The aquatic biome is the largest of all the biomes, covering about 75 percent of Earth’s surface. This biome is usually divided into two categories: freshwater and marine. Typically, freshwater habitats are less than 1 percent salt. Marine life, however, has to be adapted to living in a habitat with a high concentration of salt. Freshwater habitats include ponds, lakes, rivers, and streams, while marine habitats include the ocean and salty seas.

Ponds and lakes are both stationary bodies of freshwater, with ponds being smaller than lakes. The types of life present vary within lakes and ponds. In the shallow, sunny waters there is an abundance of life, such as various species of fish. In the deep, dark waters, however, decomposers thrive.

Rivers and streams are moving bodies of freshwater. The water in a river or stream is largely made up of runoff from sources such as melting glaciers or rainwater. Rivers and streams usually empty into a lake or the ocean. At the beginning of a fast-moving river or stream, the water is clear and oxygen is abundant. As the water flows, however, it may pick up debris, making the river or stream increasingly cloudy. Oxygen levels may subsequently be affected.

The ocean is a large body of saltwater that spans most of Earth’s surface. Like ponds and lakes, life in the ocean is adapted to certain regions of the water. For example, the deepest parts of the ocean are too dark to support photosynthesis, but many creatures still manage to survive here. In these regions, the food chain is based on bacteria that perform chemical reactions to obtain energy, also called chemosynthesis. In shallow ocean waters, coral reefs can form. These structures look like shelves of rock, but they are actually made of living animals, called corals, with a calcium carbonate skeleton. Coral reefs are incredibly diverse, hosting over a thousand species of fish. Currently, coral reefs are in danger due to human-caused climate change, which has led to the ocean growing hotter and more acidic.

Estuaries are regions where freshwater and ocean water mix. Life in estuaries must be adapted to this mixture of saltwater and freshwater. Estuaries are home to many species of fish and shellfish, as well as several species of migratory birds that depend on estuaries for a place to nest and raise their young.