The freshwater biome includes aquatic ecosystems defined by a low salt concentration, typically less than one percent, encompassing nearly all non-marine surface waters on Earth. Despite holding less than three percent of the planet’s total water, this biome supports immense biodiversity. Its presence is determined by the continuous availability of water and the geological structures that contain it. The freshwater biome is found on every continent, except Antarctica’s deep interior, spanning environments from mountains and deserts to tropical rainforests.
Defining the Global Distribution
The geographic distribution of freshwater biomes is primarily governed by precipitation and local topography. Sufficient rainfall or snowmelt supplies the water, while the shape of the land dictates where that water collects, flows, and is stored. These biomes are not limited by a single climate type, appearing in high-precipitation tropics and water-scarce xeric basins alike. Freshwater systems are classified into two types: lentic (standing waters like lakes and ponds found in closed basins) and lotic (flowing waters like rivers and streams along drainage networks driven by gravity).
Standing Water Systems (Lentic)
Lentic ecosystems are bodies of standing water, varying in size from small, temporary pools to massive, deep lakes. Large lakes often form due to major geological processes, such as the tectonic activity that created the African Rift Valley lakes or the glaciation that carved the basins of North America’s Great Lakes. These large systems are defined by internal zones, including the shallow, light-receiving littoral zone near the shore and the open-water limnetic zone. Smaller ponds and temporary pools are geographically ubiquitous, located in almost any depression where the water table is high or where local runoff collects.
Flowing Water Systems (Lotic)
Lotic ecosystems, encompassing rivers, streams, and creeks, are defined by water that moves continuously in one direction, driven by elevation differences and gravity. Their location is linear, originating at a headwater source and flowing toward a mouth, which might be another river, lake, or the ocean. The characteristics of these systems change along their path, a concept known as longitudinal zonation. The headwaters, or upper reaches, are typically located in higher elevations where the water is colder, clearer, and has higher oxygen levels. As the water flows downstream, the river widens, the current slows, and the water becomes warmer and more turbid as it picks up sediments. Large river basins, like the Amazon or Nile, span vast geographical areas, often crossing multiple climate zones and countries.
Saturated Land Systems (Wetlands)
Wetlands, including marshes, swamps, and bogs, are transitional zones between aquatic and terrestrial environments. They are located where the water table intersects or is near the land surface for a significant part of the year, leading to saturated soil. This saturation creates unique hydric soil conditions that support specialized plant life. These systems are often situated adjacent to lentic or lotic bodies, such as in river floodplains or along the vegetated fringes of lakes. Wetlands can also form independently in areas of low topographic relief or poor drainage, relying solely on precipitation or groundwater. Examples include the prairie potholes of North America and the large peatlands of the boreal forest.