Page 170 - (DK) Ocean - The Definitive Visual Guide

168 THE OPEN OCEAN AND OCEAN FLOOR OCEAN ZONES

SUNLIT ZONE 0–660 ft
Seawater rapidly absorbs sunlight, so only
Zones of the one percent of light reaches 660 ft (200 m) Light
below the surface. Phytoplankton use the
light to photosynthesize, forming the base of
food chains. This zone drives all ocean life. Temperature
Open Ocean TWILIGHT ZONE 660–3,300 ft 660 ft 68ºF Pressure People and Machines

(200 m)
Too dark for photosynthesis, but with just
1
deepest safe scuba diving
enough light to hunt by, many animals move 41ºF atm
from this zone into the sunlit zone at night. sea level • 164 ft (50 m)
CONDITIONS IN THE OCEAN vary greatly
deep freedive
with depth. Light and temperature changes DARK ZONE 3,300 ft–13,100 ft 3,300 ft • 702 ft (214 m)
Almost no light penetrates below 3,300 ft (1,000 m) 35–39ºF • 1,082 ft (330 m) scuba
occur quickly, while pressure increases (1,000 m). From here to the greatest depths, record by French diver
incrementally. Although many of these it is dark, so no plants can grow, and 100 • 1,960 ft (600 m) nuclear-
atm
virtually the only source of food is the powered submarine
changes are continuous, the ocean can “snow” of waste from above. Temperatures
o
down here are a universally chilly 35–39 F 6,500 ft
be divided into a series of distinct depth (2–4 C), and the pressures so extreme that
o
zones, each of which produces very only highly adapted animals can survive. (2,000 m)
The dark zone is defined as continuing
different conditions for living things. down to the abyssal plain, below 13,100 ft
(4,000 m). Technically, all the water below
3,300 ft (1,000 m) is a dark zone, where the
The Surface Layer only light comes from bioluminescent 9,800 ft
animals (see p.224). However, for (3,000 m)
The top three feet of the ocean is the richest in convenience, the waters below the dark
nutrients. This upper layer is sometimes called the zone can be further subdivided.
neuston, although this term is also used for the
animals that live there, such as jellyfish. Amino acids, ABYSSAL ZONE 13,100–19,700 ft 13,100 ft
fatty acids, and proteins excreted by plants and Beyond the continental slope, the sea bed (4,000 m)
animals float up into this surface layer, as do oils from flattens out. In many areas, it forms vast 35–39ºF
the decomposing bodies of dead animals. These plains at depths below 13,100 ft (4,000 m).
produce a rich supply of nutrients for phytoplankton. Some areas drop deeper to a sea floor that 400
undulates down to depths of 19,700 ft
The top three feet of seawater is also the interface (6,000 m). Around 30 percent of the total atm
where gas exchange takes place between the ocean seabed area lies between these depths. 16,400 ft
and the atmosphere. This is vitally important to all Animals living here move up and down (5,000 m)
life on Earth, as half of the oxygen animals need for through a narrow column above the sea
survival comes from the ocean. Not surprisingly, bed, called the abyssal zone. • 14,800 ft (4,500 m)
phytoplankton gathers in this surface zone in daylight, Alvin submersible
as do the animals that feed on them. This zone is also HADAL ZONE 19,700–36,100 ft 19,700 ft
The sea floor plunges below 19,700 ft
highly susceptible to chemical pollution and floating (6,000 m) in only a few deep ocean trenches. (6,000 m)
litter, which can be deadly for marine life. This hadal zone makes up less than 35–39ºF
2 percent of the total seafloor area.
NOCTURNAL AND DIURNAL DISTRIBUTION Fewer than 10 human beings have ever 600
Only a small proportion of marine life inhabits the deep zone; the visited this zone (see p.183), and the atm
majority live above 3,300 ft (1,000 m). The sunlit zone is dangerous pressures are so high that only a few 23,000 ft
for animals—many stay in the twilight zone by day and only go submersibles are able to operate here. (7,000 m)
upward at night. The sunlit zone is much emptier by day. As yet, little is known about life at these
depths, although anemones and jellyfish
have been observed at a depth of 27,000 ft
DAY NIGHT • 21,000 ft (6,400 m)
(8,221 m) and a fish has been dredged from
10% 40% Shinkai submersible
sunlit zone sunlit zone a depth of 27,500 ft (8,370 m). Amphipods, 26,300 ft
as well as amoebae and various other
microbes, have been found living at the (8,000 m)
bottom of the Mariana Trench, the deepest
75% 50%
twilight zone twilight zone point in the oceans.
SCALE
15% 10% 29,500 ft
deep zone deep zone Burj Khalifa (9,000 m)

OCEAN ENVIRONMENTS When divers breathe compressed air deep trenches plunging to (10,000 m) • Trieste submersible
HUMAN IMPACT
(2,722 ft/829.8 m)
FREE DIVING
32,800 ft

underwater, excess nitrogen dissolves in
their blood, and they risk the bends if
they surface too fast. Free divers avoid
this by holding their breath
underwater. Pressure squeezes their
36,100 ft
lungs, but the surrounding blood
(11,000 m)
vessels swell to protect them, and blood
nitrogen levels stay safe. Trained free
36,100 ft (11,000 m) cover just
divers can hold their breath long
2 percent of the sea bed
35,797 ft (10,911 m)
enough to reach 660 ft (200 m), using
aids to help them descend and ascend.