The Earth's hydrosphere contains almost 1.4 billion km ³ of water, which is the combined mass of water on the planet including its surface, atmospheric, and subsurface sources. There are many different types of aquatic habitats on Earth, of which the major types are estuaries, lakes, oceans, rivers and wetlands. Water, the context for all of these habitats, has unique chemical and physical properties which in turn affect the species and communities that live in these aquatic habitats.

Unique properties of water and aquatic habitats
The molecular structure of water is the reason for many of its unique attributes, such as its density, specific heat, high surface tension, and versatility as a solvent, all of which are critical to life. Water is a polar molecule that consists of one oxygen and two hydrogen atoms. When water molecules are in contact with each other, they form strong bonds between the positive and negative poles. Frozen water is less dense than liquid water because of these bonds, which when frozen create a lattice-like structure that spreads the molecules out, enabling ice to float. This surface ice in turn allows organisms in lakes and streams to survive cold winter temperatures, because the surface of ice acts as an insulator, preventing the water beneath from freezing. Water has a high specific heat, meaning it takes much more heat to raise the temperature of water than the surrounding air and soil. This enables water to buffer changes in temperature in the natural world, allowing for a relatively stable environment for aquatic life. Water has the second highest surface tension of any liquid on Earth, also due to its strong molecular bonds. This creates a special environment at the air-water interface, giving some aquatic insects the option to skate on the surface of water. Another of water's important characteristics is its capability as a nearly universal solvent. This property allows the waters of the world to have mixtures of dissolved gases, salts, and organic compounds, which are used by aquatic organisms to sustain life as they know it.

Adaptations to life in water
Aquatic animals have evolved adaptations to deal with the issue of respiration under water. Many use gills to breathe, which are areas of high surface area that allow the animal to take up dissolved oxygen from water through gas exchange with their circulatory system. Some small animals, such as invertebrates, may use their entire bodies to respire. In addition to animals that live under water, other animals such as shorebirds and wetland birds have life history adaptations and behaviors to utilize aquatic habitats.

Adaptations to salinity
Marine and freshwater environments are very different in part due to the different properties of salt and fresh water. Adaptations to these differences are most notable in fish. Freshwater fish do not drink water, but instead absorb it through their gills by osmosis. They solve the issue of living in water less saline than themselves by having very efficient kidneys to aid in excretion of excess water. In contrast, marine fish must drink large amounts of water in order to replace what has been lost by osmosis to their more saline surroundings.

Sources:
Closs, G., B. Downes, and A. Boulton. 2004. Freshwater Ecology. Blackwell Publishing. Victoria, Australia.
Hornberger, G.M., J.P. Raffensperger, P.L. Wiberg, and K.N. Eshleman. 1998. Elements of Physical Hydrology. Johns Hopkins University Press. Baltimore, MD.
Moyle, P. and J. Cech. 2004. Fishes: An Introduction to Ichthyology. Fifth Edition. Prentice Hall. Upper Saddle River, NJ.

Find out more about the major types of aquatic habitats, including classification, subhabitats, and human impacts:

	Estuaries Read about the diversity of estuaries, where rivers meet the ocean.
	Lakes Learn about lakes, inland reservoirs of water.
	Oceans Find out about the habitat that covers 71% of the Earth.
	Rivers Learn about the rivers, flowing freshwater habitats.
	Wetlands Discover the saturated world of wetlands.