Coral bleaching responses in Sodwana Bay, South Africa

Bachelor Thesis


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University of Cape Town

This study assessed the bleaching response (BR) of coral colonies within the central reef complex in Sodwana Bay, South Africa. Bleach surveys were conducted at 16 sites on 8 reefs over the period of 2007 to 2013. A total of 12 858 coral colonies from 30 taxa were randomly sampled and colonies were placed into 7 categories of bleaching response. This allowed for the calculation of taxon-specific BR as a weighted percentage of coral cover bleached. Continuous temperature records from a permanent temperature gauge on Two Mile Reef were used to assess thermal stress over this period. The percentage of coral colonies that bleached in 2007, 2008, 2011, 2012 and 2013 were 37.4%, 17.4%, 23.8%, 33.6% and 38.8% respectively. A binomial GLM model framework was used to separate the effects of year, reef and taxon on the bleaching response. Due to inconsistent sampling of sites over time, only data from the seven sites on Two Mile Reef (TMR) and the two sites on Nine Mile Reef (NMR) were included in the model. A total of 6758 coral colonies from the nine most abundantly sampled taxa were used in the assessment of bleaching response for TMR and NMR over the sample period. Taxon was shown to explain most of the variability in the bleaching response of TMR and NMR over time (40.9%). The standardized reef bleaching response of TMR and NMR indicated the same temporal trends with a range of 5% to 28% of live coral surface bleached. Standardized reef-specific BR showed periods of high (2007, 2012 and 2013) and low (2008 and 2011) bleaching response. Low BR in 2008 and 2011 did not correspond to thermal stress (≥27.5°C) suggesting that local upwelling buffered the effects of thermal stress experienced. Standardized taxon-specific bleaching response for TMR and NMR displayed large variability over time and ranged from 2.5% to 45% of live coral surface bleached, with Montipora being the most susceptible and Galaxea and Playgyra being the least. Mean site-specific BR averaged over all years, including all sites, found that Coscinaraea, Montipora, Astreopora and Anomastrea were the most susceptible taxa. This study and other recent studies draw attention to a trend of either episodic or an increasing frequency and intensity of bleaching in southern African reefs.