Climate change effects on cold-water coral reefs and their associated communities

Abstract

The distribution of biodiversity on the planet faces dramatic spatial reorganization from climate change. This is especially true in the marine realm, where species often live near their physiological limits. Thus, effective conservation agendas for marine biodiversity must be predicated upon robust multi-scenario projections of climate-driven changes in oceanographic conditions. However, much of the theory and empirical work on distributional changes in marine biodiversity comes from shallow-water ecosystems. The deep seafloor (> 200 m) has received comparably little attention despite mounting evidence of the accrual of climatic changes within this largest habitable area of the planet. Here, I present a number of case studies predicting the effects of climate change on the distributions of cold-water coral (CWC) reef habitats and their associated fauna, using both modelling approaches and empirical data collected on multiple oceanographic cruises to the CWC reefs of the southeast USA (SEUS) in 2018-2019. These reefs are persistent features of continental margins (~200 – 4000 m ) around the globe, important biodiversity and biogeochemical cycling hotspots, and sentinels of marine climate change. In Chapter 2, I fit global habitat suitability models (HSMs) using publicly available oceanographic and biogeographic products to predict the occurrence of reef-forming CWC species and the reef habitat they form, testing for taxonomic and regional differences in their ecological niches. I then use an ensemble of global climate model outputs as inputs for ensemble HSMs projecting the distributions of these same taxa to 2100 in a range of climate scenarios, and test for differences in distribution changes across species and bioregions. In Chapter 3, I use higher-resolution regional and global climate products and data from multiple oceanographic cruises to the SEUS to build HSMs for this region; this data collation revealed the largest known, essentially continuous CWC reef province on the planet. The models located pivotal climate refugia primarily at deeper (> 600 m) eastward reef sites – notably including those outside of areas designated to protect coral from bottom-contact fisheries – that may remain suitable to 2100 while shallower sites are projected to experience catastrophic declines. In Chapters 4 and 5 I present community ecological work based from research expeditions to CWC reefs of the SEUS described in Chapter 2. In Chapter 4, I use video imagery and in situ collections of intact seafloor communities to test how the abundance, taxonomic and functional diversity, and community structure of invertebrate communities in hard-substratum ecosystems along the SEUS margin, including CWC reefs and submarine canyons change along biocomplexity (e.g. the percentage of live coral cover), bathymetric, and oceanographic gradients. In Chapter 5, I synthesize invertebrate and fish data from SEUS CWC reefs to fit Bayesian community-level joint HSMs predicting the occurrence and abundance of these faunas as functions of their ecological traits. These models reveal strong distinctions fish and invertebrates in their climate and habitat preferences at CWC reefs, suggesting opposing responses to climate change. Overall, Chapters 3-5 expand upon baseline descriptions of reef habitats and coral-associated fauna in the SEUS, testing for mechanisms driving observed ecological patterns across large environmental gradients. Together, this volume improves our understanding of the ecological drivers of vulnerable marine ecosystem occurrence and biodiversity, augmenting conservation efforts for these critical components of the global ocean.