Depth as refuge: depth gradients in ecological pattern, process, and risk mitigation among coral reef fishes

The impacts of anthropogenic habitat disturbance are often asymmetric along environmental gradients and among taxa. For species that cannot successfully utilize post disturbance habitats, the ability to occupy positions on spatial gradients that fall outside of disturbance regimes may offer a key refuge. However, decreasing resource availability or quality, and changing ecological and behavioural dynamics along gradients may result in substantial physiological costs for fringe-dwelling organisms. Assessments of potential refuges therefore require nuanced spatially gradated ecological assessments that are often absent and difficult to attain. Coral reefs are now heavily impacted by climate related disturbance, and the greatest rates of biotic attrition among reef fishes generally occur within species obligated to associate with live corals. Because key drivers of future coral loss (i.e. warm water bleaching and storm events) may attenuate with depth, deep reefs hypothetically offer a refuge to vulnerable fishes. However, because of access difficulties, most ecological studies on coral reef organisms occur in shallow waters of <15 m. In Chapter 2, I investigated the natural depth distributions, depth-related variation in community structure and coral habitat associations for 123 reef fish species at 6 depths between 0m and 40m, and from inner-bay to offshore reefs. The results indicated that depth is a stronger driver of reef fish assemblages than cross shelf gradients, though complex coral habitats and some associated fish species more frequently occupy deeper depths further from shore. Total live hard coral cover did not decline with depth in Kimbe Bay, though the cover of habitat-providing complex corals declined with depth. The major break in the community assemblage of reef fishes occurred between 5 m and 10 m, and 25% of species were limited to the shallowest 5m. However, 25% of species occurred at all depths between 0m and 30m, and 12% between 0m and 40m. In addition, I show that 85% of species with strong associations with live complex coral habitats occurred at depths of 20m or below. I therefore conclude that deep reef habitats in Kimbe Bay can provide a substantial refuge potential if reef degradation does attenuate with depth and the ecological costs of occupying deep periphery habitats are not prohibitive to long-term population maintenance. In Chapters 3 to 6, I utilized the Chaetodontidae family (Butterflyfishes) to further investigate how a broad suit of behaviours and ecological dynamics that influence the distribution, vulnerability and success of a wide range of taxa in multiple biomes interrelate and vary among reef fishes along a broad coral reef depth gradient, from 0 – 40 m. Interrelationships among distribution breadth, abundance, and degree of resource specialization form the basis of many general models in ecology, as well as extinction-risk assessments in conservation biology. Species with narrow distributions, low abundance and high resource specialisation are more vulnerable to environmental change and risk increases when vulnerability traits are combined. In Chapter 3, I evaluate whether depth may mediate these risks in coral-specialist fishes. Contrary to expectation, the most coral-specialized species were also the most abundant and the most broadly distributed. Further, no specialist-species had combined vulnerability traits, and no specialists were wholly restricted to shallow-water. Chapter 3 demonstrates that interrelationships among vulnerability traits and occupancy depths do not necessarily follow traditional ecological expectations on coral reefs, but they do work to mediate substantial risks for species vulnerable to shallow-reef habitat declines. Chapters 2 and 3 demonstrate that many ecologically vulnerable reef fish species may offset the risks associated with shallow-water habitat losses by utilising deep habitats. However, the refuge potential of deep peripheral habitats may be mediated by the potentially substantial costs of securing sparsely distributed resources, which can limit survival and reproductive output. Further, depth-related resource shifts are likely to be more detrimental to dietary specialists than to generalists. In Chapter 4, I use extensive and intensive in-situ behavioural observations in combination with physiological condition measurements to examine the costs and benefits of resource-acquisition along the depth-gradient in two obligate corallivore reef fishes with contrasting levels of dietary specialisiation. I demonstrate that the space utilised to secure coral-resources increases towards deeper depths, as expected. However, increased territory sizes result in equal or greater total resources secured within deep territories. Foraging-distance, pairing-behaviour, body condition and fecundity did not decline with depth, but competitive interactions did. Unexpectedly, therefore, coral-specialist fishes selecting high-quality coral patches in deep water access equal or greater resources than their shallow-reef counterparts, with no extra costs. As demonstrated in Chapter 4, the capacity for species to successfully occupy range peripheries is enhanced by their ability to mediate costs related to decreases in quantities and quality of key resources. In Chapter 5, I investigate the capacity to of species to employ variation in dietary strategies and energy acquisition along depth gradients. I focus on two obligate corallivores with differing levels of dietary specialization, as well as their mixotrophic coral prey. Total resource availability and total feeding effort did not decline toward deep-range peripheries in either fish species, but availability of preferred Acropora resources did decline. The more specialized species exhibited limited feeding plasticity along the depth gradient, and selective feeding effort on the preferred coral genus Acropora increased rather than decreased with depth. In contrast, the generalist's diet varied greatly with depth, reflecting changes in prey composition. Unexpectedly, the nutritional content of Acropora did not decline with depth, with shifts in δ¹³C and δ¹⁵N indicating increased coral heterotrophy in deeper water may offset declines in light energy. Mixed modelling of stable isotopes in amino acids of fish muscle tissue revealed a parallel increase in plankton-sourced carbon among deep-resident fish. Therefore, deep ranges appear to be supported by multiple mechanisms of dietary versatility, but for specialist species this versatility occurred at the resource level (corals), rather than among the consumers (fish). This dietary variability and trophic plasticity may act to buffer costs and bolster refuge potentials associated with dwelling at deep range peripheries, even among taxa with differential functional strategies. In Chapter 6, I utilize two natural experiments to 1) demonstrate that a natural habitat disturbance event (a crown of thorns sea-star outbreak) can result in differential impacts and outcomes on shallow and deep populations of the coral obligate reef fish Chaetodon baronessa that favour the persistence of deep population; and 2) individual fishes are able to migrate downward, away from territories in degrading shallow-water habitats to inhabit healthy deep-reef habitats when made available via experimental competitor removal. Overall, my thesis highlights how interrelationships among vulnerability traits, occupancy depths, and deep coral habitats, offer some risk mitigation among taxa currently thought to express high vulnerability to global-scale coral declines in shallow-water. The thesis further demonstrates how various combinations of stability and plasticity in resource specialization, space use, effort, food availability and quality, diet, feeding behaviour, and body condition, may aid the successful exploitation of deep refuges by species with contrasting functional traits. Finally, severe habitat disturbance can differentially impact fish and habitat survival between shallow and deep reefs, and individual fish are demonstrably able to utilize downward vertical migration away from declining shallow-water habitat to access higher-quality deep-water habitats where prior residence is not established

