Undersea

An illustration of some old-timey divers in the ocean.

"The sea is everything.  
 It covers seven tenths of the terrestrial globe.  
 Its breath is pure and healthy.  
 It is an immense desert, where man is never lonely,  
 for he feels life stirring on all sides.  
 The sea is only the embodiment 
 of a supernatural and wonderful existence.  
 It is nothing but love and emotion."  

- Jules Verne, Twenty Thousand Leagues Under the Sea (1870)

Links

Smithsonian Ocean
NOAA Ocean Service
NOAA Ocean Exploration
Deep Ocean Education Project
Discovery of Sound in the Sea
The Open Ocean (archived)
My Legendary Quest: The Ocean Is The Most Terrifying Place In The World And You Should Be Scared Of It

Biology

Youtube: Monterey Bay Aquarium Research Institute
Youtube: The Secret Life of Plankton

Pelagic Zone

(Edited from this Wikipedia article, and the articles for each zone)

Epipelagic (sunlight)

The epipelagic zone (also called the photic zone) is the uppermost layer of the sea, recieving enough sunlight for phytoplankton to perform photosynthesis. These phytoplankton grow extremely quickly because of the light's heavy influence, enabling it to be produced at a fast rate. In fact, ninety five percent of photosynthesis in the ocean occurs in the photic zone. Therefore, if we go deeper, beyond the photic zone, such as into the compensation point, there is little to no phytoplankton, because of insufficient sunlight.

The photic zone is home to the majority of aquatic life due to the activity (primary production) of the phytoplankton.

Mesopelagic (twilight)

The twilight zone at the depth where only 1% of incident light reaches and ends where there is no light; the depths of this zone are between approximately 200 to 1,000 meters below the ocean surface. So although some light penetrates this zone, it is insufficient for photosynthesis. This is why it's diverse biological community is very adapted to life in a low-light environment.

Many organisms in the mesopelagic zone...

recycle the organic matter sinking from the epipelagic zone.
move up into the epipelagic zone at night, and retreat to the mesopelagic zone during the day, which is known as diel vertical migration (DVM). There is so much biomass in this migration that sonar operators in World War II would regularly misinterpret the signal returned by this thick layer of plankton as a false sea floor!

Bathypelagic (midnight)

The ocean is pitch black at this depth, apart from occasional bioluminescent organisms. No plants live here. Most animals survive on detritus (also called "marine snow") falling from the zones above or by preying on other inhabitants of this zone.

Hydrothermal vents are a common feature in some areas, playing an important role in global ocean chemical processes, supporting unique ecosystems that have adapted to utilize chemicals as energy (via chemoautotrophy) to sustain themselves, and facilitating precipitation of minerals on the seafloor, making them regions of interest for deep-sea mining.

Abyssopelagic (abyss)

The word "abyss" is derived from the Ancient Greek word "ἄβυσσος", a holdover from times when the deep ocean was believed to indeed be bottomless. At depths of 4,000 to 6,000 metres (13,000 to 20,000 ft), this zone remains in perpetual darkness. The abyssal zone has temperatures around 2 to 3 °C (36 to 37 °F) through the large majority of its mass.

Due to the absense of light, there isn't a single plant producing oxygen here. That primarily comes from ice which melted long ago in the polar regions. The water along the seafloor of this zone is actually devoid of oxygen, resulting in a death trap for organisms unable to quickly return to the oxygen-enriched water above. When this happens (or when any dead organism drifts down from the higher levels), they provide food for the organisms in the deep. When a whale carcass falls down here in a whale fall, complex ecosystems can be created around it.

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. 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.

The abyssal zone's ecosystem are dominated by planktonic detritivores, with low densities of gelatinous invertebrates and scavenging and predatory fish. These organisms had to evolve to withstand the sheer cold and intense pressure found at this level. They also had to not only find ways to hunt and survive in constant darkness but to thrive in an ecosystem that has less oxygen and biomass, energy sources or prey items, than the upper zones.

To survive in a region with so few resources and low temperatures, many fish and other organisms developed a much slower metabolism and require much less oxygen than those in upper zones. Many animals also move very slowly to conserve energy. Their reproduction rates are also very slow, to decrease competition and conserve energy. Animals here typically have flexible stomachs and mouths so that when scarce food items are found they can consume as much as possible.

Other challenges faced by life in the abyssal zone are the pressure and darkness caused by the zone’s depth. Many organisms living in this zone have evolved to minimize internal air spaces, such as swim bladders. This adaptation helps to protect them from the extreme pressure, which can reach around 75 MPa (11,000 psi).

The absence of light also spawned many different adaptations, such as having large eyes that allow the detection/use of any light avaliable. Animals in the abyssal zone are commonly bioluminescent, with internal organs that produce blue light, because the blue wavelength of light is attenuated over greater travel distances than other wave lengths. These lights are used to attract prey or mates or to defend themselves. Complex designs and bright colors are not needed in these depths, so most fish species have evolved to be transparent, red, or black to better blend in with the darkness and avoid wasting energy on developing and maintaining bright/complex designs.

