Cold-water coral sensitivity: growth, morphology and reef-forming potential of Desmophyllum pertusum in response to environmental factors

Abstract

The deep ocean is home to vast and flourishing cold-water coral (CWC) reefs, formed by a handful of widely distributed scleractinian CWC species. These structurally complex habitats host a rich diversity of associated species (ranging from microbes to fish), contribute significantly to the carbon cycle and deeply affect local sediment deposition. Over thousands and millions of years, CWC buildups can develop into massive carbonate mounds. All these fundamental geo-ecological functions are intimately tied to the morphology of CWCs, because the skeletal framework of these ecosystem engineers directly shapes the reef habitat. Hence, understanding how reef-building CWCs grow in response to environmental conditions can shed light on deep-sea reef formation, ecosystem functioning and vulnerability to environmental (including human-induced) change. CWCs exhibit a high degree of morphological variability (often regarded as plasticity), but despite its ecosystem significance, patterns and drivers of this variability are still poorly understood and challenging to study, due to the limited accessibility and slow growth of these species. This thesis aims to improve our understanding of how the growth, morphology and reef-forming potential of CWCs vary in response to their environmental setting. To this end, the focus is on Desmophyllum pertusum (syn. Lophelia pertusa), one of the most widespread and best-known reef-building CWC species, which occurs over a broad range of environmental conditions. The intraspecific variation in growth and morphology of D. pertusum and its physicochemical hydrographic drivers were qualitatively and quantitatively investigated across multiple structural levels (from individual polyps to whole colonies) and geographic scales (from microhabitat to ocean basin), using a large Atlantic-wide collection of underwater imagery and skeletal samples. Characterisation of global patterns of morphological variation in D. pertusum led to the identification of three main colony morphotypes, which indicate that polyp budding is a key mechanism of colony shape determination and suggest a major influence of hydrodynamics on coral growth. This latter hypothesis was explicitly tested by creating digital 3D models of corals from adjacent Norwegian sites with contrasting flow regimes and comparing their 3D framework architecture. Polyp phenotypic variation in relation to flow speed and other physiologically relevant hydrographic factors was then analysed on a global scale. Findings reveal high phenotypic plasticity and suggest that variation in polyp budding and corallite morphology are widespread responses of D. pertusum to various hydrographic factors (dissolved oxygen, temperature and flow speed in particular). Such responses are highly relevant for the growth and morphology of the CWC reef framework, although CWC sensitivity to hydrographic factors seems to somewhat differ among regions, and the interaction between multiple forcing factors may also play an important role. In particular, climate change stressors (ocean warming and deoxygenation) and increasing current speeds could have contrasting effects on CWC reef growth, by respectively hampering and favouring polyp growth and framework compactness. The present work represents the first characterisation of species-wide morphological variation and its environmental drivers in a reef-building CWC, and advances our knowledge of the relationship between CWCs and their physicochemical environment. This knowledge can be used to model CWC reef responses to environmental change and, vice versa, to infer past and present environmental conditions and reef health from CWC phenotypic traits.