Variability in the functional composition of coral reef fish communities on submerged and emergent reefs in the central Great Barrier Reef, Australia

On coral reefs, depth and gradients related to depth (e.g. light and wave exposure) influence the composition of fish communities. However, most studies focus only on emergent reefs that break the sea surface in shallow waters (<10 m). On the Great Barrier Reef (GBR), submerged reefs (reefs that do not break the sea surface) occupy an area equivalent to all emergent reefs. However, submerged reefs have received comparatively little research attention, and fish communities associated with submerged reefs remain poorly quantified. Here, we quantify fish assemblages at each of three depths (10, 20 and 30 m) on eight submerged reefs (four mid-shelf and four outer-shelf) and two nearby emergent reefs in the central GBR where reef habitat extends from 0-~25 m depth. We examine how total fish abundance, the abundance of 13 functional groups, and the functional composition of fish communities varies among depths, reef types (submerged versus emergent reefs), and shelf position (mid-shelf versus outer-shelf). Overall fish abundance decreased sevenfold with depth, but declined less steeply (twofold) on outer-shelf submerged reefs than on both mid-shelf submerged reefs and emergent reefs. The functional composition of the fish assemblage also varied significantly among depths and reef types. Turnover in the functional composition of the fish community was also steeper on the mid-shelf, suggesting that shallow-affiliated groups extend further in deeper water on the outer-shelf. Ten of the 13 functional groups were more strongly associated with the shallowest depths (the upper reef slope of emergent reefs or the 'crests' of submerged reefs), two groups (soft coral/sponge feeders and mesopredators) were more abundant at the deepest sites. Our results confirm that submerged reefs in the central GBR support a wide range of coral reef fishes, and are an important component of the GBR ecosystem

Back-to-back coral bleaching events on isolated atolls in the Coral Sea

Severe bleaching events caused by marine heatwaves over the past four decades have now affected almost every coral reef ecosystem in the world. These recurring events have led to major losses of coral cover, with adverse consequences for tropical reef ecosystems and the people who depend on them. Here, we document two consecutive and widespread coral bleaching events on remote atolls in the Coral Sea in 2016 and 2017. In each year, the proportion of colonies that bleached was strongly related to heat exposure (measured as Degree Heating Weeks, DHW, °C-weeks), depth and coral assemblage structure. Bleaching was more severe at higher DHW exposure, and at sites with a higher proportion of susceptible taxa. Bleaching was also lower at 6 m than at 2 m depth. Despite the severe bleaching in 2016 on reefs in the central section of the Coral Sea Marine Park, total coral cover was not significantly reduced by 2017, suggesting that most bleached corals survived. Moreover, bleaching was less severe in 2017 despite a higher exposure to heat stress. These results indicate that while the isolation of these oceanic reefs provides no refuge from bleaching, low nutrient levels, high wave energy and proximity to cooler deeper waters may make coral on these reefs more resistant to bleaching induced mortality

