- Title
- Toward a framework for predicting structural decay on coral reefs following extreme marine heatwaves
- Creator
- Fordyce, Alexander John
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2021
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Marine heatwaves (MHWs), discrete periods of anomalous ocean warming, are becoming longer and more frequent because of Anthropogenic climate change. As a direct result of increased MHW intensity, coral reefs are being degraded worldwide as these temperature anomalies drive localised or mass coral bleaching. There is considerable heterogeneity in heating within these large events, and some areas can experience intense, localised heating that is greater than an event’s average intensity. These are “MHW hotspots” (Chapter 1) and it was recently discovered that the extremity of heating in these hotspots causes unprecedented rates of skeletal degradation by promoting microbially-mediated bioerosion of coral skeletons (Appendix D. iv). At the reef scale, the outcome is an acute loss of structural complexity as the physical framework is eroded and this affects a range of coral reef ecosystems services such as coastal protection, tourism, and fisheries. Coral reefs, like MHWs, are highly variable and so being able to predict reef-scale risk of MHW-driven structural degradation would, in theory, allow the targeted management of these areas to prevent this extreme outcome. This thesis represents the first steps toward constructing a predictive framework aimed at identifying the risk of an acute loss of structural complexity, due to extreme thermal stress on coral reefs. In Chapter 2, new interdisciplinary methods are first designed for the purpose of studying acute skeletal dissolution in corals and these reveal changes to the internal structure of the skeleton during dissolution not previously described (Chapter 3). Specifically, skeletal permeability is highlighted as being an important internal feature of the skeleton affecting the extent of microbioerosion in Porites cylindrica (Chapter 3), and in Acropora aspera (Chapter 2) compared to Pocillopora damicornis. There is a need to create a rapid assay for coral skeletal permeability so that this biomechanical feature can be examined in more detail and used in study of reef-scale dissolution. Chapter 3 also demonstrates that the unprecedented rates of skeletal degradation during extreme thermal stress are, in part, due to the post-mortality coral tissue decomposition enriching the endolithic microenvironment with nutrients (Chapter 3). Identifying this mechanism of promoting microbioerosion highlights the potential utility of tissue thickness as a predictive measure of the extent of bioerosion across species. It contrasts our understanding of how thick tissues increase coral resilience to moderate thermal stress and suggests that ‘winners’ during moderate coral bleaching events may be most at risk of rapid degradation during extreme MHW hotspots. Further trait-based studies found that tissue thickness was predictive of total microbial biomass in the skeleton but not the concentration of chlorophyll in phototrophic skeletal microbes (Chapter 4). This suggests that tissue thickness affects heterotrophic microbial taxa in the skeleton only, such as saprotrophic fungi which dominate the heterotrophic community and are likely the primary decomposers of dead coral tissue (Chapter 3). More information on metabolic exchanges between corals and their endolithic microbiome is likely to highlight new pathways through which coral responses to thermal stress may affect microbial bioerosion of their skeletons. Chapter 4 also highlights positive relationships between skeletal density, the size and complexity of corallites and the biomass of phototrophic microbes in the skeleton. Corallite structure is indicative of the capacity of a coral skeleton to capture and scatter light on a micromorphological level. The more effective a coral is at light capture, the more susceptible it is to coral bleaching, so in contrast to tissue thickness we see an alignment in features that predict microendolithic biomass and bleaching resilience. In addition to internal and external skeletal morphology, Chapter 5 reveals the need to account for other endolithic associates in the coral skeleton when predicting how much microbioerosion might occur under extreme thermal stress. A novel association was identified in that phototrophic and total microbial biomass in the skeleton of Isopora palifera was twice as high when it was also inhabited by macroboring bivalve (Chapter 5). This reflects nutrient enrichment of the coral skeleton due to the bivalve’s waste products and supports the assertion the endolithic microbiome is responsive to nutrient enrichment on the scale of skeletal microhabitats (Chapter 3, 5). Similar relationships have been reported between microborers and other macroborers, but more data is needed on how thermal stress might disrupt or amplify interactions within the bioerosion loop. During a natural bleaching event, Isopora palifera colonies whose endolithic microbiomes have relatively low biomass (Chapter 4) frequently experienced blooms of endolithic algae (Chapter 6), and these corals are almost always inhabited by at least one endolithic bivalve (Chapter 5). Further, branching Acropora corals most frequently experienced endolithic algal blooms during this MHW (Chapter 6), and these frequently formed near an interaction zone between corals and epilithic algae. Together these findings highlight the need to consider these two aspects of endolithic algal ecology: the form the coral they are inhabiting (both macromorphological and micromorphological, internal and external), their co-habitants in the endolithosphere, and the identity of the benthic organism living above them. An acute loss of structural complexity on coral reefs has a long-lasting effect on reef function and maintaining structural complexity is a key target for coral reef managers, particularly in the context of safeguarding ecosystem provisioning. This thesis presents a framework, based on generalisable rules of what makes a coral susceptible to rapid microbioerosion, which can theoretically be used to identify high-risk areas. This can support the effective allocation of conservation resources to avoid reef structural collapse. There is considerable scope to adapt frameworks that already exist for describing community patterns in thermal tolerance, toward predicting the risk of structural decay on coral reefs during extreme MHWs and viewing coral traits through the lens of endolithic microbial ecology can help identify traits of interest. The distinct ecological outcome of an acute loss of structural complexity, requires a distinct approach toward predicting when and where it might occur. While regional-scale warming cannot be prevented without addressing global climate change, offsetting structural degradation in localised marine heatwave hotspots can protect a key ecological characteristic of coral reefs and maximise their resilience during the Anthropocene.
- Subject
- coral reefs; marine heatwaves; bioerosion; microbiology; endolith; Great Barrier Reef; community ecology; thesis by publication
- Identifier
- http://hdl.handle.net/1959.13/1504040
- Identifier
- uon:55435
- Rights
- Copyright 2021 Alexander John Fordyce
- Language
- eng
- Full Text
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| Thumbnail | File | Description | Size | Format | |||
|---|---|---|---|---|---|---|---|
| View Details Download | ATTACHMENT01 | Thesis | 20 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 674 KB | Adobe Acrobat PDF | View Details Download |