Mesophotic Coral Ecosystems

7.5. What are the impacts of natural and anthropogenic threats on mesophotic coral ecosystems?

knobby cactus coral, Mycetophyllia aliciae , was documented going from a healthy appearance to dead within five months. We know this because it happened within a research study’s photographic time-series, but we don’t know what caused it, or whether it occurred in only this coral colony or was found throughout colonies in the area. In general, the specific impacts from climate change and increasing carbon dioxide levels, fishing, pollution and invasive species and the effects of extreme events (such as tropical cyclones, earthquakes and tsunamis) on MCEs require documentation and study if resource managers are to address them in a meaningful way. Research Need: Determine the anthropogenic and natural threats to MCEs and assess the ecological impacts and their subsequent recovery, if any, from them. replenishment is dependent on a number of factors, including whether the same species are present at both depths, the extent of species adaptation at particular depths, and whether there is oceanographic connectivity between them. Data on connectivity between shallow and mesophotic reefs is limited (Bongaerts et al. 2010a, Kahng et al. 2014). With the exception of a few studies, the validity of the deep reef refugia hypothesis can only be evaluated on known species distributions. Considering this, there is potential that many fish species are connected between shallow and mesophotic habitats, as has been shown for the threespot damselfish, Chromis verater , in the Hawaiian Islands (Tenggardjaja et al. 2014) using genetics, and for commercially-important snappers and groupers in the Caribbean (Bejarano et al. 2014). However, for coral species, the possibility of connectedness only exists for those living in the upper mesophotic zone (30–50 m) to mid- mesophotic zone (50–70 m) in clear waters, because the deeper mesophotic zone tends to be populated by coral species that are not found in shallow waters (Bongaerts et al. 2010a, Pochon et al. 2015). Determining the degree of connectivity of MCEs with shallow reefs and other MCEs for key sessile andmobile species is crucial to ensuring that effective management measures, such as marine protected areas (Lesser et al. 2009), are implemented. Research Need: Understand the genetic, ecological and oceanographic connectivity of MCEs with shallow reefs and other MCEs. Research Need: Determine whether MCEs can serve as refugia and reseed shallow reefs (or vice versa).

Worldwide, shallow coral reef ecosystems are facing an array of natural and anthropogenic threats, including fishing, pollution, invasive species, climate change and extreme events (e.g. tropical cyclones), which are contributing to their decline (Wilkinson 2008). MCEs face similar threats, albeit to differing degrees. For light-dependent mesophotic organisms living at low light levels (1 per cent of that found at the sea surface), anything that inhibits light reaching the depths (e.g. sedimentation, turbidity or pollution) has a marked impact on their survival. As we learned in Chapter 6, little is known or understood about the extent of the impact fromnatural and anthropogenic threats to MCEs. In many cases, our knowledge of these impacts is incidental. For example, in Puerto Rico, a single colony of the With the documented decline of shallow coral reefs, there has been strong interest in determining the level of ecosystem connectivity between shallow and mesophotic reefs. Ecosystem connectivity in the broadest sense is the exchange of materials (nutrients, organisms, and genes) between ecosystems. Connectivity can be further broken down into three types: genetic (exchange of genes and organisms), ecological (exchange of individuals) and oceanographic (water circulation patterns and material flow) connectivity. The potential that MCEs may be ecologically or genetically connected to shallow reefs, and may serve as refugia for shallow reef species in decline from multiple natural and anthropogenic stressors, has brought hope to resource managers that all may not be lost. The ‘deep reef refugia’ hypothesis, first postulated in the mid-1990s, was based on the premise that MCEs may serve as a refuge or population source for replenishing shallow reef species being impacted by thermal stress induced by climate change (Glynn 1996). This hypothesis has since been expanded to also include serving as a refuge from fishing, pollution and other threats. The idea is that depth and distance from shore buffer or protect MCEs from the direct impacts associated with these threats, thereby allowing mesophotic populations to survive through disturbances primarily affecting shallow-water reefs, and reducing the likelihood that a species would be extirpated from a region by a severe disturbance event. In addition to serving as a refuge, a second premise of the hypothesis is that surviving populations could assist the recovery of shallower reefs by reseeding or replenishing shallower populations. Such

7.6. Aremesophotic coral ecosystems connected to shallow reefs and can they serve as refuges for impacted shallow species?

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 85