Hadopelagic (hades)

This is the deepest part of the ocean at more than 6,000 m (20,000 ft) or 6,500 m (21,300 ft), depending on authority. Such depths are generally located in trenches. The cumulative area occupied by the 46 individual hadal habitats worldwide is less than 0.25% of the world's seafloor, yet trenches account for over 40% of the ocean's depth range.

The deepest ocean trenches are considered the least explored and most extreme marine ecosystems. They are characterized by complete lack of sunlight, low temperatures, nutrient scarcity, and extremely high hydrostatic pressures. The major sources of nutrients and carbon are fallout from upper layers, drifts of fine sediment, and landslides.

Most of the organisms are scavengers and detrivores. Over 400 species are currently known from hadal ecosystems, many of which possess physiological adaptations to the extreme environmental conditions. There are high levels of endemism, and noteworthy examples of gigantism in amphipods, mysids, and isopods and dwarfism in nematodes, copepods, and kinorhynchs.

Marine life decreases with depth, both in abundance and biomass. Most trench communities probably originated from the abyssal plains. Although they have evolved adaptations to high pressure and low temperatures, there is no consistent relationship between pressure and metabolic rate in these communities. Increased pressure can instead constrain the ontogenic or larval stages of organisms.

The only known primary producers in the hadal zone are certain bacteria that are able to metabolize hydrogen and methane released by rock and seawater reactions (serpentinization), or hydrogen sulfide released from cold seeps. Some of these bacteria are symbiotic, otherwise the first link in the hadal food web are heterotroph organisms that feed on marine snow, both fine particles and the occasional carcass.

Seamounts

Seamounts, undersea mountains typically formed by volcanic activity, were once thought to be little more than hazards to submarine navigation. Today, scientists recognize these structures as biological hotspots that support a dazzling array of marine life.

The biological richness of seamount habitats results from the shape of these undersea mountains. Thanks to the steep slopes of seamounts, nutrients are carried upwards from the depths of the oceans toward the sunlit surface, providing food for creatures ranging from corals to fish to crustaceans.

New estimates suggest that, taken together, seamounts encompass about 28.8 million square kilometers of the Earth's surface. That's larger than deserts, tundra, or any other single land-based global habitat on the planet!

(Edited from the original.)

Lexicon

Chemistry

gas hydrate: ice-like substance that forms in deep-sea sediments when low-density gas combines but does not chemically bond with water and freezes into a solid under low temperature and moderate pressure conditions
serpentinization: a heat-producing chemical reaction between seawater and mantle rocks

Ecosystem

chemosynthesis: the process by which food is made by bacteria or other living things using chemicals as the energy source, typically in the absence of sunlight
chemotroph an organism that obtains energy by the oxidation of electron donors in their environment
cold seeps: places throughout the global ocean where various hydrocarbon-rich fluids and/or gases escape from cracks/fissures in the ocean floor. At these seep sites, the escaping fluid or gas is most often found to have temperatures close to that of the surrounding seawater. They are referred to as cold seeps to differentiate them from the super-heated fluids released from hydrothermal vents
doldrums: a region over the ocean near the equator abounding in calms, squalls, and light baffling winds
hydrothermal vent: habitat where geological activity has opened cracks on the ocean floor, expelling superheated and chemical-rich water
seamount: undersea mountain typically formed by volcanic activity

Slang

bubblehead: US military slang for a submariner
sun dodger: submariner in any of the world's navies

Sonar

transducer: electrical device that transforms energy from one form to another
active sonar: emit an acoustic signal or pulse of sound into the water. if an object is in the path of the sound pulse, the sound bounces off the object and returns an "echo" to the sonar transducer (if the transducer is equipped with the ability to receive signals, it measures the strength of the signal. By determining the time between the emission of the sound pulse and its reception, the transducer can determine the range and orientation of the object)
passive sonar: used primarily to detect noise from marine objects/animals. (advantageous for scientists who wish to listen, and millitary who wish to avoid revealing themselves with an active sonar signal)
sidescan sonar: sonar method, uses sound waves to create images of the seafloor
multibeam sonar: sonar method, uses multiple, simultaneous sonar beams (sound waves) at once in a fan-like pattern or "swath"
synthetic aperture sonar (SAS): sonar method, uses an artificial array to generate higher resolution imagery than traditional side-scan sonars

Art

Ludens

Subnautica by Unknown Worlds Entertainment (Steam)
Fisher Diver by Eli Pilonen (Flash)

Audio

Albums

Journey Of The Deep Sea Dweller I by Drexciya (Bandcamp)
The Deep by clipping. (Bandcamp)
Bones in the Ocean by The Longest Johns (Bandcamp)
Assasin's Creed IV: Sea Shanties by Various Artists (Youtube)