Freshwater Biome
The freshwater biome covers all the parts of the world dominated by non-salty water: rivers, lakes, ponds, streams, marshes and underground water. Only approx. 3% of all the water on Earth is fresh (the rest is salty ocean water), and most of that fresh water is locked up in ice caps and glaciers. Less than 1% is available as the rivers and lakes that almost every land creature, including every human, depends on for drinking. That tiny fraction is the most fought-over resource on Earth.
- % of Earth's Waterapprox. 3%The rest is salty ocean
- Available to drink<1%Most is locked in ice or underground
- Longest riverNile6,650 km, Africa
- Largest lake (area)Caspian Seaapprox. 371,000 km², Asia and Europe
- Deepest lakeBaikal1,642 m, Siberia
- Iconic animalBeaverBuilds dams that create wetlands
Where is Earth's freshwater?
Most fresh water is actually locked up where it is hard to use.
Approx. 99% of Earth's fresh water is either frozen in glaciers or hidden underground. Just 1% sits in lakes and rivers where land creatures can reach it.
Types of freshwater
Scientists usually split the freshwater biome into two main types, based on whether the water moves or sits still.
- Lentic (standing water): lakes, ponds, marshes. Water sits in one place. Nutrients stay where they are, so plants can root and oxygen levels change with depth.
- Lotic (flowing water): rivers, streams, springs. Water moves continually downhill towards the sea, washing nutrients downstream and keeping oxygen levels high. Different creatures live in fast and slow stretches.
Wetlands like marshes, swamps and bogs sit between the two: shallow, slow-moving water often flooded only part of the year. They are some of the most productive ecosystems on Earth, packed with fish, birds and insects, and they store huge amounts of carbon.
Where Earth's freshwater is
The continents are not equally watered. South America holds the most freshwater per person (the Amazon alone carries roughly 20% of all the water that flows into the world's oceans). The Middle East and North Africa are the poorest, with countries like Yemen and Libya running out of water completely.
The five biggest fresh water lakes by area are all in North America and Africa: Lake Superior (USA/Canada), Lake Victoria (East Africa), Lake Huron (USA/Canada), Lake Michigan (USA) and Lake Tanganyika (East Africa). The deepest and oldest lake in the world is Lake Baikal in Siberia, which contains approx. 20% of all the world's liquid fresh water in one single lake. The longest rivers are the Nile in Africa (6,650 km) and the Amazon in South America (6,400 km), which together drain about a fifth of all the land on Earth.
The water cycle
All the world's fresh water is part of one giant loop called the water cycle. Sunlight evaporates water from the oceans, lakes and even plants (a process called transpiration). The water vapour rises, cools and forms clouds, which travel on the wind. Eventually the clouds drop their water as rain or snow. Some of that water flows back into rivers and lakes; some seeps into the ground to become groundwater; some is taken up by plants; and the rest eventually finds its way back to the sea.
The water you drink today has been around for approximately 4.5 billion years and may once have been swallowed by a dinosaur. The total amount of water on Earth does not really change; it just keeps recycling through the atmosphere, the land and the sea. What does change is where the water ends up, and human activity (dams, irrigation, climate change) is shifting that balance fast.
Plants and producers
Freshwater plants come in three styles. Emergent plants like reeds, cattails and water lilies are rooted in shallow water but grow above the surface. Floating plants like duckweed and water hyacinth drift on top with their roots dangling underneath. Submerged plants like pondweed and watermilfoil live entirely under the surface, photosynthesising in the dim filtered light. In fast rivers, large plants struggle, and tiny algae coating the rocks become the main food source instead.
Microscopic plant plankton (phytoplankton) drift in still water and form the base of the freshwater food web, just as they do in the ocean. They are eaten by tiny animals like water fleas, which are eaten by small fish, which are eaten by big fish or birds, all the way up to ospreys, otters and people.
Wildlife of freshwater
Freshwater is full of life. Fish like salmon, trout, pike and carp live their whole lives in or around it. Salmon make some of the great journeys of the natural world, swimming hundreds of kilometres upstream to spawn in the exact stream where they hatched years before. Mammals like beavers, otters, river dolphins (Amazon, Yangtze) and hippos depend on it. Many bird species (kingfishers, herons, ducks, kingfishers, ospreys) hunt along the banks. Frogs, toads, newts and salamanders all need fresh water to lay their eggs.
Some lakes have a unique cast of animals that exist nowhere else. Lake Baikal has the only freshwater seal in the world, the nerpa. Lake Malawi and Lake Tanganyika in Africa each have hundreds of species of brightly coloured cichlid fish, all evolved from a tiny number of ancestors in just a few million years. These isolated lake ecosystems are sometimes called "natural laboratories of evolution".
People and freshwater
People depend on freshwater for almost everything: drinking, washing, cooking, growing food, building things, and disposing of waste. Around 70% of the freshwater pumped from rivers, lakes and aquifers is used to grow crops; roughly 20% for industry; and only 10% for households. Almost every great civilisation in history grew up along a major river: Egypt on the Nile, Mesopotamia on the Tigris and Euphrates, China on the Yellow and Yangtze, India on the Indus and Ganges.
The biggest threats to the freshwater biome are pollution (farm runoff, plastic, sewage and industrial chemicals), over-use (whole rivers like the Colorado in the USA, the Yellow River in China and the Murray in Australia now run dry before reaching the sea in some years), dams (which fragment rivers and block migrating fish), and climate change (changing rainfall patterns and shrinking the glaciers that feed many of the world's rivers). Roughly 2 billion people still drink water from sources contaminated with sewage, and waterborne disease kills approximately 1 million people every year.
Deeper dive: aquifers, groundwater and the world's hidden rivers
Most of the world's liquid fresh water is invisible. It hides beneath your feet, in aquifers: layers of porous rock or gravel saturated with water like an underground sponge. The biggest is the Great Artesian Basin in Australia, an underground sea covering 22% of the continent. The Ogallala Aquifer beneath the central United States is another giant, providing irrigation water for one of the world's most productive farming regions.
Aquifers fill up very slowly. Rainwater that lands today may take hundreds or thousands of years to seep down through the soil to refill an aquifer. But people are pumping that water out at far higher speeds: in many regions, water tables are dropping by metres per year. The Ogallala has lost approximately one-third of its volume since pumping began in the 1940s. When an aquifer is over-pumped, the ground above can collapse in a process called subsidence: parts of Mexico City have sunk 9 m, and parts of California's Central Valley have sunk over 8 m.
Groundwater also feeds the surface. Many rivers stay flowing through dry summers only because groundwater seeps in to top them up at springs. Some places have remarkable underground rivers: the Sistema Sac Actun in Mexico is the world's longest known underwater cave system, an interconnected network of over 350 km of flooded passages carved into ancient limestone. Aquifers, rivers, lakes and the atmosphere are all part of one connected freshwater system; what happens to one part affects all the rest.
For famous individual rivers including the Nile, the Amazon, the Yangtze and the Mississippi, see the Rivers section.