Mesophotic Coral Ecosystems

Chapter 1.

Introduction

Peter T. Harris , GRID-Arendal, Norway Thomas C.L. Bridge , Australian Research Council Centre of Excellence for Coral Reef Studies, James Cook University & Australian Institute of Marine Science, Australia

1.1. Coral reefs in peril

Globally, coral reefs are deteriorating rapidly due to elevated sea surface temperatures, coastal development, pollution and unsustainable fishing practices (Hughes et al. 2003, Pandolfi et al. 2003). About 19 per cent of coral reefs have already been lost, with a further 35 per cent expected to disappear in the next 40 years (Wilkinson 2008). Unless something changes, almost all shallow-water coral reefs will experience thermal stress sufficient to induce severe bleaching every year by the 2050s. Coral reefs most likely to survive the twenty-first century include those that sustain low impact from terrestrial runoff and that occur in locations safeguarded from extreme sea surface temperatures. These include large areas of intermediate depth reefs, also known as mesophotic coral ecosystems (MCEs; Glynn 1996, Riegl and Piller 2003). Occurring at depths greater than 30–40 m, MCEs may be buffered from some human and natural disturbances that negatively affect shallow-water reefs (Bongaerts et al. 2010a, Bridge et al. 2013), but not all stressors (Stokes et al. 2010, Lesser and Slattery 2011). Science has shown that MCEs are far more widespread and diverse than previously thought (Locker et al. 2010, Harris et al. 2013). However, they remain largely understudied in most parts of the world and there is little awareness of their importance among policy makers and resource managers The notion that MCEs could provide a refuge for coral reef biodiversity from natural and human impacts has been formalized in the ‘deep reef refugia hypothesis’ (Glynn 1996, Bongaerts et al. 2010a). Some disturbances affecting coral reefs are most acute in shallow waters (Figure 1.1): for example, wave energy attenuates with increasing depth, making MCEs less likely to be affected by storm waves (De’ath et al. 2012). Similarly, warm-water coral bleaching, resulting from overheating of the upper few metres of surface waters (in calm, stratified water columns) and a synergistic effect between heat and light, has less of an impact on MCEs located in deeper water (> 30–40 m to over 150 m) and receiving lower irradiance. In addition, many MCEs occur in remote, offshore locations, such as along the edge of the continental shelf or on remote, submerged patch reefs. These isolated MCEs are less exposed to many stressors commonly affecting

Mesophotic coral ecosystems are characterized by the presence of light-dependent corals and associated communities typically found at depths ranging from30–40mand extending to over 150m in tropical and subtropical regions.The dominant communities providing structural habitat in the mesophotic zone can be comprised of coral, sponge, and algal species (Puglise et al. 2009, Hinderstein et al. 2010). (Bridge et al. 2013, Madin and Madin 2015). Consequently, they are for the most part not considered in conservation planning, marine zoning and other marine policy and management frameworks. This report aims to raise awareness of the importance of MCEs in order to improve their protection and catalyze appropriate policy, management and research responses. The potential that MCEs may act as “refugia” and a source of replenishment for some shallow reef species (Glynn 1996, Riegl and Piller 2003, Bongaerts et al. 2010a) or, in other words, “lifeboats”, offers a glimmer of hope that MCEs may aid in the recovery of degraded shallow reefs. This report provides an accessible summary on MCEs, including a discussion of the ecosystem services they provide, the threats they face, and gaps in our understanding, as well as addressing the question of whether MCEs can serve as lifeboats for coral reefs. shallower reefs, such as terrestrial runoff. MCEs may also offer a refuge from fishing pressure, particularly for commercially- important species (Bejarano et al. 2014, Lindfield et al. 2014). The concept of ecological refugia as a potential option for mitigating biodiversity loss under climate change has been increasingly debated in the scientific literature of recent years (Ashcroft 2010, Keppel et al. 2012), including defining the spatial and temporal scales of what is termed a refugium (Keppel et al. 2012). It is now accepted that the term ‘refuge’ refers to short timescales (e.g. a particular MCE may be a refuge from the effects of a tropical cyclone), whereas ‘refugia’ operate on longer temporal scales. Most studies addressing refugia in relation to MCEs are actually referring to their role as a refuge; that is, whether mesophotic habitats were less affected by a particular disturbance, such as a cyclone or a

1.2. Mesophotic coral ecosystems — a refuge for shallow- water coral reefs?

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 9