Aquatic Environments

Just as the land has many different environments, water also provides opportunities for the survival of a great variety of life forms.

     It is believed that life began in the ocean, possibly around deep volcanic vents in the ocean floor, or perhaps in warm, shallow seas. The chemicals of life move easily in water, and water environments support many forms of animals and plants.

     Aquatic life is dependent on access to sunlight and nourishing chemicals that are dissolved in the water. Living things need phosphorus, nitrogen, oxygen, carbon (dissolved as carbon dioxide), sulfur, calcium, and hydrogen. Some of these chemicals are carried into the ocean from the land, some are reclaimed from the decaying bodies of dead animals and plants. The availability of these elements limits the numbers of life forms that live in different areas.

     Light is an important factor in determining the makeup of aquatic populations. In some waters, such as muddy rivers, light may barely penetrate below the surface of the water: in the deep, clear waters of the ocean, it may go down to 100 meters. However, the light (and the nutrients) are most abundant in the first meter of water below the surface. Plants can only grow where they have light.

     Because of limitations in the supply of nourishing chemicals and light, some parts of the ocean are almost lifeless, while other parts are thickly populated with life forms. There are many varieties of life forms near the land, especially where parts of the ocean are shallow, and where nutrients are being washed off the land into the water.

Aquatic environments share certain characteristics.

  • Temperatures in water change slowly and are not so extreme as temperature changes on land.
  • Water supports the bodies of living things. Water life forms do not need to be as concerned about supporting their bodies as land life forms.
  • Living things that live in water are not in danger of dehydration.
  • Water is more dense than air. Creatures that need to move swiftly in water develop streamlined shapes.
  • Light does not penetrate far into water.
Nutrients in the water come from the decomposition of existing life forms or are washed into the water from the land.
Some Types of Communities

 Conditions  Life Forms
 Intertidal Zone Between the high and low tide levels.  Alternately wet and dry. Challenging! Crabs, barnacles, snails -- exoskeletons protect from predators and dehydration
Estuary Water is mixture of fresh and saline. Where a river flows into the ocean. Lots of nutrients in water Highly productive: a nursery for many life forms
Oceanic Shallow Water Mud Flats  Warm water, gentle tides  Clams, flounders, burrowing worms, sea grass
Oceanic Shallow Water Rocks  May have strong waves, tide pools  Star fish, anemones, crustaceans, small plants
Plants and many animals cling to rocks
 Shallow Water Over  Sand Sand is easily moved by the water. It may be hard for plants to anchor in Clams --life forms may dig down into the  sand or hide by burying themselves
Coral Reefs Warm, clear, deep water Very diverse community: fishes, corals, sea anemones, eels, octopus
Kelp Forests Deep water but still receiving light Gigantic plants shelter small fish, snails, sea urchins, abalone
Abyssal (deep bottom) Zone Cold, dark, enormous pressures, very little food, life forms few in number, but very varied Star fish, nemotodes, occasional fish: no plants
Hydrothermal Vents Volcanic eruptions in sea floor: hot water and sulphur, dissolved minerals. Blood worms, bacteria, clams
Streams and Rivers Fresh water: cold in the mountains Fishes, crayfish, water plants
Lakes Fresh water: zones similar to ocean Fishes, water lilies, rushes, insect larvae
Copyright 1999,2003   Elizabeth Anne Viau and her licensors.  All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at