THE MESOPHOTIC ZONE

Nils Lucas Jacobsen completed his master’s thesis in marine biology, investigating the effects of marine animal forests (MAFs) on the underlying benthic community in the upper mesophotic zone. For two months, he and his team collected photographic samples in the beautiful marine protected area (MPA) of Portofino, Italy. His GUE Tech 1 and GUE Scientific Diver certifications proved to be invaluable tools in his research.

Portofino is characterized by rich and diverse fauna and flora, making decompression stops after sample collection highly rewarding.

Getting cooked in my drysuit and carrying so much gear that I look like a swimming Christmas tree can only mean one thing: We are going for another scientific dive!

Getting here took multiple months spent writing research proposals and planning logistics. A big part of the preparation was honing my diving skills and acquiring the necessary certificates. When I was first inspired to complete the project, I was an advanced open water diver and far from able to perform scientific work underwater. Coming from a different diving organization, I completed the GUE Fundamentals course first. It was a real game-changer for me and provided the solid foundation that I needed for my future endeavors.

In 2021, I completed the Scientific Diver course, gaining the skills and confidence to start my journey as a scientific diver. The courses were demanding, but also thorough, and incredibly valuable to me not only as a diver, but also as a marine researcher.

To work in the upper mesophotic (40-60 m/130-200 ft) zone, I needed to go further—or, rather, deeper—so I set my sights on GUE’s Tech 1 course. As my skills did not yet meet the high standards demanded by technical diving, I practiced my buoyancy, my kicks, and GUE’s standard operating procedures many times during a “training camp” in Sagres, Portugal. Finally, I reached my goal—the technical pass. The Tech 1 course was an incredible experience. My instructor and my team were wonderful. Together, we faced many challenges, simulated various failures, dived in low visibility, and explored the historic wrecks of the Adriatic Sea. GUE’s courses gave me the necessary skill set to perform scientific work underwater and opened a new world of exciting technical scientific diving.

The mesophotic zone

In scientific literature, the mesophotic zone has most often been described as the area between 40-120 m/130-400 ft. These limits are based on convention and do not necessarily reflect a change in biological parameters. Therefore, a more meaningful and unambiguous definition has been introduced for the Mediterranean me- world of exciting technical scientific diving. sophotic zone: as the upper limit, the depth at which 1% of the surface irradiance arrives, and as the lower limit, the deepest depth at which primary producers can be found. As a result, the mesophotic zone is placed between the light-intense euphotic zone (where light is sufficient for the growth of plants) and the dark aphotic zone (with little or no sunlight).

The good visibility and, for winter time, relatively warm water temperatures make for an amazing workplace underwater.

Now that we have the definition out of the way, what is so important about this zone?

It is no secret that coral reefs are in decline worldwide. This decline results from a combination of global and local stressors such as overfishing, storm damage, sedimentation, eutrophication, toxic discharge, disease, ocean acidification, and thermal stress. These factors are believed to be less impactful at deeper depths, not (or only partially) affecting mesophotic ecosystems. Furthermore, mesophotic ecosystems are less exposed to thermal anomalies and the related mass-mortality events that are occurring in the Mediterranean Sea, especially during recent decades.

In theory, the mesophotic zone could serve as an important refugium for organisms escaping unfavorable conditions in the upper water layers and could replenish shallower reefs by providing a nursery ground for larvae and juveniles. These functions are highly debated and urgently require more scientific attention. Moreover, there is information about impacts on mesophotic ecosystems that has been reported, particularly regarding physical damage from hurricanes, impacts of fishing activities, sedimentation, coral bleaching, and invasive species. In contrast, undiscovered, pristine ecosystems characterized by minimal human pressure could serve as a valuable baseline against which we can compare the status of threatened and degraded ecosystems. Therefore, the mesophotic zone may prove an important tool to fight the so-called “shifting baseline syndrome.”

Widely underexplored

But, mesophotic zone exploration is easier said than done. Due to its depth, accessing the mesophotic zone is costly and requires specialized equipment, training, and experience.

Recreational divers can usually reach a maximum depth of 40 m/130 ft, and beyond that, the realm of technical diving begins. Therefore, many studies focus on shallow-water habitats only.

