Under Pressure

What are the challenges to exploring the deep ocean? Dr Anjani Ganase considers light and pressure beyond 300 metres deep. Hard to imagine the weight of water at a thousand metres deep; far less deep in the abyss.

We’ve all experienced that first plunge under the surface as a child; the frantic kicking of the legs to propel the body downwards; then the blurriness and burning of the eyes followed quickly by the build up of pressure in the ears to make us give up and return to the surface. I would spend hours at the beach, heels above my head trying to push the limits just to catch a view of the bottom and any fish or creature below. The first lesson was how to equalise the pressure in the ears to go a little bit deeper and to stay longer just to catch glimpses of the underwater world through short windows of 30-second breaths at 4 or 5 m deep. 

If we wish to go beyond ten metres, we generally need SCUBA (Self-Contained Underwater Breathing Apparatus) equipment to observe and experience the marine world. 

Let's go diving! Photo by Anjani Ganase

The upper 300 metres of the ocean’s water column which is sunlit is aptly known as the photic (light) zone and is the area best known. Along coastlines and on continental shelves, the ocean meets the sea floor at these depths creating perfect locations for marine benthic communities, such as coral reefs, kelp forests, sea grass beds, all of which depend on light (photosynthetic pathways) for food. Note that on land we also depend on light from the sun for food. In the open ocean, phytoplankton draws close to the surface to soak up the light and brings those that feed on it in tow. 

So you have put on your SCUBA gear. Once you roll back into the water, it takes about a minute or two to descend past brightly lit waters teeming with coral reefs growing along the bottom, and busy brightly coloured fish communities foraging among the corals, algae and sponges. 

By the time we reach about 30 m deep, the light is mostly blue as the rest of the light spectrum has been absorbed and dissipated in the water.  Here, coral communities tend to flatten out like an open flower trying to capture as much of the more limited light as possible. Nevertheless this blue world teems with life. If you wear a scuba tank, the air inhaled feels thicker and your weight belt is now a size too big. For every 10 meters of depth you feel the added equivalent pressure of 14 pounds per square inch; the pressure exerted on our bodies at this depth is about 42 pounds per square inch. But you can be comfortable once all airways are equalised by popping our ears and maintaining a constant breath.  

If we continue to dive beyond 30 m we’ll definitely need more air and even more specialised equipment to help our bodies. The mesophotic zone (literally means “middle light” or “twilight” zone) in the tropics shares a lot of similarities with the shallower benthic communities, yet there are marine organisms that are specially adapted to relatively low light conditions. There is still enough sun energy for organisms – algae and corals - to harness the lower light energy and maximise absorption with specially adapted algal cells, as well as changing the overall growth morphology. More fragile filter feeding sponges are also abundant at these depths, away from the surface waves. The coral communities present in the mesophotic zone can extend to 150 m in locations where the water is transparent enough to let the light penetrate these depths. This environment makes for unique communities of fish species and invertebrates, including sponges that have also adapted to being at these depths. 

By simply strapping a tank onto our back, we’ve extended our view of our underwater world by about 40 m but to go farther requires more equipment. Photo by Anjani Ganase

We would not be able to dive beyond the mesophotic zone without a serious swap over of diving equipment. The extreme pressures exerted on the body at those depths,  ~ 210 pounds of pressure per square inch at 150 m depth. Submersibles are the way to go! So let’s keep diving past 150 m. Apart from the change in the light and the pressure, you also notice that you’re getting colder. The source of our light is also the source of our heat. The waters are much colder as we get to depths with minimal light. In some locations where light may penetrate, scientists have recently discovered a distinct community of fish that are adapted to the meagre light conditions, as well as lower temperatures and higher pressures even though they share common ancestors with their shallow water relatives (as oppose to the deep-sea fish communities), and therefore deserve their own community zone, called the rariphotic zone (150m – 300m) (Baldwin et al 2018). The structures that were formed by corals are no longer present and the substrate is flatter except for the scattered presence of sponges and deep-sea corals that are not light dependent and much slower growing. 

The rest of the ocean below 300 m is known as the aphotic (no light) zone, but with over 10,000 m of ocean below known as the deep abyss. Let us continue to drift downwards into the dark deep, off the continental shelf. As both Trinidad and Tobago sit on the continental shelf you would have to swim several kilometres out to the northeast to get to water that is over 100 m deep. From the edge of the continental shelf, let us jump into a submersible and follow the slope down to depths of 4500 m with one or two daring attempts to go farther.  

Substation Curaçao allows for exploration for extended periods down to about 300 m (1000 feet) for research in minimal light ecosystems. Photo by Underwater Earth, XL Catlin Seaview Survey

Our personal experience of the marine world in the abyss is scarce with only a handful of manned vessels. To be able to view the deep, most of the time this is done remotely with the use of remotely operated vehicles (ROVs) managed from the surface. The most famous manned descent was done by James Cameron (director of the movie Avatar), who went into the Challenger Deep, the deepest known point on earth in the Mariana Trench (10,898 m) in the Pacific Ocean, in a custom-made deep sea submersible appropriately named the Deepsea Challenger. The journey took him two hours and 37 minutes, the same as a flight to Turks and Caicos, yet it looked as if he was landing on the moon. With about 16,000 pounds per square inch of pressure this submersible actually shrank a few centimetres at this depth. 

At 10,898 m, what does life look like in the abyss? Mostly scavenging communities were found including species of sea cucumbers, worms and amphipods (Gallo et al. 2015). Marine life down here has special adaptations. No light means having no eyes or specially adapted eyes (large round roving eyes that can capture any light for “sight” in the darkness). The light that they would focus on is bioluminescence (light being emitted by marine organisms typically through a chemical reaction). Bioluminescence is used for communicating, defence and even attracting mates. Prey, such as small fish and worms, use bioluminescence to distract predators, while the predators, such as the firefly squid and anglerfish, uses it to lure prey. There are adaptations of other senses, such as changes in pressures in the water column and vibrations because of movement. The use of sound/ echolocation allows them to “see” in the dark but dense water. Fish also have large stomachs to hold larger prey as food is much more scarce. In the very deep, the water nears freezing, and so the metabolism and the release of energy are also very slow. 

Time to return to the surface.

References: 

Baldwin CC, Tornabene L, Robertson DR. Below the mesophotic. Scientific reports. 2018 Mar 20;8(1):4920.

Gallo ND, Cameron J, Hardy K, Fryer P, Bartlett DH, Levin LA. Submersible-and lander-observed community patterns in the Mariana and New Britain trenches: influence of productivity and depth on epibenthic and scavenging communities. Deep Sea Research Part I: Oceanographic Research Papers. 2015 May 1;99:119-33.