Ecological insights from environmental disturbances in mesophotic coral ecosystems

Mesophotic coral ecosystems (MCEs) have historically been considered more stable than shallow reefs and thus suggested to provide refuge to coral reef communities against natural and anthropogenic impacts. Despite this assumption, a growing body of literature has shown that deep reefs are not immune to natural disturbance. Here, based on our in situ observations, we propose that disturbance may actually represent an important mechanism for maintaining biodiversity in MCEs, as is the case for shallow reefs. Our observations suggest that disturbances can provide microhabitat and space necessary for the recruitment and occurrence of different species, increasing overall diversity. Since bioerosion rates are lower at depth, and most well‐developed coral reefs on MCEs are formed by dense aggregations of a single or a few species, intermediate levels of disturbance could represent a critical driver of community structure balancing. Therefore, instead of long‐term stability, intermediate disturbances should be expected on MCEs. However, high frequency and intensity of natural disturbances, or their association with anthropogenic stressors, might have stronger negative impacts on MCEs than on shallower reefs due to slower coral growth and calcification rates. Mesophotic coral ecosystems (MCEs) represent the deeper portions of coral reefs and a transition zone between photic and aphotic realms, found between 30 and 150 m depth (Weiss 2017, Rocha et al. 2018). In recent years, improvements in technical diving have allowed researchers to explore and study MCEs more than ever before and have resulted in the discovery of several previously unknown species and descriptions of unique habitats and communities (Pyle 2000, Pinheiro et al. 2014, 2016, Anderson et al. 2016, Rocha et al. 2018). Historically, MCEs have been considered more stable than shallow reefs (Lesser et al. 2009, Bridge et al. 2011, Slattery et al. 2011, Reardon 2018), and depth has been suggested to provide refuge to coral reef communities against natural and anthropogenic impacts, due to attenuation of wave energy (Kahng et al. 2010), and reduced temperature and ambient light (Glynn 1996, Bridge et al. 2013), leading to the concept known as the “Deep Reef Refuge Hypothesis” (Bongaerts et al. 2010). However, current observations suggest that deep reefs are not immune to disturbance. In a recent expedition to the Bahamas, diving with closed‐circuit rebreathers down to 135 m depth, we observed impacts of Hurricane Matthew and bleaching events from shallow to lower mesophotic habitats of the coral reef