Ulrich Krieger - Aphotic III — Bathyal

From Ulrich Krieger

For a long time, I have been fascinated by the ‘alienness’ of oceanic depths. Together with its opposite, the large deserts, it is the closest to an alien environment and possible alien life we will find here on planet earth.

The aphotic zones (from Greek: lightless) relate to the oceanic depths roughly between 1.000 – 11.000 meters. While the epipelagic (sunlit, 0-200m) and mesopelagic (twilight, 200-1000m) zones have sunlight, and therefore photosynthesis, no light reaches down into the endless darkness of the three deep-sea aphotic, ‘midnight’ water layers: Bathyal (1.000-4.000 m), Abyssal (4.000 – 6.000 m) and Hadal (6.000 – 11.000 m). It is the realm of huge, incredible flat abyssal plains and large deep-sea trenches, like the Mariana Trench.

The fauna of the deep sea is for the most part still unexplored. Down here many fish are luminescent, producing their own light either to attract prey or defend from predators. And the deeper we go pressure gets stronger, water gets colder and less animal life can be found.

Being able to move in three dimensions is one of the new experiences of scuba diving as well as the feel of changing pressure and its effect on our nervous system. As a recreational scuba diver, I only have been diving as deep as 53 m in a canyon in the Red Sea—as part of my special deep dive training. This is already pretty deep considering that the regular depth for most recreational divers is 18 m. But already in shallow depths one can feel changes in perception and the slowing down of brain function—subjectively everything feels slower. My 60-minute dives feel like 20-30 minutes to me.

But underwater also has its own, different acoustic that irritates our senses and creates a new, unique experience. The speed of sound in salt water is 4 times faster than in air, resulting in a loss of directional hearing—sounds seem to come from everywhere, meaning we cannot orientate by sound anymore. Further there is the filter effect of water, making the underwater soundscape filtered, darker and quieter.

With this edition, we are now ascending and reach the Abssyal (from greek: ábyssos – bottomless) layer of oceanic waters. The pressure has eased off, we are now 4.000-6.000 m deep.

And we are ascending further into the upper most aphotic zone, the Bathyal (from greek: bathys - deep) layer. This relates to the oceanic depths roughly between 1.000–4.000 meters.

Our sounds are getting a bit brighter, but it is still part of the lightless and cold aphotic zones. Hydrothermal vents, so called Black Smokers, are a common feature in some areas of the bathypelagic zone. This upper aphotic zone is also called the midnight zone. It is the first aquatic layer into which absolutely no light reaches down anymore. The fauna of this part of the deep sea is for the most part still unexplored. But we know about giant squids, octopods, bioluminescence angler fish and hunting sperm whales, but most about this zone is still unknown.