Regional Variations in Live Proportions of Southwest Pacific Cold-Water Coral Solenosmilia variabilis Reefs

Reef-building cold-water-corals (CWC) form deep-sea habitats that can create biodiversity hotspots. As live coral and dead intact framework provide disparate ecosystem services and are vulnerable to different anthropogenic stressors, it is important to quantify the proportions of each on CWC reefs. We analysed 1,160 images of Solenosmilia variabilis reefs at four sites off New Zealand (Valerie and Forde Guyots at the Louisville Seamount Chain & Ghoul and Gothic Seamounts at the Graveyard Seamount Complex) to determine the ratio of live coral to the whole reef area (termed live:reef). We found live:reef ratios are significantly different between sites at the offshore Louisville Seamount Chain and onshore Graveyard Seamount Complex. This could be driven by reef position relative to the aragonite saturation horizon (ASH) as corals in the Louisville Seamount Chain live below the ASH in colder and deeper waters than those at the Graveyard Seamount Complex, which live above the ASH. In the southwest Pacific, depth is a driver of live:reef ratios, with a larger proportion of live coral at shallow depths and dead intact framework at deeper depths. The live:reef ratios at Gothic Seamount within the Graveyard Seamount Complex remained stable between 2015 and 2020 despite significant differences in live coral, dead intact framework, and reef structure surface area. Our results indicate live:reef ratios can be used to estimate the amount of dead intact framework threatened by shoaling ASH due to ocean acidification at each site, which can help inform which sites could be protected as possible climate change refugia.