Ronan Roche

Spatial variation in porosity and skeletal element characteristics in apical tips of the branching coral Acropora pulchra (Brook 1891)

Micro-CT scanning techniques were used to investigate fine-scale variation in porosity along branch tips of Acropora pulchra. Porosity variation is a result of progressive thickening of skeletal elements away from the apical tip of branches, rather than changes in the spacing of skeletal elements. A linear fit was found to describe the relationship between distance along the tip and both porosity and skeletal thickness. The slope of the line obtained may relate to branch extension rates and allow retrospective data to be obtained from Acropora specimens. Skeletal morphology examined by 2D and 3D imaging shows a progressive gradation in thickness occurring in the axial corallite wall and thickness changes at a site of incipient branch formation. The application of the micro-CT technique to museum and fossil specimens is illustrated.

Mid-Holocene sea surface conditions and riverine influence on the inshore Great Barrier Reef

We present measurements of Sr/Ca, δ18O, and spectral luminescence ratios (G/B) from a mid-Holocene Porites sp. microatoll recovered from the nearshore Great Barrier Reef (GBR). These records were used as proxies to reconstruct sea surface temperature (SST), the δ18O of surrounding seawater (δ18Osw), and riverine influence, respectively, and compared with records from a modern Porites sp. microatoll growing in the same environment. Strong riverine influence in the mid-Holocene record is indicated by (1) an increased annual δ18Osw range in the mid-Holocene record, (2) negative peaks in δ18O characteristic of flood events, and (3) a higher G/B luminescence ratio. Seasonal cycles in G/B suggest that humic acid inputs were elevated for a longer portion of the year during the mid-Holocene. The seasonal cycle of δ18Osw peaked earlier in the year in the mid-Holocene record relative to the modern, while mean δ18Osw values from the mid-Holocene record were similar to modern values. These records provide an insight into the oceanographic conditions the nearshore GBR experienced during mid-Holocene climatic shifts and are consistent with a strong Australian–Indonesian Summer Monsoon (AISM) system at ~ 4700 cal. yr BP.

Patterns in reef fish assemblages: Insights from the Chagos Archipelago

Understanding the drivers of variability in the composition of fish assemblages across the Indo-Pacific region is crucial to support coral reef ecosystem resilience. Whilst numerous relationships and feedback mechanisms between the functional roles of coral reef fishes and reef benthic composition have been investigated, certain key groups, such as the herbivores, are widely suggested to maintain reefs in a coral-dominated state. Examining links between fishes and reef benthos is complicated by the interactions between natural processes, disturbance events and anthropogenic impacts, particularly fishing pressure. This study examined fish assemblages and associated benthic variables across five atolls within the Chagos Archipelago, where fishing pressure is largely absent, to better understand these relationships. We found high variability in fish assemblages among atolls and sites across the archipelago, especially for key groups such as a suite of grazer-detritivore surgeonfish, and the parrotfishes which varied in density over 40-fold between sites. Differences in fish assemblages were significantly associated with variable levels of both live and recently dead coral cover and rugosity. We suggest these results reflect differing coral recovery trajectories following coral bleaching events and a strong influence of ‘bottom-up’ control mechanisms on fish assemblages. Species level analyses revealed that Scarus niger, Acanthurus nigrofuscus and Chlorurus strongylocephalos were key species driving differences in fish assemblage structure. Clarifying the trophic roles of herbivorous and detritivorous reef fishes will require species-level studies, which also examine feeding behaviour, to fully understand their contribution in maintaining reef resilience to climate change and fishing impacts.

Towards Developing a Mechanistic Understanding of Coral Reef Resilience to Thermal Stress Across Multiple Scales

Coral reefs are a globally threatened ecosystem due to a range of anthropogenic impacts. Increasing sea surface temperatures associated with global warming are a particular threat, as corals grow close to their upper thermal limit. When this limit is exceeded for a sufficient length of time during thermal stress events, corals lose their algal symbionts, resulting in coral bleaching and possible mortality. Coral reefs have experienced the most severe and extended global bleaching event to date from 2014 to 2017. The most recent global climate models predict that similar global bleaching events are likely to become an annual occurrence by the middle of the present century. Current understanding of coral reef recovery following disturbance events is based around decadal to sub-decadal impacts, making the adaptive capacity of corals as bleaching events approach an annual frequency unknown. However, there is considerable spatial heterogeneity in bleaching impacts across a range of scales, from global reef provinces to local reef areas and between coral species. Understanding of the mechanisms responsible for this observed coral resilience to thermal stress is increasing in a variety of disciplines, with particular recent advances at the sub-cellular level, facilitated partly by technological developments. This understanding suggests that some resilience factors have the potential to operate within the predicted annual frequency of thermal stress events, whilst others act over longer time-scales. The ability of coral reef management actions to successfully support coral resilience is a significant challenge and requires increased empirical evidence to support and refine actions. However, in addition to essential global actions to reduce carbon dioxide emissions, protective actions can be strengthened by a focus on identifying reef locations that have the potential to exhibit resilience to thermal stress events, either via resisting them or recovering quickly following impact. Here, we present a spatially explicit overview of the potential resilience factors and mechanisms that can be considered in such an approach.

First record of coralline fungal disease (CFD) in the Indian Ocean

Crustose coralline algae (CCA) play a key role in calcification and consolidation of substrate on coral reefs, with some species also providing important settlement substrate for coral recruits. Like corals, CCA suffer diseases that threatens their survival and persistence (Vargas-Ángel 2010), including a fungal disease that results in rapid tissue necrosis, particularly during ocean warming events (Williams et al. 2014). Coralline fungal disease (CFD) was first reported at the island of Tutuila in American Samoa in 1998 (Littler and Littler 1998) and later at the islands of Swains and Rose in American Samoa (VargasÁngel 2010). Other reports of CFD are restricted to the remote central Pacific at Kingman Reef and Palmyra Atoll in the northern Line Islands (Vargas-Ángel 2010; Williams et al. 2014). Here we report on the first field sightings of CFD in the Indian Ocean at islands within the remote and highly protected Chagos Archipelago, British Indian Ocean Territory (Fig. 1). Of the 29 reefs surveyed during an expedition in April 2018 across ~ 200 km of latitude, CFD was documented at 8 of them. CFD sightings were restricted to shallow (< 15 m depth) fore reef habitats and not observed on backreef or patch reef habitats. At two reefs, CFD was at outbreak levels, with > 5 cases m of CCA. These are among the highest densities of CFD ever reported and appeared to correlate with high host density at these locations (> 50% host CCA cover). The coral reefs of the Chagos Archipelago suffered from back-to-back ocean warming events in 2015, 2016 and 2017 that have reduced live hard coral cover. The high densities of CFD recorded here may represent a residual impact of these warming events but whether CFD significantly alters key reef processes such as accretion, coral recruitment and substrate consolidation across the region requires further study.