Every living thing on Earth needs water to survive, but more than one hundred thousand species, including our own, need a special kind of water that can only be found in certain places and is in very rare supply: fresh water. The plants, animals, microbes, rocks, soil, sunlight, and water found in and around this valuable resource are all part of what is called a freshwater ecosystem. Less than three percent of our planet’s water is fresh water, and less than half of that is available as a liquid; the rest is locked away as ice in polar caps and glaciers. For these reasons, freshwater ecosystems are a precious resource.


Where is Fresh Water?


Fresh water starts out as water vapor that has evaporated from the surface of oceans, lakes, and other bodies of water. When this vapor rises, it leaves salts and other contaminants behind and becomes “fresh.” The water vapor collects in drifting clouds that eventually release the water back to Earth in the form of rain or snow.


After fresh water reaches the ground through precipitation, it flows downhill across a landscape called the watershed to lakes, ponds, rivers, streams, and wetlands. But fresh water can be found in less-obvious places too. More than half of all freshwater on our planet seeps through soil and between rocks to form aquifers that are filled with groundwater. The top surface of an aquifer is called the water table, and this is the depth where wells are drilled to bring fresh water into cities and homes.


Studying Freshwater Ecosystems

On the volcanic island nation of Iceland, explorer Jónína Herdís Ólafsdóttir studies freshwater ecosystems that develop from groundwater seeping into fissures. These fissures are large cracks, which are caused by the tectonic plates underneath the country shifting and pulling the bedrock apart. The crystal blue water in these fissures is barely above freezing temperature. Wearing scuba gear, Olafsdóttir drops into the water and collects biological samples, recording notes about the species of fish, crustaceans, algae, and other microbes that she finds. She was one of the first scientists ever to describe the biodiversity in these Icelandic fissure ecosystems.

Scientists who study freshwater ecosystems are called limnologists. Limnologists want to learn what creatures live in an ecosystem and how they interact with each other through the ecosystem’s food web, as well as how they interact with their environment. This knowledge can help the researchers know when a freshwater ecosystem is healthy and when it may be in danger.


Balancing Change


Freshwater ecosystems naturally share resources between habitats. The ecosystems in rivers and streams, for example, bring salts and nutrients from the mountains to lakes, ponds, and wetlands at lower elevations, and eventually they bring those nutrients to the ocean. These waterways also enable migrating species, like salmon, to bring nutrients from the ocean to upstream freshwater ecosystems. 


Lakes and ponds, on the other hand, can exchange nutrients in a seasonal cycle. Cold water is denser than warm water, so it sinks to the bottom, where a fairly steady temperature is maintained. However, as the air temperature drops with the arrival of winter, the water that is closest to the surface may drop below the temperature of the water at the bottom of the lake, causing it to sink and the warmer bottom water to rise. The same process happens as floating surface ice melts into very cold water in the spring. During these periods, nutrients are churned from the floor and brought to the surface.


It is normal for ecosystems to encounter change. Temperatures may fluctuate, populations may rise and fall, and rain may bring an abundance of water, then tapering during drought. The plants, animals, and microbes in healthy freshwater ecosystems are resilient and have adaptations that allow them to adjust appropriately until ideal conditions resume. However, if any element of the ecosystem varies too far outside of the norm, the balance of the whole system can start to fail.


Signs of Danger


Humans use fresh water in many ways, but these activities can be dangerous for freshwater ecosystems when we are not careful. Overfishing, pollution, and disruption of the landscape through projects like dams and deforestation are just a few ways we can put these ecosystems—and ultimately, our own access to fresh water—at risk. When the changes we cause are too great or too sudden, then ecosystems struggle to bounce back.


An example of this kind of sudden change is when an invasive species enters an area, which happened in 2009 near the city of Madison, Wisconsin, when the spiny water flea (Bythotrephes longimanus) was detected in Lake Mendota. The spiny water flea, native to Russian and European lakes, came to North America in the 1980s with cargo ships that had traveled across the Atlantic and down the St. Lawrence River to the Great Lakes. Eventually, these tiny stowaways were carried over land to Lake Mendota, and that is where they unleashed a cascade of havoc.


Spiny water fleas love to eat Daphnia pulicaria plankton, which are important to the Lake Mendota ecosystem, because they eat green algae that would otherwise grow out of control. D. pulicaria is also a key food source for fish in the lake. As the population of spiny water fleas increased, algae began to overgrow and lower the oxygen content of the water, causing the fish to die and the lake water to grow murky. Ten years later, spiny water fleas are still thriving in Lake Mendota and now, a new invasive species, the zebra mussel (Dreissena polymorpha), is taking over the lake floor. Once an invasive species becomes established in a freshwater ecosystem, it is nearly impossible to get it out.


Scientists like Canadian aquatic ecologist Dalal Hanna can help avoid disasters like these by studying ecosystems and identifying points of human interaction that might cause trouble. Hanna has researched freshwater fish in African streams and birds that live near freshwater ecosystems in Canada. Today, she is developing useful measures and management strategies so communities can gauge how to balance their need for “ecosystem services” like drinking water, recreation, and flood prevention with the health of the freshwater ecosystems upon which they rely.


Freshwater Ecosystem

A mountain stream flowing through Inverpolly, Scotland. 


an underground layer of rock or earth which holds groundwater.


wetland of soft ground mostly made of partially decayed plant matter called peat.


process by which liquid water becomes water vapor.


narrow opening or crack.


all related food chains in an ecosystem. Also called a food cycle.


type of plant or animal that is not indigenous to a particular area and causes economic or environmental harm.


fresh water that is relatively still, such as in lakes, ponds, and wetlands.


study of freshwater ecosystems.

littoral zone

shallow area of water where sunlight can penetrate to the bottom.


fresh water that flows in a single direction, such as in rivers, streams, and creeks.


having to do with the open ocean.


all forms in which water falls to Earth from the atmosphere.


entire river system or an area drained by a river and its tributaries.


underground area where the Earth's surface is saturated with water. Also called water level.


area of land covered by shallow water or saturated by water.