The deep sea is our planet’s largest and least explored ecosystem. Once thought to be a barren abyss devoid of life, we have learned the deep sea is home to diverse ecosystems. One such deep-sea ecosystem that supports biodiversity is cold-water coral (CWC) reefs. Less studied than their tropical counterparts, CWC reefs provide a range of ecosystem services such as carbon storage, pharmaceutical development, and fisheries. Threats from climate change and increasing anthropogenic deep-sea activity make protecting and managing CWC reef futures exceptionally important. Scientific advances over the last decades have allowed us to better understand CWC reef ecosystems, though we remain far off from achieving effective ecosystem management. The overall aim of this thesis is to guide CWC reef management through expanding knowledge of these ecosystems.
Despite the majority of CWC reef area being composed of dead framework, most research has focused on understanding live coral responses to climate change. Dead framework supports the highest levels of biodiversity in a CWC reef system and contributes significantly to carbon and nitrogen cycling. Live corals and dead coral skeletons are vulnerable to different environmental stressors. To understand reef composition and accurately predict their futures under climate change, the proportion of live coral colonies to the entire reef structure must be quantified. Chapter 2 of this thesis guides CWC reef management through increasing our knowledge of depth’s role in driving live and dead reef proportions. In Chapter 2, the live and dead proportions of CWC Solenosmilia variabilis reefs are quantified across four seamount features in the southwest Pacific Ocean. Images of CWC reefs are analysed with significant differences in the proportions of live coral between reefs at the Louisville Seamount Chain and Graveyard Seamount Complex. Depth is identified as a driver of live and dead reef proportions in these regions, with a larger proportion of live coral at shallow depths and dead intact framework at deeper depths. Additionally, the proportion of live coral in the Graveyard Seamount Complex remained stable between 2015 and 2020, despite significant differences in the surface areas of live coral, dead intact framework, and the reef. These results indicate reef proportions can be used to estimate the amount of dead intact framework threatened by the shallowing aragonite saturation horizon (ASH) due to ocean acidification at each site, which can help inform which sites could be protected as possible climate change refugia. Identifying depth as a driver of reef proportions quantifies reef health, identifies reef threats, and predicts reef impacts, all of which increases our knowledge of current and future reef conditions.
Ocean acidification (OA) is a critical stressor and leads to dissolution of exposed calcium carbonate (CaCO3) in aragonite-undersaturated waters — a direct threat to dead CWC skeletons. This increased coral porosity from climate stressors threatens the structural integrity of the entire reef framework and could lead to a direct loss of habitat through crumbling. Laboratory studies have largely explored how skeletons grown under favourable conditions respond to exposure in OA conditions. We do not yet know if CWC skeletons grown in OA conditions are as robust as those grown in favourable conditions or if CWC skeletal structures are built differently in ideal and OA conditions. Chapter 3 of this thesis guides CWC reef management through increasing our knowledge of the 3-dimensional (3D) crystallographic structure of CWC. In Chapter 3, 3D volumes of CWC skeletal samples from above and below the ASH are compared using Electron Backscatter Diffraction (EBSD). Aragonite needles grow radially from Rapid Accretion Deposits (RAD) which join with neighbouring crystal structures to create the skeletal building blocks called sclerodermites. From large RADs, sequential imaging shows aragonite needles radiate with a preferred growth direction perpendicular to the calcification interface before rotating out of plane. Crystal size and orientation are compared between samples collected from above and below the ASH to understand differences in skeletal structure and better predict reef futures under OA. Neither aragonite crystal size or sclerodermite length were significantly different between the sample taken from below and above the ASH. Increasing our understanding of CWC 3D crystallographic structure above and below the ASH quantifies reef health, identifies reef threats, and predicts reef impacts, increasing our knowledge of current and future reef conditions.
There are more CWC reefs in the world which have not yet been discovered, and thus cannot be managed. Therefore, increasing global capacity to carryout baseline deep-sea research is crucial to ensuring adequate protection and management for these vulnerable marine ecosystems. Limited at-sea training opportunities make it difficult to ensure the next generation of deep-sea scientists are properly trained in sea-going research methods. However, telepresence and remote learning can be used to increase the number of active participants on deep-sea expeditions. Chapter 4 of this thesis guides CWC reef management through increasing our knowledge of the effectiveness of virtual ship-to-shore training for increasing deep-sea capacity. Chapter 4 explores the 2021 iMirabilis2 expedition’s use of telepresence to virtually involve early career researchers from several countries in deep-sea science. Post expedition, a survey of onshore participants was conducted to assess and quantify the effectiveness of the peer-to-peer early career researcher ship-to-shore scheme. During the expedition, live, interactive training via WhatsApp and Zoom was accessed more than traditional static, unidirectional methods of blog posts and pre-recorded videos. All respondents either agreed or strongly agreed the scheme provided an inclusive and accessible platform to share deep-sea science. These results suggest similar schemes could be used to supplement shorter duration at-sea training or used prior to a seagoing experience to better prepare early career researchers, increasing inclusivity. Creating an inclusive and accessible platform to virtually share at-sea deep-sea science increases capacity for deep-sea exploration which can lead researchers to discover more CWC reefs.
Through quantifying reef health, identifying reef threats, predicting reef impacts, and increasing capacity for deep-sea exploration, this thesis expands knowledge of CWC reef ecosystems to guide deep-sea ecosystem management
