A population genetic assessment of coral recovery on highly disturbed reefs of the Keppel Island archipelago in the southern Great Barrier Reef.

Madeleine J.H Van Oppen,Vimoksalehi Lukoschek,Alison M Jones,Ray Berkelmans,Lesa M Peplow

doi:10.7717/peerj.1092

Abstract

Coral reefs surrounding the islands lying close to the coast are unique to the Great Barrier Reef (GBR) in that they are frequently exposed to disturbance events including floods caused by cyclonic rainfall, strong winds and occasional periods of prolonged above-average temperatures during summer. In one such group of islands in the southern GBR, the Keppel Island archipelago, climate-driven disturbances frequently result in major coral mortality. Whilst these island reefs have clearly survived such dramatic disturbances in the past, the consequences of extreme mortality events may include the loss of genetic diversity, and hence adaptive potential, and a reduction in fitness due to inbreeding, especially if new recruitment from external sources is limited. Here we examined the level of isolation of the Keppel Island group as well as patterns of gene flow within the Keppel Islands using 10 microsatellite markers in nine populations of the coral, Acropora millepora. Bayesian cluster analysis and assignment tests indicated gene flow is restricted, but not absent, between the outer and inner Keppel Island groups, and that extensive gene flow exists within each of these island groups. Comparison of the Keppel Island data with results from a previous GBR-wide study that included a single Keppel Island population, confirmed that A. millepora in the Keppel Islands is genetically distinct from populations elsewhere on the GBR, with exception of the nearby inshore High Peak Reef just north of the Keppel Islands. We compared patterns of genetic diversity in the Keppel Island populations with those from other GBR populations and found them to be slightly, but significantly lower, consistent with the archipelago being geographically isolated, but there was no evidence for recent bottlenecks or deviation from mutation-drift equilibrium. A high incidence of private alleles in the Keppel Islands, particularly in the outer islands, supports their relative isolation and contributes to the conservation value of the archipelago. The lack of evidence for genetic erosion, in combination with our observation that the North Keppel Island population samples collected in 2002 and 2008, respectively, exhibited a pairwise genetic distance of zero, supports previous published work indicating that, following bleaching, Acropora corals in the Keppel Islands predominantly recover from regrowth of small amounts of remaining live tissue in apparently dead coral colonies. This is likely supplemented by recruitment of larvae from genetically similar, less disturbed populations at nearby reefs, particularly following extreme flood events.

Highlights

Coral reefs along the East Australian coastline are shaped by a range of factors and forces that include coastal geomorphology, freshwater inundation and sediment runoff, hydrodynamics, unusually warm summer sea surface temperatures caused by climate warming, as well as local weather patterns
Three multilocus genotype (MLG) in the Keppel Islands data set were repeated twice each; two of these MLGs occurred at Barren Island and one at Man & Wife Rocks
Limited gene flow between inner and outer island clusters The Barren Island population is a genetic outlier with Dest values ranging from 0.175 to 0.253 (Table 1), and most of the individuals sampled belong to a genetic cluster distinct from any other cluster observed on the Great Barrier Reef (GBR) (Fig. 1)

Summary

Introduction

Coral reefs along the East Australian coastline are shaped by a range of factors and forces that include coastal geomorphology, freshwater inundation and sediment runoff, hydrodynamics, unusually warm summer sea surface temperatures caused by climate warming, as well as local weather patterns. These forces cause recurring perturbations and in some regions result in frequent, high levels of coral mortality. The Keppel Islands are renowned for their high disturbance regime, causing repeated widespread coral mortality. Thermal mass coral bleaching affected >60% of the corals in this area in 1998, 2002 and 2006 and caused significant coral cover loss (e.g., ∼40% loss in 2006; Jones, Berkelmans & Houston, 2011), in shallow (0–6 m) reef areas (Berkelmans et al, 2004; Jones et al, 2008; Diaz-Pulido et al, 2009)

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Conclusion