The challenges posed by equine arboviruses

Equine populations worldwide are at increasing risk of infection by viruses transmitted by biting arthropods including mosquitoes, biting midges (Culicoides), sandflies and ticks. These include the flaviviruses (Japanese encephalitis, West Nile and Murray Valley encephalitis), alphaviruses (eastern, western and Venezuelan encephalitis) and the orbiviruses (African horse sickness and equine encephalosis). This review provides an overview of the challenges faced in the surveillance, prevention and control of the major equine arboviruses, particularly in the context of these viruses emerging in new regions of the world

Loss of coral reef growth capacity to track future increases in sea level

Water-depths above coral reefs is predicted to increase due to global sea-level rise (SLR). As ecological degradation inhibits the vertical accretion of coral reefs, it is likely that coastal wave exposure will increase but there currently exists a lack of data in projections concerning local rates of reef growth and local SLR. In this study we have aggregated ecological data of more than 200 tropical western Atlantic and Indian Ocean reefs and calculated their vertical growth which we have then compared with recent and projected rates of SLR across different Representative Concentration Pathway (RCP) scenarios. While many reefs currently show vertical growth that would be sufficient to keep-up with recent historic SLR, future projections under scenario RCP4.5 reveal that without substantial ecological recovery many reefs will not have the capacity to track SLR. Under RCP8.5, we predict that mean water depth will increase by over half a metre by 2100 across the majority of reefs. We found that coral cover strongly predicted whether a reef could track SLR, but that the majority of reefs had coral cover significantly lower than that required to prevent reef submergence. To limit reef submergence, and thus the impacts of waves and storms on adjacent coasts, climate mitigation and local impacts that reduce coral cover (e.g., local pollution and physical damage through development land reclamation) will be necessary

Solid-state nuclear magnetic resonance spectroscopy of cements

Cement is the ubiquitous material upon which modern civilisation is built, providing long-term strength, impermeability and durability for housing and infrastructure. The fundamental chemical interactions which control the structure and performance of cements have been the subject of intense research for decades, but the complex, crystallographically disordered nature of the key phases which form in hardened cements has raised difficulty in obtaining detailed information about local structure, reaction mechanisms and kinetics. Solid-state nuclear magnetic resonance (SS NMR)spectroscopy can resolve key atomic structural details within these materials and has emerged as a crucial tool in characterising cement structure and properties. This review provides a comprehensive overview of the application of multinuclear SS NMR spectroscopy to understand composition–structure–property relationships in cements. This includes anhydrous and hydrated phases in Portland cement, calcium aluminate cements, calcium sulfoaluminate cements, magnesia-based cements, alkali-activated and geopolymer cements and synthetic model systems. Advanced and multidimensional experiments probe 1 H, 13 C, 17 O, 19 F, 23 Na, 25 Mg, 27 Al, 29 Si, 31 P, 33 S, 35 Cl, 39 K and 43 Ca nuclei, to study atomic structure, phase evolution, nanostructural development, reaction mechanisms and kinetics. Thus, the mechanisms controlling the physical properties of cements can now be resolved and understood at an unprecedented and essential level of detail

AcroLipidR

Percent weight of lipids from coral tissues of the genus Acropora, collected along a depth gradients from 0-40m in Kimbe Bay, Papua New Guinea

ChaetAcroSelec

Feeding selectivity data along a depth gradient from 0-30m for two obligate coral feeding butterflyfishes (Chaetodon baronessa and Chetodon octofasciatus) in Kimbe Bay, PNG

AcroBSIA

Bulk Stable Isotopes (13C, 15N) of shallow (<5m) and deep (30-40m) corals from the genus Acropora and with Hispidose morphology

CSIAAAMixingModelData

Data for mixing models of baseline carbon contributions to Acropora corals and Chaetodon baronessa in shallow (<5m) and deep (20-40m) depths in Kimbe Bay, PNG. Compound Specific Stable Isotope Analyises of Amino Acids