Uncovering drivers of juvenile coral density following mass bleaching

Jan-Claas Dajka,James P W Robinson,Karen M Chong-Seng,Nicholas A J Graham,Alasdair Harris,Shaun K Wilson

doi:10.1007/s00338-019-01785-w

Abstract

Thermally induced mass coral bleaching is globally responsible for major losses of coral cover. Coral recovery from mass coral disturbances like the 2016 bleaching event hinges on successful recruitment of new coral colonies to the existing population. Juvenile corals as a life history stage represent survival and growth of new recruits. As such, habitat preferences of juvenile corals and how environmental parameters interact to drive coral recovery following a mass bleaching disturbance are important research areas. To expand our knowledge on this topic, we compared juvenile coral densities from before the 2016 bleaching event with those after the disturbance and identified abiotic and biotic characteristics of 21 reefs in the inner Seychelles that predict juvenile coral densities. Our results show that following the 2016 bleaching event, juvenile coral densities were significantly reduced by about 70%, with a particularly large decline in juvenile Acropora. Macroalgae present a large obstacle to survival of juvenile corals in a post-bleaching setting, but their influence varies as a function of herbivore biomass, reef structure, and reef type. Higher biomass of herbivorous fish weakens the negative effect of macroalgae on juvenile corals, and structural complexity on granitic reefs is a strong positive predictor of juvenile coral density. However, structural complexity on carbonate or patch reefs was negatively related to juvenile coral density, highlighting the importance of considering interactive terms in analyses. Our study emphasises the importance of habitat for juvenile coral abundance at both fine and seascape scales, adding to the literature on drivers of reef rebound potential following severe coral bleaching.

Highlights

Mass coral bleaching events resulting from ocean warming have led to significant losses of coral cover across many of the world’s reefs (Goreau et al 2000; Hughes et al 2017)
The densities of juvenile corals have been proposed as an important predictor of coral reef recovery from mass bleaching events
Following the 2016 bleaching event, we recorded a significant reduction in juvenile coral abundance in the inner Seychelles

Summary

Introduction

Mass coral bleaching events resulting from ocean warming have led to significant losses of coral cover across many of the world’s reefs (Goreau et al 2000; Hughes et al 2017). Two major drivers of early post-settlement mortality are competition with other benthic organisms, such as macroalgae (Rasher et al 2011; Johns et al 2018), and predation, for instance by corallivores and incidental predation by some herbivores (Cole et al 2008; Doropoulos et al 2012). Unstable substrates such as rubble have been suggested to cause major coral recruit die-offs (Fox et al 2003; Chong-Seng et al 2014). Mortality rates gradually reduce with coral growth, and most corals escape mortality once they have reached sizes above 5 cm (Doropoulos et al 2015), allowing the corals to grow to reproductive sizes and contribute to the adult population (Hughes et al 2010; Gilmour et al 2013)

Methods

Results

Conclusion