title: Investigating the diversity and distribution of Mesophotic Coral Ecosystems in the Chagos Archipelago
Creators: name: Diaz, Clara
Publisher: name: University of Plymouth
Year: 2023
Subjects: subject: Chagos Archipelago
subject: central Indian Ocean
subject: community structure
subject: environmental drivers
subject: habitat suitability modelling
subject: benthic diversity
subject: deep coral bleaching
subject: Mesophotic Coral Ecosystems
subject: PhD

description: Mesophotic coral ecosystems (MCEs, 30-150 m) have received increasing attention from the scientific community over the past two decades, as their significance has been recognised in terms of biodiversity, distribution, and ecological value. However, MCEs remain relatively unexplored compared to their shallow-water counterparts. They are also under-protected and under-represented in marine spatial planning, despite meeting the international criteria used to identify priority areas for conservation policies. One reason for the imbalance between their high ecological value and their low level of protection is that baseline data on MCEs is largely missing to date. Furthermore, research efforts have been unevenly distributed, with studies predominantly focussing on scleractinian corals in specific geographic regions. The Indian Ocean is particularly under-represented in terms of research on MCEs. Comprehensive information on MCEs, including the overall benthic community composition, their distribution, the environmental drivers shaping them, and their susceptibility to human pressures are largely unknown in the Indian Ocean. Yet, such data are crucial to make evidence-based decisions for management and conservation actions. The studies carried out in this thesis aimed to contribute to our understanding of the ecology of benthic MCEs in the Chagos Archipelago, central Indian Ocean; and participate in enhancing MCE conservation policies. More specifically, the research goals of the individual data chapters were to investigate variation in MCEs between depths and sites in terms of i) community structure and environmental drivers, ii) alpha and beta diversity, iii) distribution and iv) vulnerability to thermal stress. Invertebrate benthic communities were surveyed along the entire mesophotic depth gradient, from shallow to lower mesophotic depths (15 m to 160 m), using digital imagery collected from Remotely Operated Vehicle (ROV) surveys, as well as environmental data using different techniques, at Egmont Atoll, Chagos Archipelago. MCEs were firstly described in terms of community structure, using a cluster analysis, as well as in terms of alpha and beta diversities, using rarefaction/extrapolation curves and Jaccard’s dissimilarity, respectively. Secondly, the variation in benthic communities along the depth gradient and across sites was investigated with the potential environmental processes driving these differences, using a distance-based linear model. Thirdly, the spatial distribution of benthic mesophotic communities within the Archipelago was predicted using habitat suitability modelling techniques. Lastly, using a multidisciplinary approach, mesophotic coral vulnerability was assessed at Egmont Atoll. MCEs of the Chagos Archipelago were found to harbour diverse, distinct, and unique benthic communities, with differences in their community structure driven primarily by irradiance and temperature. A community break was observed at ~60 m deep, and newly defined MCE boundaries for the Chagos Archipelago were proposed, based on a combination of biological and environmental data. In addition, an extensive distribution of MCEs across the entire Archipelago was revealed, driven by both environmental and topography-derived variables. Finally, evidence of severe coral bleaching was observed down to 90 m, the deepest ever observed in the Indian Ocean, in the absence of shallow-water bleaching. Sub-atoll scale heterogeneity in bleaching prevalence and severity was also observed. This deep coral bleaching demonstrates the susceptibility of MCEs to thermal stress and emphasizes the need for oceanographic knowledge to predict bleaching susceptibility and heterogeneity. In conclusion, the results of this PhD thesis provide a comprehensive understanding of the ecology of MCEs in the Chagos Archipelago. The results show that MCEs harbour a high diversity and are unique communities compared to shallow-water reefs; with specific environmental processes that are driving their structure and extensive distribution. Furthermore, MCEs are vulnerable to climate change stressors, which may have previously been overlooked. Finally, this thesis highlights the need for targeted monitoring programs and management plans for MCEs, and aims to be used as a baseline to develop evidence-based protection of these precious ecosystems in a rapidly changing world.