Modelling environmental drivers of black band disease outbreaks in populations of foliose corals in the genus Montipora

Carla C.M Chen,Christopher C Drovandi,M Julian Caley,David G Bourne,Kerrie Mengersen,Yui Sato,Bette L Willis

doi:10.7717/peerj.3438

Abstract

Seawater temperature anomalies associated with warming climate have been linked to increases in coral disease outbreaks that have contributed to coral reef declines globally. However, little is known about how seasonal scale variations in environmental factors influence disease dynamics at the level of individual coral colonies. In this study, we applied a multi-state Markov model (MSM) to investigate the dynamics of black band disease (BBD) developing from apparently healthy corals and/or a precursor-stage, termed ‘cyanobacterial patches’ (CP), in relation to seasonal variation in light and seawater temperature at two reef sites around Pelorus Island in the central sector of the Great Barrier Reef. The model predicted that the proportion of colonies transitioning from BBD to Healthy states within three months was approximately 57%, but 5.6% of BBD cases resulted in whole colony mortality. According to our modelling, healthy coral colonies were more susceptible to BBD during summer months when light levels were at their maxima and seawater temperatures were either rising or at their maxima. In contrast, CP mostly occurred during spring, when both light and seawater temperatures were rising. This suggests that environmental drivers for healthy coral colonies transitioning into a CP state are different from those driving transitions into BBD. Our model predicts that (1) the transition from healthy to CP state is best explained by increasing light, (2) the transition between Healthy to BBD occurs more frequently from early to late summer, (3) 20% of CP infected corals developed BBD, although light and temperature appeared to have limited impact on this state transition, and (4) the number of transitions from Healthy to BBD differed significantly between the two study sites, potentially reflecting differences in localised wave action regimes.

Highlights

Coral disease has contributed to localised declines in coral cover and changes in benthic communities (Weil, Smith & Gil-Agudelo, 2006; Harvell et al, 2007)
On only two occasions did corals in the cyanobacterial patches’ (CP) state die without a Black band disease (BBD) lesion being observed (Table 1), transitions from CP to death were omitted from the subsequent MSM analyses (Table 2)
This study demonstrates the use of a multi-state analysis to understand the dynamics of a BBD disease within a Montipora spp. coral assemblage and elucidate how the covariate effects of light and temperature influence lesion state-transitions within individual colonies

Summary

Introduction

Coral disease has contributed to localised declines in coral cover and changes in benthic communities (Weil, Smith & Gil-Agudelo, 2006; Harvell et al, 2007). The impacts of coral disease on reefs in other regions are not as extensively documented, outbreaks have been observed across the Indo-Pacific (Raymundo et al, 2005; Weil et al, 2012) and in some areas of Great Barrier Reef (GBR) (Willis, Page & Dinsdale, 2004; Page & Willis, 2006; Sato, Bourne & Willis, 2009; Haapkylä et al, 2010). On the GBR, BBD is one of the most widespread coral diseases (Page & Willis, 2006) It appears as a darkly pigmented microbial mat occurring as a band at the interface between apparently healthy coral tissue and freshly exposed skeleton. Even though BBD is potentially part of the natural ecology of coral assemblages (Page & Willis, 2006), an outbreak of BBD is capable of reshaping a coral community (Bruckner & Bruckner, 1997)

Methods

Results

Conclusion