The Online Library of Knowledge

Ocean life

Surface waters

The sunlit surface waters of the oceanThe sunlit surface waters of the ocean The richest variety of life in the oceans is found in the top 200 metres (about 660 feet): the “roof” of the ocean. Here, light from the sun penetrates the water, allowing tiny plants, called phytoplankton, to grow. This is food for tiny animals called zooplankton, which forms a vital part of the ocean food web, to which all ocean creatures are linked. Small fish, for example, feed on the zooplankton and are themselves preyed upon by larger predatory fish, such as tuna or sharks. They also fall prey to sea turtles, and mammals such as seals, dolphins and whales

Humpback whales feeding in surface watersHumpback whales feeding in surface waters
Diatoms, kinds of phytoplanktonDiatoms, kinds of phytoplankton


Unlike land plants, oceanic plants cannot put down roots into the ground. Instead, ocean plants, called phytoplankton, are microscopic in size and float around in the ocean currents, living off the chemical nutrients dissolved in the water. 

Phytoplankton are fed upon by tiny animals called zooplankton. These include the larvae (young) of fish, as well as tiny relatives of crabs and shrimps, known as copepods. Both kinds of plankton provide food for a wide range of animals, including jellyfish, shrimp, fish and even huge whales. Other kinds of fish, mammals, turtles and diving birds prey on some of the plankton-eaters. 

The top 200 m (660 ft) of the ocean is known as the photic zone, after the Greek word phos, meaning "light". About 90% of all marine life lives in the photic zone.

© 2020 Q-files Ltd. All rights reserved. Switch to Mobile