Assessing the living and dead proportions of cold-water coral colonies: implications for deep-water Marine Protected Area monitoring in a changing ocean.

Johanne Vad,J Murray Roberts,Covadonga Orejas,Helen S Findlay,Juan Moreno-Navas

doi:10.7717/peerj.3705

Abstract

Coral growth patterns result from an interplay of coral biology and environmental conditions. In this study colony size and proportion of live and dead skeletons in the cold-water coral (CWC) Lophelia pertusa (Linnaeus, 1758) were measured using video footage from Remotely Operated Vehicle (ROV) transects conducted at the inshore Mingulay Reef Complex (MRC) and at the offshore PISCES site (Rockall Bank) in the NE Atlantic. The main goal of this paper was to explore the development of a simple method to quantify coral growth and its potential application as an assessment tool of the health of these remote habitats. Eighteen colonies were selected and whole colony and dead/living layer size were measured. Live to dead layer ratios for each colony were then determined and analysed. The age of each colony was estimated using previously published data. Our paper shows that: (1) two distinct morphotypes can be described: at the MRC, colonies displayed a ‘cauliflower-shaped’ morphotype whereas at the PISCES site, colonies presented a more flattened ‘bush-shaped’ morphotype; (2) living layer size was positively correlated with whole colony size; (3) live to dead layer ratio was negatively correlated to whole colony size; (4) live to dead layer ratio never exceeded 0.27. These results suggest that as a colony develops and its growth rate slows down, the proportion of living polyps in the colony decreases. Furthermore, at least 73% of L. pertusa colonies are composed of exposed dead coral skeleton, vulnerable to ocean acidification and the associated shallowing of the aragonite saturation horizon, with significant implications for future deep-sea reef framework integrity. The clear visual contrast between white/pale living and grey/dark dead portions of the colonies also gives a new way by which they can be visually monitored over time. The increased use of marine autonomous survey vehicles offers an important new platform from which such a surveying technique could be applied to monitor deep-water marine protected areas in the future.

Highlights

Some species of cold-water corals (CWC) can form complex 3-dimensional reef frameworks supporting biodiversity hotspots (e.g., Freiwald, 2002; Roberts et al, 2009; Henry, Davies & Roberts, 2010; Buhl-Mortensen et al, 2010), but several anthropogenic activities are putting them at risk
We explore a new approach to assess L. pertusa colony size, and the proportion of live and dead coral in each colony by using opportunistic measurements from high definition video footage recorded from two sites in the NE Atlantic, one inshore and one offshore
This study reveals the presence of two distinct L. pertusa colony morphologies at Mingulay Reef Complex (MRC) (‘‘cauliflower’’) and PISCES area (‘‘bush-shaped’’)

Summary

Introduction

Some species of cold-water corals (CWC) can form complex 3-dimensional reef frameworks supporting biodiversity hotspots (e.g., Freiwald, 2002; Roberts et al, 2009; Henry, Davies & Roberts, 2010; Buhl-Mortensen et al, 2010), but several anthropogenic activities are putting them at risk. In deep waters (>100 m), local hydrodynamics and energy supply (Mienis et al, 2007), as well as temperature (Thresher, 2009) play a central role for survival and growth of CWC. For many years, these factors have been known to modify tropical coral phenotype. Video footage and still images obtained with ROVs have become powerful non-destructive approaches to study several aspects of CWCs and the communities they support including their occurrence, density and geographic distribution (e.g., Orejas et al, 2009; Arnaud-Haond et al, 2015), bathymetric distribution, coral size classes and orientation (Gori et al, 2013) and relationship with associated species (Purser et al, 2013)

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