A mechanistic approach to understanding and predicting hydrodynamic disturbance on coral reefs

Transcription

1 A mechanistic approach to understanding and predicting hydrodynamic disturbance on coral reefs Thesis submitted by Joshua Simon MADIN B.Sc. (Hons.) March 2004 For the degree of Doctor of Philosophy in Marine Ecology within the School of Marine Biology and Aquaculture James Cook University

2 STATEMENT OF ACCESS I, the undersigned, author of this work, understand that James Cook University will make this thesis available for use within the University Library and, via the Australian Digital Theses network, for use elsewhere. I understand that, as an unpublished work, a thesis has significant protection under the Copyright Act and; I do not wish to place any further restriction on access to this work. 2

3 ABSTRACT To understand and predict community dynamics in habitats where physical disturbance is a major cause of mortality, we must understand the frequency and intensity of these events, as well as their differential effects on the community s structural species. Using a tropical coral reef as a study system, the aim of this thesis was to quantify the mechanical vulnerability of the habitat-forming structural species, scleractinian corals, and build a framework to estimate size- and species-specific mortality rates based on the return time and magnitude of hydrodynamic disturbances. To accomplish this aim, a geometric model and classical engineering theory were used to identify the factors upon which colony strength depends. These factors were 1) the tensile strength of the limiting material at the colony/substrate interface, 2) the projected shape of the colony perpendicular to water motion and 3) the maximum water velocity per wave cycle. To investigate the first of these factors, the strength of coral skeleton from three morphologically disparate species (submassive Acropora palifera [Subgenus Isopora], corymbose Acropora gemmifera and tabular Acropora hyacinthus) and the strength of the reef substrate were investigated spatially at a hydrodynamicallyexposed shallow reef platform (Lizard Island, Australia) to determine whether overall colony strength (i.e., a colony s ability to withstand physical stress) is limited by skeletal strength or the reef substrate to which it is attached. To investigate the second factor, colonies of the three study species were photographed from the exposed reef crest along belt transects shoreward towards the relatively sheltered reef back and mechanically quantified using a novel technique for calculating the maximum predicted stress (MPS) at the base of a colony as a function of its projected shape for a given water velocity (MPS can be thought of as an objective quantification of mechanical 3

4 vulnerability). Finally, to examine the third factor, a 37-year meteorological record of hourly wind conditions, in conjunction with a field-calibrated oceanographic modelling procedure, was used to comprehensively quantify maximum water motion (i.e., displacement, velocity and acceleration per wave cycle) on the study reef platform at scales ranging from seconds to decades and from metres to the entire reef. Using the measurements of each colony s limiting strength (factor 1) and MPS (factor 2), the maximum water velocity that each colony is predicted to be able to withstand was calculated. The expected mortality rates of colonies from physical disturbances were calculated by fitting the exponential probability density function to the frequency distribution of times between wave events which produced maximum water velocity (per wave cycle; factor 3) greater than that which the colony is predicted to be able to withstand at a given location on the reef. The carbonate substrate of coral reefs served as the limiting factor to the strength of mechanically threatened coral reef colonies and raises the question of why corals invest resources into building skeletons that are stronger than mechanically necessary. The robust, submassive Acropora palifera has a significantly lower MPS than the corymbose Acropora gemmifera, which in turn has a significantly lower MPS than the competitively superior, yet more mechanically vulnerable, tabular Acropora hyacinthus. In addition to these inter-specific differences, these three species display distinctly different intra-specific patterns of MPS when examined with relation to colony size. Acropora palifera demonstrates approximately equal MPS values regardless of size, whereas Acropora gemmifera and Acropora hyacinthus become significantly more mechanically vulnerable with increased size. In addition, the lognormal distribution of MPS values of the mechanically inferior species, Acropora hyacinthus, was truncated at the reef crest, suggesting the imposition of a mechanical threshold by a recent 4

5 hydrodynamic disturbance which caused colonies with an MPS above the threshold to have been dislodged from their position on the reef. Superimposition of the disturbance thresholds modelled from the meteorological record onto the three study populations illustrated that larger colonies of Acropora hyacinthus are indeed predicted to be mechanically removed at regular intervals from the reef at Lizard Island, and such removal regimes are predicted to occur in a manner consistent with present distributions of this species on the reef. Finally, estimated mortality rates for coral colonies in general (i.e., based on their MPS) vary significantly over the reef s hydrodynamic gradient and suggest that a mechanical refuge exists approximately 40-60m back from the reef crest. This refuge is in addition to the expected refuges found at the reef base and reef back, each of which should theoretically promote mechanical, and therefore morphological, diversity. This study has developed a framework for which the change in community structure of scleractinian corals caused by recurrent physical disturbance can be estimated. Building on this framework will significantly enhance ecological understanding of the relationship between physical disturbance and biodiversity on coral reefs, and will facilitate the estimation of general future community changes resulting from changes in the intensity and frequency of physical disturbance that may be associated with global climate change. 5

