76 11.4 Deep Sea
Introduction
The deep sea is an environment that is unfriendly to humankind. As a result, it represents one of the least explored areas on Earth. It has even been said that the deep sea is less explored than the surface of the moon. The deep sea region begins about 1000 meters beneath the ocean’s surface, located between the thermocline and the seafloor (Gage 1991). Here, light is limited, and after a depth of 100-500 meters, it disappears completely. Temperatures in the deep sea can significantly drop — nearing 0°C at the very bottom. However, its temperature typically ranges between 2-3°C. The salinity level is relatively stable, with concentrations ranging between 34-35 parts per thousand (ppt). On the other hand, its pressure steadily rises with depth, increasing approximately 10 atmospheric pressure per meter (Gage 1991; George 2013).
Figure 1. https://upload.wikimedia.org/wikiped…HERMOCLINE.png
This figure depicts the thermocline. Temperature of the surface water remains high until it reaches the thermocline where the temperature drops rapidly as the lack of heat from the sun dissipates. Temperature continues to decline, though much less rapidly, at lower depths.
Life at the Depths
While many scientists believed the deep sea was devoid of life due to its harsh conditions, multiple expeditions have demonstrated the opposite. They found that the deep sea possessed intricate and lively ecosystems, despite being thousands of meters below the surface. In the 1870s, the Challenger expedition revealed the existence of a variety of deep water species. More recently, in 1977, scientists aboard DSV Alvin discovered diverse deep sea communities around hydrothermal vents.
Caused by the absence of light, life in the deep sea faces multiple challenges, such as having difficulty producing energy, attracting prey, and finding mates. At these depths, primary producers can no longer rely on the sun for energy, so they use alternative sources, such as falling detritus from above. These falling particles are known as marine snow, consisting of dead or decaying plant, animal material, fecal matter, and other organic and inorganic particulate matter (Silver 2015). The continuous rain of marine snow provides food for many deep-sea creatures, giving them plenty of carbon and nitrogen to sustain the scavengers of the deep and fuel the rest of the ecosystem.
Physical Adaptations
The animals of the deep sea have adapted to survive in their habitats’ extremely harsh conditions: no light, low amounts of food, low amounts of oxygen, high pressure, and cold temperatures. Advantageous physiological changes have developed to enable them to survive in this kind of environment.
Many organisms in the deep sea have adapted a useful physiological trait to aid in the environment’s low light conditions: bioluminescence. Bioluminescence is the production and emission of light by a living organism. The ability to produce their own light allows organisms to lure pre