ENCYCLOPEDIA BRITANNICA:
Water molecules, dissolved salts, organic substances, and suspended
particulates combine to cause the intensity of available solar radiation to
decrease with depth.
In
general, any species of fishes (class Osteichthyes) that are found at extreme
ocean depths, usually more than 600 m and even to as much as 2,700 m (2,000 to
9,000 feet). The species, which represent more than a dozen families of marine
fishes, are characterized by huge mouths, and the presence of luminous organs
on some or several parts of the body. The light-producing organs serve to attract
either prey or potential mates. These and other peculiar traits of deep-sea
fishes represent evolutionary adaptations to the extreme pressure, cold, and
particularly the darkness of their environment.
Waves also occur on internal “surfaces” within oceans. These surfaces
represent strata of rapidly changing water density with increasing depth, and
the associated waves are called internal waves.
The
cause of internal waves may lie in the action of tidal forces or in the action
of a wind or pressure fluctuation. Sometimes, a ship may cause internal waves
if there is a shallow, brackish upper layer.
He
(Ekman) displayed his theoretical and experimental talents in his study of
so-called dead water, which causes slow-moving boats to become stuck because of
a thin layer of nearly fresh water spreading over the sea from melting ice.