ENCYCLOPEDIA BRITANNICA:

Water molecules, dissolved salts, organic substances, and suspended particulates combine to cause the intensity of available solar radiation to decrease with depth.

ENCYCLOPEDIA BRITANNICA:

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.

ENCYCLOPEDIA BRITANNICA:

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.

ENCYCLOPEDIA BRITANNICA:

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.