A study on the recovery of Tobago's coral reefs following the 2010 mass bleaching event by Salome Buglass BSc. Geography, University College London, United Kingdom, 2009 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF Master of Science in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Geography) The University of British Columbia (Vancouver) December 2014 © Salome Buglass, 2014 Abstract The rise of ocean temperatures globally has become a grave threat to coral reefs, as it is increasing the severity and frequency of mass coral bleaching events and post-bleaching coral mortality. The continued eXistence of productive coral reefs will rely on corals’ ability to undergo recovery. In 2010, Tobago’s coral reefs were exposed to severe heat stress leading to mass bleaching of up to 29-60% of colonies at observed sites. This study evaluated the impact of coral bleaching and recovery of coral communities across three major reef systems in Tobago that differ in their eXposure to terrestrial runoff. Assessments were done on the 1) density and composition of coral juveniles to characterise the levels of recruitment, 2) sedimentation rates and composition to understand its potential impact on recovery, and 3) species’ size frequency distributions in 2010, 2011 and 2013 to eXamine temporal changes among population size structure. In 2013, low juvenile densities were observed (5.41 ± 6.31 m-2) at most reef sites, which were dominated by brooding genera while broadcasting genera were rare. Sediment material, measured in May and June (end of Tobago’s dry season) was mostly terrigenous and deposited at rates below coral stress threshold levels at most sites. Out of 27 species populations assessed between all sites, 4 populations mean colony size had significantly changed by the bleaching event, and only changed 5 populations over the two following years. The few populations that were significantly altered (mainly S. siderea and M. faveolata) after the bleaching saw a rise in small sized colonies, mostl likely as a result of colony fragmentation. This study highlights that recovery via seXually produced recruits among broadcasting species was limited. While sedimentation rates were low, it is likely they are significantly higher throughout the rainy season, thus a long-term sedimentation study is highly recommended. Most coral populations resisted significant alteration from heat stress in 2010. However, given that future thermal stress is projected to become more intense, this study shows that mass bleaching disturbance could lead to decline coral population’s mean colony size, which could affect coral recovery as smaller colonies are less fecund. ii Preface This thesis is based on field data collected in 2013 in collaboration with the Institute of Marine Affairs (IMA) under the guidance and supervision of Dr. Simon Donner. All data from the field were collected by myself with the assistance of volunteers. Benthic percent cover and coral colony size frequency data for 2010, 2011 and 2012 were previously collected by the IMA as part of their Biodiversity and Ecology Research Programme. Laboratory sample processing was performed partly in the IMA’s Biodiversity and Ecology Laboratory and on UBC Vancouver campus. I undertook all laboratory sample processing, analysis of data, and writing of the thesis manuscript. iii Table of Contents Abstract ................................................................................................................................... ii Preface ................................................................................................................................... iii Table of Contents ................................................................................................................... iv List of Tables .......................................................................................................................... vi List of Figures ........................................................................................................................ vii Acknowledgements .............................................................................................................. viii Chapter 1. Introduction .......................................................................................................... 1 Chapter 2. An assessment of coral juvenile community and sediment deposition among three major reef systems in Tobago ............................................................................ 4 2.1 Introduction ........................................................................................................................ 4 2.2 Methods ............................................................................................................................. 6 2.2.1 Study area ................................................................................................................... 6 2.2.2 Sedimentation assessment ......................................................................................... 8 2.2.3 Juvenile community survey ...................................................................................... 10 2.2.4 Statistical analysis ..................................................................................................... 11 2.3 Results .............................................................................................................................. 11 2.3.1 Juvenile density and composition ............................................................................ 11 2.3.2 Characterization of sedimentation ........................................................................... 14 2.4 Discussion ......................................................................................................................... 16 Chapter 3. Using coral size distribution to assess the recovery from mass bleaching in the southern Caribbean ......................................................................................................... 21 3.1 Introduction ...................................................................................................................... 21 3.2 Methods ........................................................................................................................... 23 3.2.1 Benthic cover survey ................................................................................................ 24 iv 3.2.2 Colony size frequency survey ................................................................................... 25 3.2.3 Statistical analysis ..................................................................................................... 25 3.3 Results .............................................................................................................................. 26 3.3.1 Changes in percent coral cover ................................................................................ 26 3.3.2 Changes in coral population structure and community composition ...................... 27 3.4 Discussion ......................................................................................................................... 32 Chapter 4. Conclusion ............................................................................................................ 36 Bibliography .......................................................................................................................... 39 Appendices ............................................................................................................................ 46 v List of Tables Table 1. Information on the siX reef sites where juvenile assessment was undertaken and sediment traps were installed ..................................................................................... 8 Table 2. Sieved size groups and sediment classes according to the Wentworth size class system. ............................................................................................................................. 10 Table 3. Coral juvenile data per site. ................................................................................ 11 Table 4. Number of juvenile taxa found at each site. ....................................................... 12 Table 5. Mean and standard deviations of sediment measurements .............................. 15 Table 6. Mean percent cover of live coral and (±) standard deviation estimated at each site and year ..................................................................................................................... 27 Table 7. Total number of coral colony and species recorded per site .............................. 27 Table 8. Summary of the colony abundance and size data collected for each dominant species present at each reef sites. ................................................................................... 46 Table 9. Significant comparison of size frequency distributions and colony size between years (2010, 2011 and 2013) determined using Kolmogorov-Smirnov test (KS) and Kruskal-Walis (KW) test respectively. ............................................................................... 47 Table 10. Significant comparison size frequency distributions between reef sites determined using Kolmogorov-Smirnov test (KS) test respectively. ................................ 48 vi List of Figures Figure 1. Map of Tobago and location of studied reef systems and site ........................................ 7 Figure