Mesophotic Research

Mesophotic Coral Ecosystems have received little research attention until recent times, primarily because they have not been easy to access. The popularisation of SCUBA in the 1950s’ led to a burgeoning use of this technology in field research, yet primarily limited by human physiology (nitrogen narcosis and the bends). To research at deeper depths requires much more funding, so the scientists who raised the funds would want to explore as deep as they could. Thus, the mesophotic depths have been typically bypassed. With technological advances in closed circuit rebreather technology, a whole new world has opened up for both scientists and technical divers alike. 

Below, are the top 10 questions frequently asked by those fascinated by these mysterious depths. Each question – and many more – will be explored in greater depth at Patreon. Sign up to learn more 🤓 

1. What are Mesophotic Coral Ecosystems (MCEs)?

In July, 2008 an international workshop was held in Jupiter, Florida, USA, where a formal definition was established:

“Mesophotic Coral Ecosystems (MCEs) are characterised by the presence of light-dependent corals and associated communities typically found at depths ranging from 30-40 m (98-131 ft) and extending to over 150 m (492 ft) 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).

This area of research is still very much in its infancy, and as our understanding of these little-known depths develops, so will the definition.

2. Why Mesophotic?

The word “mesophotic” is derived from Greek. It combines “meso-” (μέσος), meaning “middle,” and “photic” (φωτός), from “phos” (φῶς), meaning “light.”

It is important to note that Greek and Latin, whilst regularly confused, Greek came first! In scientific nomenclature (the system of naming organisms), the two are used interchangeably, many researchers consider it heresy, which leads us onto the true definition of a mesophotic ecosystem; are shallow turbid (low water clarity) reefs mesophotic?

Thank you primarily to Stratis Kas, and the ‘Composition of Scientific Words‘, which have been my guides on the precise nomenclature of mesophotic.

3. Why are MCEs also called the Twilight Zone?

The first reference to the term ‘Twilight Zone‘ was adapted from an article by Starck and Starck (1972) in the National Geographic magazine.

This article was the first of its kind. Yet, whilst popular and somewhat intuitive, the term ‘Twilight Zone‘ is scientifically incorrect. It may also be a little confusing. This is because it was popularised by the 1959 Rod Serling Sci-Fi TV series, and also used as an alternative term for the Mesopelagic Zone, which is a fascinating area of the open ocean between 200-1000 m (656-3281 ft) depth.

4. How do we research at Mesophotic depths?

Until recent years mesophotic depths (>30 m/98 ft depth) have been largely inaccessible. Mini submarines (e.g., Pisces, Triton etc.), or unmanned underwater vehicles such as an ROV (tethered, therefore, a remotely operated vehicle) and AUV (untethered, autonomously operated vehicle), have been the principle means of visual access to the depths below that of conventional SCUBA.

Rapid advances in closed circuit rebreather diving technology (latest news at InDEPTH) using mixed gases has enabled the exploration of mesophotic depths. Human physiology limits our time on the seabed, yet this time is of paramount importance for research and discovery. Thus, we are now in an age of incredible developments in subsea environments (i.e., DEEP, Proteus).

5. How dark are Mesophotic Ecosystems?

By definition, light (or more precisely, solar irradiance) is the critical parameter of the mesophotic environment. This can vary between location and depth. For example the oceanic islands and atolls of the Indo-Pacific can have remarkable light penetration across depth (the image here was taken at 90 m/295 ft depth, at Pohnpei, Federated States of Micronesia; you can still see the sun at this depth!). Compare this to continental regions or areas of high sedimentation where light is unable to penetrate past the particulates in the water column.

Irradiance is thus, a function of depth, water quality, and/or geomorphology. Taken together, or in isolation, the reef communities particularly of the upper-mesophotic depths are dependent on irradiance levels, whereas other factors, such as physical oceanography, can increase their influence with depth.