6 ACKNOWLEDGEMENTS I would like to thank my supervisors, Terry Hughes and Sean Connolly, for their excellent advice and support. I also thank Terry for his helpful suggestions, for financial support during my first year of candidature, and for helping to cover the cost of field trips. I also thank Sean for his help in formulating the probability densities in Chapter 3, and for helping me financially with my work in New Zealand with Kerry Black. I thank Kerry Black for help with the oceanographic modelling portion of this thesis. Thank you to my thesis examiners Mark Denny (Stanford University) and Emily Carrington (University of Rhode Island) for their helpful comments and suggestions. I am grateful to the funding bodies that provided financial support for my project. These organizations are: the Australian Research Council (Australian Postgraduate Award), the Packard Foundation (for support to attend Mark Denny s biomechanics course at Hopkins Marine Station), James Cook University (Write-up Scholarship), Sigma Xi (Grant-in-aid of Research), the Great Barrier Reef Marine Park Authority (Augmentative Research Grant), the Cooperative Research Centre for the Great Barrier Reef World Heritage Area (Augmentative Research Grant), the Australian Coral Reef Society (travel grant) and James Cook University s School of Marine Biology and Aquaculture (Doctoral Merit Research Scheme Grant). I would like to thank the staff of the Lizard Island Research Station for their support: Anne Hoggett, Lyle Vail, Bob and Tania Lamb, and Marianne and Lance Pearce. I would also like to thank everyone who helped me in the field: Andrew Baird, Morgan Pratchett, Tony Dell, Elizabeth Madin, Michael Berumen, Maria Dornelas and Chris Dudgeon. I also thank the support staff of the JCU Biological Sciences Department, and Rob Gegg in particular for his help in the workshop. Thanks also to my friends and colleagues. In particular, I would like to thank Tony Dell, Jackie Wolstenholme, Jason McKenzie, Morgan Pratchett, Andrew Baird and all the members of the Centre for Coral Reef Biodiversity and the Theoretical Ecology Lab Group for all of their help and advice. I am forever grateful to my family for their support and love. I am especially grateful to my wife Elizabeth for her love and for being my best friend. On behalf of all of Elizabeth s help and patience, I dedicate this thesis to her. 6

7 TABLE OF CONTENTS CHAPTER 1: General introduction CHAPTER 2: The mechanical integrity of coral colonies 2.1 Introduction 2.2 Methods Relative importance of stress-types Mechanical testing of coral skeleton and reef substrate Spatial gradients in strength of coral skeleton and reef substrate Limiting strength of coral colonies: dislodgement versus pruning 2.3 Results Relative importance of stress-type Mechanical testing of coral skeleton and reef substrate Spatial gradients in strength of coral skeleton and reef substrate Limiting strength of coral colonies: dislodgement versus pruning 2.4 Discussion Limits to colony size The limiting strength of coral colonies Spatial gradients of substrate and skeletal strength over the reef CHAPTER 3: Measuring and comparing biomechanical variation in reef corals 3.1 Introduction 3.2 Methods Calculating the maximum predicted stress Data collection Data analysis 3.3 Results Study species and sites Maximum predicted stress and colony size Maximum predicted stress over a hydrodynamic exposure gradient 3.4 Discussion Biomechanical differences between the study species Relationships between maximum predicted stress and colony size

8 3.4.3 Maximum predicted stress and distance over the reef CHAPTER 4: Wave-induced water motion on shallow coral reef platforms 4.1 Introduction 4.2 Methods Study location Expected return time of maximum significant wave heights Expected largest wave during a given wave event Expected largest wave during an organism s life Wave attenuation and water motion over the reef 4.3 Results Expected return time of maximum significant wave heights Expected largest waves through time Wave attenuation and water motion over the reef 4.4 Discussion The return-time of large wave climates The largest waves on the reef The attenuation of waves over the reef The motion of water on the reef CHAPTER 5: Hydrodynamically induced mortality rates of reef corals 5.1 Introduction 5.2 Methods A 37-year estimate of maximum flow velocity over the reef Substrate strength over the reef Instantaneous rate of mortality The maximum predicted stress of colonies from three local populations 5.3 Results Coral mortality resulting from hydrodynamic disturbance is a Poisson process Variation in mortality rate over the reef Projecting model mechanical thresholds onto real coral populations 5.4 Discussion Hydrodynamic disturbance approximates a Poisson process Hydrodynamic disturbance and mortality Magnitude of disturbance effects over the reef Mechanical thresholds

9 CHAPTER 6: General discussion 6.1 Biomechanical limitations of corals 6.2 The dominant hydrodynamic force acting on coral colonies 6.3 Predicting the maximum stress at the colony/substrate interface Maximum predicted stress as a function of taxonomic identity Maximum predicted stress as a function of intraspecific colony size Maximum predicted stress as a function of position over a the hydrodynamic gradient 6.4 Predicting colony mortality rates APPENDICES REFERENCES

10 STATEMENT OF SOURCES DECLARATION I declare that this thesis is my own work and has not been submitted in any form for another degree or diploma ay any university or other institution of tertiary education. Information derived from the published or unpublished work of others has been acknowledged in the text and a list of references is given. 10