Submersibles offer another option for the exploration of the mesophotic zone. They can reach deep depths and carry heavy equipment like high-resolution cameras. Due to their high operating cost, submersibles have mostly been used in deeper environments, leaving the mesophotic zone largely unexplored. However, in recent years, the scientific interest regarding this research area has been increasing thanks to the improvement (and the decrease in costs) of technologies such as autonomous underwater vehicles (AUVs) and remotely operated vehicles (ROVs).

The good news for all divers and curious people out there is that advanced scuba diving technologies—such as mixed gases (trimix) and closed-circuit rebreathers (CCRs)—make observations and physical experiments at mesophotic depths possible. Given the unmatched capability of direct observations and sample collection, scientific diving is a powerful tool that we should use to its fullest potential.

While the mesophotic zone remains widely underexplored, a number of studies have revealed high levels of habitat complexity and important species richness in both temperate and tropical environments. While data indicate diverse mesophotic ecosystems with high endemism, the underlying processes have not been clearly identified. Nonetheless, these characteristics may be attributed to the high environmental stability over glacial sea level changes, as well as the restricted dispersal at deeper depths due to slower water movements. Both hypotheses (especially the latter) require more scientific attention to draw any conclusions. Earlier studies suggest that the upper mesophotic zone especially (<60 m/200 ft) is inhabited by shallow-water species, as well as species from deeper waters, forming an important transition zone between the two, and possibly resulting in increased diversity levels.

A beautiful Paramuricea clavata in the foreground as the measuring tape is carefully collected. The only thing we take is pictures and we leave nothing behind.

Marine animal forests

At deeper depths, environmental parameters tend to be less variable than at shallower depths. This is especially beneficial for the slow-growing, long-lived species that can thrive and reach large sizes and densities there—the organisms that form the so-called marine animal forests (MAFs). Additionally, due to the lower light availability, fast-growing autotrophs tend to be less competitive, resulting in a higher success rate of others.

If you have ever stood in a terrestrial forest, you have experienced its influence on the surrounding environment. It provides shade, im- portant habitats, and shelter (from high winds, for example). Similarly, MAFs can alter current velocity, light intensity, and sedimentation rates. They contribute greatly to the 3D complexity of the underwater world and have a positive effect on biodiversity by increasing the amount of available micro-habitats.

One of the most peculiar features in the MPA of Portofino is the beautiful gorgonian forests, mostly made up of the gorgonian Paramuricea clavata. Larger structures, such as the ones created by P. clavata, can offer protection from mechanical damage to the understory. It has been reported that colonies of this species are vulnerable to aggregations of mucilage, but at the same time may protect the organisms living below from suffocation by trapping those aggregations. Additionally, gorgonian forests can hinder the spread of invasive species.

Sadly, the ecologically valuable MAFs are facing multiple threats such as mechanical damage from lost fishing gear and recreational diving, the aggregation of mucilage (and the implications of this phenomenon), invasive spe- cies, and increased sedimentation rates. Additionally, marine heatwaves are likely to increase with ongoing global climate change, and multiple mass mortality events of gorgonians—linked to high temperatures—have already been documented for the northwestern Mediterranean Sea. The loss of gorgonian forests could lead to major shifts in the associated benthic community and could result in lower diversity, as well as lower resistance against (and resilience to) disturbances.

The study

Mesophotic coral ecosystems (MCEs) represent an important but highly understudied research area. Without knowledge of the distribution and biomass of MCEs, their global role cannot be described, and therefore effective management and protection measures cannot be implemented, making the exploration of such ecosystems highly important.

To understand the functional role of MCEs and protect these valuable habitats, information about their community composition and structure is needed. Thus, the goal of our research was to study differences in epibenthic communities in relation to the density of forest-building species— in this study represented by Paramuricea clavata—at two different depths: 35 m/115 ft and 50 m/165 ft. To highlight the potential of Structure from Motion (SfM) photogrammetry, a 3D reconstruction of one of the analyzed forests was created, extracting the height frequencies of the gorgonian population from it.