6. What lives there?

Each dive introduces us to something new; new species to science, new species depth and/or location records, and interactions previously unknown. As we descend the dominant communities that are encountered shift from light-dependent photosynthetic coral and algae to suspension feeding heterotrophs.These include gorgonian octocorals, black corals, sponges, and many more enigmatic species. A lot of what we see is determined by ecological and environmental dynamics, and particularly so for organisms that live on the sea floor (benthic); unable to simply swim away when the going gets tough. So, they have adapted not only to survive, but to thrive.

7. Why are they important?

Mesophotic ecosystems are extremely important because they support a wealth of biodiversity, the majority of which is yet to be discovered and described. So, if we know very little of these intriguing environments, how do we know they’re important? Simple:

1. The footage and images speak for themselves – these ecosystems are vibrant and alive.

2. Biodiversity:  These ecosystems are festooned with many different creatures of all sorts of shapes and sizes.

3. We know from the groups of animals and plants present that they possess natural products for pharmaceuticals and other resources.

4. Animal forests provide essential habitat for many species of commercial fishes.

5. Adaptation: Mesophotic ecosystems are not calm they are highly dynamic ecosystems with remarkable variations in temperature, dissolved oxygen, chlorophyll, and other variables. Organisms that thrive here possess adaptations that hold keys to understanding thermal tolerance and survival over the millennia.

As far as mesophotic ecosystems are concerned deep means dynamic!

8. Is it dangerous?

Yes. Not only are they dynamic, but of course, they are deep! To visit here, you need to be armed with the right equipment, training, physical fitness, and motive.

• The limitations of human physiology can require long hours of decompression and various gas mixes.

• The intricacies of human thought require vigilance and rigorous self-honesty.

• The challenges of environmental change such as depth, strong and unpredictable water currents, sharks, temperature… significantly compromise safety and ability.

These factors and more can be overlooked whether due to complacency or frustration over limitations. A significant amount of training and experience are necessary to research mesophotic environments; and whilst MCEs are not for the faint hearted, there’s still much that can be achieved with the fast pace of technological development.

9. Are shallow and mesophotic reefs connected?

Shallow coral reefs (0-30 m/0-98 ft) and mesophotic ecosystems are different. The deeper you go the greater the contrast. The upper mesophotic (30-60 m/98-197 ft) can bear a strong resemblance to shallow coral reef ecosystems, and as you descend the reduction of light lends itself to completely different inhabitants that are adapted to distinct conditions.

Much discussion and research have been devoted to the deeper depths being a refugia for shallow-water species, particularly reef-building corals. A persistent procession of global climatic anomalies leading to catastrophic bleaching events has led to the hope that the shallow reefs of the tropical will be reseeded by those from deeper depths. The majority of evidence suggests that this is species-specific and in the majority of cases not so. And thus, human-derived destruction can only be reversed when we change our thinking and our actions, wholesale. There is no magic formula.

10. Conservation: Are they in danger?

Yes. Mesophotic ecosystems are not immune to the vicissitudes of environmental disturbance, whether natural or anthropogenically derived. What we have now is the confounding effects of both natural and human-induced disturbance, which can be direct or indirect.

Natural stressors include hurricane or storm damage, and volcanism, that can cause destruction and smothering by sedimentation across depth. The removal of mangroves, coral mining, construction, and pollution also lead to smothering by sedimentation, but also the release of impurities on to mesophotic reefs.

Overfishing whether for fish or jewellery, destructive fishing practices, and global warming, are all sinister devistating impacts that are actively destroying mesophotic reefs today. It is important to share the beauty and uniqueness of these ecosystems to mitigate their destruction.

Natural thermal anomalies have existed for millennia, and the mesophotic depths of many Indo-Pacific locations are subject to remarkable ranges in temperature and other environmental variables. However, prolonged exposure to multiple stressors leads to algal-dominated or dying reefs at depth.

To pursue active research at mesophotic depths in a safe and effective way, a small yet growing number of committed scientists and technical divers are creating ways to make these remarkable ecosystems safely accessible for active research. Click on the links below to learn, train, or contribute towards this research.