Written By Brooke E. Benson
Over the past few decades, rapid, global coral reef decline has spurred scientists to study factors that may allow reefs to persist into the next century. Though reefs all over are being impacted by both natural and human-induced stressors, patterns of degradation are not always consistent: some reefs, or even regions within a reef, tend to fare better than others under stress, and the factors contributing to these differences in response are numerous. One factor that has captured more attention in recent years is that of depth. While one’s mental image of a coral reef may be that of a vibrant, brightly illuminated coral forest glimmering just beneath the ocean’s surface, many corals can live at depths exceeding 200 feet (60 m). At these depths, light is scarce and photosynthesis is limited. The algae on which corals rely for the majority of their nutrition (found living in the coral tissue), may not provide as much energy here as they do in shallower reefs. However, the benefits of living at these depths ensure that at least some corals make it their home.
On deeper reefs, also known as mesophotic (meaning “middle-light”) coral ecosystems or MCEs, temperatures tend to be more stable, partially buffering the corals from the thermal stress that is so often the demise of their shallower counterparts. Mesophotic reefs also tend to escape the majority of storm damage, and are generally further removed from land-based pollution and nutrient runoff (Menza et al. 2008, Smith et al. 2008) since they are usually further from the shoreline. For these reasons, researchers have wondered if mesophotic reefs may have the potential to restore degraded shallow reefs by supplying them with larvae. Promisingly, some studies have demonstrated that these special ecosystems may indeed be able to supply certain corals and other organisms with new population members. In the Northwest Gulf of Mexico, off the coast of Texas, the coral species Montastraea cavernosa has been shown to have a population that is well-connected across shallow and mesophotic reefs (Studivan and Voss 2018), suggesting regular exchange of larvae between shallow and mesophotic zones. In contrast, other studies on the same coral species found genetic differentiation between individuals in shallow and mesophotic – an indication that shallow and deep populations don’t often reproduce together or receive larvae from reefs in a different depth zone. Brazeau et al. 2013). As is always the case in ecology, there is no silver bullet, and the phenomena that we can observe are the result of highly complex interactions between organism and environment.
Overall, research has suggested that the ability of mesophotic reefs to restore degraded shallow reefs by supplying larvae may be limited and case-specific. For starters, only ~25% of reef-building coral species are depth-generalists, or corals that are able to thrive equally well across both shallow and deep reef habitats (Bongaerts et al. 2010). Second, mesophotic reefs, though buffered from certain stressors that batter shallow reefs, come with their own unique challenges: reduced light can limit corals’ ability to reproduce at high levels, cascading sedimentation can smother colonies, and cold-water bleaching may occasionally decimate some mesophotic reefs (Bongaerts et al. 2010). Yet this doesn’t mean that all deep reefs are irrelevant when thinking about shallow reefs. In a study of hydrocorals of the genus Millepora, which reproduce similarly to stony corals, researchers found that mesophotic populations twice restored shallow populations that were depleted by thermal stress (Smith et al. 2014). These mesophotic rescue populations, however, were found at depths less than 80 ft (25 m). Substantial evidence suggests that genetic and phenotypic (physical characteristic) divergence across greater depth ranges may at times prevent this type of refuge zone in most environments (Brazeau et al. 2013, Serrano et al. 2014).
Though the potential of mesophotic reefs to support shallow reef health and recovery may seem ambiguous, the complexities shouldn’t surprise us. Corals are incredibly unique animals, and the environments they inhabit are more diverse and varied than we ever thought possible. These intricacies don’t mean that mesophotic reefs are universally unimportant; while deep reefs may not be a universal backup for the world’s shallow reefs, they may indeed play critical roles on local scales. In this case, the complex picture of deep reef-shallow reef dynamics demonstrates that, time after time, understanding how corals are responding to their environments requires studying reef processes at all scales and across diverse sites. The main takeaway is—and always will be: more research is needed.
Brazeau DA, Lesser MP, Slattery M. (2013). Genetic structure in the coral, Montastraea
cavernosa: assessing genetic differentiation among and within mesophotic reefs. PLoS One 8(5): e65845. https://doi.org/10.1371/journal.pone.0065845
Bongaerts P, Ridgway T, Sampayo EM, Hoegh-Guldberg O. (2010). Assessing the ‘deep
reef refugia ’hypothesis: focus on Caribbean reefs. Coral Reefs 29(2): 309-327. https://doi.org/10.1007/s00338-009-0581-x
Menza C, Kendall M, Hile S. (2008). The deeper we go the less we know. Rev Biol Trop 56:11-24.
Serrano X, Baums IB, O’Reilly K, Smith TB, Jones RJ, Shearer TL, Baker AC. (2014). Geographic
differences in vertical connectivity in the Caribbean coral Montastraea cavernosa despite high levels of horizontal connectivity at shallow depths. Molecular ecology, 23(17), 4226-4240. https://doi.org/10.1111/mec.12861
Smith TB, Glynn PW, Mate JL, Toth LT, Gyory J. (2014). A depth refugium from catastrophic
coral bleaching prevents regional extinction. Ecology 95(6): 1663-1673. https://doi.org/10.1890/13-0468.1
Smith TB, Nemeth RS, Blondeau J, Calnan JM, Kadison E, Herzloeb S. (2008). Assessing coral ref
health across onshore to offshore stress gradients in the US Virgin Islands. Mar Pollut Bull 56: 1983-1991. https://doi.org/10.1016/j.marpolbul.2008.08.015
Studivan MS, Voss JD. (2018). Assessment of mesophotic coral ecosystem connectivity for
proposed expansion of a marine sanctuary in the Northwest Gulf of Mexico: population genetics. Front Mar Sci 5:152. https://doi.org/10.3389/fmars.2018.00174