In practice, we collected photographic samples at three different sites (two depth levels each) along the south coast of the Portofino promontory. During technical dives, we placed 30 m/100 ft transects on the seafloor and filmed the gorgonian forests as well as the understory. The dives were challenging but very rewarding. As a Tech 1 diver, I was limited to around 20 minutes of bottom time at the 50 m/165 ft level. That was barely enough time to find a gorgonian forest, place the 30 m/100 ft transect, film the gorgonian forest from above, film the understory during a second pass, and collect the measuring tape. At the same time, it was incredibly important to swim very slowly and keep the camera steady. The margin for error was low, and finding the balance between working fast and swimming slowly wasn’t always easy.

The highest forest density that we observed was around five colonies per 1 m²/10 ft². In comparison, pristine gorgonian Paramuricea clavata forests can easily reach 20 colonies per 1 m²/10 ft² and sometimes even over 50 colonies per 1 m²/10 ft². Therefore, the maximum densities observed here are low compared to other areas. The frequent mass mortality events of gorgonians recorded in the northwestern Mediterranean Sea have likely reduced the densities further.

Additionally, since P. clavata is a filter feeder, it thrives where currents are strong enough to ensure a high food supply, which is often the case at ridges and vertical walls. Since one of the aims of this study was to investigate the shading effect of animal forests, vertical walls and steep slopes were avoided, resulting in lower recorded densities. Due to the density dependence of the forest effect, most of its influence stayed undetected. Nonetheless, we found some important trends in individual morpho-functional groups, such as the increase in cover of encrusting sponges as well as massive and erect sponges with higher forest densities. MAFs may positively influence the cover of corals and sponges through shading of the understory, therefore increasing the competitiveness of heterotrophic species and protecting smaller organisms from mechanical damage.

The biggest differences in the overall community structure were detected between the two depth levels—35 m/115 ft and 50 m/165 ft— which can be mostly attributed to the change in light availability. We recorded a significant de-

A team working closely together makes the process of sample collection easier, more efficient, and safer. It also it increases the fun immensely.

New technologies

Excitingly, the photogrammetry approach resulted in valuable information regarding the height frequencies of gorgonians, providing a 3D baseline of the surveyed area. The forest at 35 m/115 ft at the “Punta del Faro” site was dominated by mostly smaller colonies (< 40 cm/15 in). Pristine and healthy forests will likely be associated with larger sizes. The present results may be an indication for sub-optimal environmental conditions or the presence of disturbances.

Evaluating height frequencies—as well as other descriptors—can give important insights into the health status of gorgonian forests. While today, the sampling process as well as the reconstruction are quite time consuming, both aspects will hopefully improve greatly in the future. With the development of new technologies (like ROVs and AUVs) and their reduction in cost, the sampling process could be sped up and even completely automated. With the advent of high- try (SfM = Structure from Motion) could offer a cost-effective and timely method to assess the structure and health of MAFs over relatively large geographical areas through time.

Currently, we are further exploring the collected data, characterizing the coralligenous habitat in the MPA of Portofino, and quantifying the impacts of lost fishing gear on the present communities. With this, we hope to create a baseline, providing information about the community composition as well as the current state of mesophotic MAFs in the area. Bringing attention to the issue of lost fishing gear is an important step towards a healthy ocean. Quantifying its impacts, working closely with all stakeholders, and informing decision-makers lays at the basis of all conservation efforts, including handling lost fishing gear.

MAFs hold a high ecological importance and the mesophotic zone represents an exciting frontier for technical scientific divers. Fundamental information on the distribution and extent of MCEs is still not available, highlight- tance. Working on my master’s thesis has been an incredible experience; it’s complete, but I will continue to explore the mesophotic zone and time (and, in turn, time to gather valuable data) by using rebreathers. There is much left to discover, so stay tuned. 

See the photogrammetric reconstruction here...

Nils Lucas Jacobsen is a marine biologist and passionate technical scientific diver. After completing a bachelor’s degree in biology, he went on to study marine biology, with specializations in conservation biology, impact assessment, and mitigation. In the framework of an international master’s program, he visited universities in Portugal, Italy, and France, and worked in research centers across Europe. The exploration and protection of undiscovered, pristine marine habitats is his main interest. Currently, he is exploring marine animal forests in the mesophotic zone. www.nils-jacobsen.com