Abyssal zone
As there is no light, photosynthesis cannot occur, and there are no plants producing molecular oxygen (O2), which instead primarily comes from ice that had melted long ago from the polar regions.
This region also contains a much higher concentration of nutrient salts, like nitrogen, phosphorus, and silica, due to the large amount of dead organic material that drifts down from the ocean zones above and decomposes.
The relative sparsity of primary producers means that the majority of organisms living in the abyssal zone depend on the marine snow that falls from oceanic layers above.
At depths greater than 4000 meters shells dissolve, leaving behind a seafloor of brown clay and silica from dead zooplankton and phytoplankton.
[3] Chemosynthetic bacteria support large and diverse communities near hydrothermal vents, filling a similar role in these ecosystems as plants do in the sunlit regions above.
Not only did they have to find ways to hunt and survive in constant darkness, but they also had to thrive in an ecosystem that has less oxygen and biomass, energy sources and prey, than the upper zones.
To survive in these conditions, many fish and other organisms developed a much slower metabolism, and require much less oxygen than those in upper zones.
Most fish species have evolved to be transparent, red, or black so that they better blend in with the darkness and do not waste energy on developing and maintaining bright or complex patterns.
[3] The abyssal zone is made up of many different types of organisms, including microorganisms, crustaceans, molluscs (bivalves, snails, and cephalopods), different classes of fishes, and possibly some animals that have yet to be discovered.
When animals from higher ocean levels die, their carcasses occasionally drift down to the abyssal zone, where organisms in the deep can feed on them.
As fish and other animals are removed from the ocean, the frequency and amount of dead material reaching the abyssal zone decreases.
Mining could increase the amount of pollution not only in the abyssal zone, but in the ocean as a whole, and would physically destroy habitats and the seafloor.
[4] Sediment plumes generated by mining activities can spread widely, affecting filter feeders and smothering marine life.
[17] The slow rate of change in deep-sea environments and the long lifespans and reproductive cycles of abyssal species mean that recovery from such disturbances could take decades or centuries.
