sambrook

doi:10.1038/s41598-018-29608-y

Abstract

Increases in the frequency of perturbations that drive coral community structure, such as severe thermal anomalies and high intensity storms, highlight the need to understand how coral communities recover following multiple disturbances. We describe the dynamics of cover and assemblage composition of corals on exposed inshore reefs in the Palm Islands, central Great Barrier Reef, over 19 years encapsulating major disturbance events such as the severe bleaching event in 1998 and Cyclone Yasi in 2011, along with other minor storm and heat stress events. Over this time, 47.8% of hard coral cover was lost, with a concomitant shift in coral assemblage composition due to taxon-specific rates of mortality during the disturbances, and asymmetric recovery in the aftermath thereof. High recruitment rates of some broadcast-spawning corals, particularly corymbose Acropora spp., even in the absence of adult colonies, indicate that a strong external larval supply replenished the stocks. Conversely, the time required for recovery of slow-growing coral morphologies and life histories was longer than the recurrence times of major disturbances. With interludes between bleaching and cyclones predicted to decrease, the probability of another severe disturbance event before coral cover and assemblage composition approximates historical levels suggests that reefs will continue to erode.

Highlights

Decadal declines in coral cover have been reported across all ocean basins[1,2,3]
The first was the 1998 bleaching event, during which live coral cover dropped from 51.7% to 15.4% followed by a roughly decade-long recovery period with few minor disturbances[41,42,43] (Fig. 1) during which coral cover increased to 27.8% (Fig. 2A)
The second was the 2011 Cyclone Yasi event, which reduced live coral cover to 4.1%, followed by ongoing decline to 1.6% in 2014, after which recovery began and coral cover increased to 27.0% by 2017 (Fig. 2A)

Summary

Introduction

Decadal declines in coral cover have been reported across all ocean basins[1,2,3]. Drivers of coral loss range from localised stressors, such as overfishing and poor water quality[4,5], to large-scale disturbances such as coral bleaching, crown-of-thorns starfish, disease outbreaks and cyclones[6]. In the 13 years between the bleaching event and Cyclone Yasi (1998–2011), and in the six years after the cyclone (March 2011 – February 2017, i.e. up until the 2017 bleaching event), there have been only minor disturbances to reefs in the Palm Island Group[21,41,42,43] (Fig. 1), including a category 2 cyclone and minor heat anomalies that were insignificant in comparison to the 1998 and 2011 events (Fig. 1) These two interludes provide an opportunity to examine the immediate impact of different acute disturbances (mass bleaching versus category 5 Cyclone Yasi), and to follow recovery trajectories of coral assemblages at affected sites. We quantify the nature of cyclone damage to these reefs, describing fine-scale recovery patterns with high taxonomic and spatial resolution

Methods

Results

Conclusion