Freshwater fuels ecosystems and enables human societies to flourish. These resources, found in rivers, lakes, and underground aquifers, constitute a small fraction of the world’s total water. Despite their importance, freshwater systems face strain from a variety of human-induced pressures. These activities are altering the quality, quantity, and health of the water that billions of people and countless species depend on.
Contamination of Freshwater Sources
One of the most direct human impacts on freshwater is contamination from agricultural activities. The use of nitrogen and phosphorus fertilizers on farms leads to runoff into nearby rivers and lakes. This influx of nutrients, also from sewage and livestock waste, can trigger a process called eutrophication. Eutrophication is characterized by algal blooms that cover the water’s surface, blocking sunlight from reaching aquatic plants below.
When these large blooms of algae die, they sink and are decomposed by bacteria, a process that consumes large amounts of dissolved oxygen. This oxygen depletion creates hypoxic areas, known as “dead zones,” where fish and other aquatic organisms cannot survive. There are now more than 500 of these dead zones in coastal areas worldwide, including a significant one in the Gulf of Mexico fed by nutrient runoff from the Mississippi River basin.
Industrial processes and everyday products are another source of freshwater contamination, introducing harmful chemicals into aquatic environments. Industrial wastewater can contain heavy metals like mercury and lead, which are toxic and persist in the environment. Similarly, persistent organic pollutants (POPs), chemicals used in pesticides and industrial applications, resist breaking down and can travel long distances from their source.
These chemical pollutants accumulate in the tissues of aquatic organisms, a process known as bioaccumulation. As larger animals consume contaminated smaller organisms, the concentration of these toxins increases up the food chain. Pharmaceuticals and personal care products also contribute to chemical pollution, as they are not fully removed by wastewater treatment plants and end up in rivers and lakes.
The pollution of freshwater systems by plastic is a growing concern. Large plastic items, such as bottles and bags, can harm wildlife through entanglement or ingestion. Over time, these larger pieces break down into smaller fragments known as microplastics, which are less than five millimeters in size. These particles originate from the degradation of larger plastic debris or are manufactured for use in products like cosmetics.
Microplastics are now found in freshwater ecosystems globally, from rivers to remote lakes. Because of their small size, they are easily ingested by a wide range of aquatic organisms, including fish, invertebrates, and plankton. This ingestion can cause physical harm and introduce toxic chemicals that may have adsorbed to the plastic’s surface into the food web.
Alteration of Natural Water Systems
Human demand for water leads to the manipulation of natural systems through diversion and over-extraction. Water is rerouted from rivers and pumped from underground aquifers to support agriculture, industries, and growing urban populations. Agriculture alone is responsible for approximately 70% of global freshwater use, with much of it lost to inefficient irrigation systems. This withdrawal can have consequences such as the shrinking of water bodies like the Aral Sea in Central Asia.
The depletion of water is not limited to surface sources; groundwater is also extracted at unsustainable rates. When water is pumped from aquifers faster than it can be replenished by rainfall, water tables drop and wells can run dry. This over-extraction can also lead to land subsidence, where the ground level sinks. This process can damage infrastructure and permanently reduce the aquifer’s ability to store water.
The construction of structures like dams and levees alters river ecosystems. Dams are built to generate hydroelectric power, store water for irrigation, and control floods, but they also create disruptions. A primary impact is river fragmentation, which blocks the migratory paths of fish species like salmon, preventing them from reaching their spawning grounds. This interruption of life cycles can lead to declines in fish populations.
Dams also change the river’s flow, temperature, and sediment transport. The water held in a reservoir becomes stagnant and warmer, which can harm species adapted to cooler, flowing water. Downstream, the river is often deprived of sediments that would naturally replenish habitats like floodplains. Levees and channelization disconnect the river from its floodplain, eliminating important seasonal habitats for many species.
Impact of Invasive Species
Human activities have facilitated the introduction of non-native species into freshwater ecosystems, often with disruptive consequences. These invasive species can be transported through pathways like the ballast water of ships or accidental releases from aquariums. Lacking natural predators, these species can outcompete native organisms for food and habitat, leading to a decline in local biodiversity and altering the food web.
The zebra mussel provides an example of an invasive species in North America. Native to the lakes of southeast Russia, these mussels were introduced to the Great Lakes in the 1980s through ballast water from a commercial ship. They have since spread, forming dense colonies that clog water intake pipes for cities and industrial facilities, causing economic damage. Ecologically, zebra mussels filter vast amounts of plankton from the water, reducing the food source for native fish and other organisms.
Another example is the water hyacinth, a floating plant native to South America. Introduced to other parts of the world as an ornamental plant, it has become an invasive weed in many tropical and subtropical regions. Water hyacinth grows quickly, forming thick mats that can completely cover the surface of lakes and rivers. These mats block sunlight and deplete oxygen levels, which harms fish and other aquatic life.
Influence of a Changing Climate
Global climate change introduces pressures on freshwater systems, compounding the effects of direct pollution and physical alterations. One impact is the warming of water temperatures. As global air temperatures rise, so does the temperature of rivers and lakes, which directly affects aquatic life. Warmer water holds less dissolved oxygen, creating stressful conditions for many fish species that depend on oxygen to survive.
Climate change is also intensifying the global water cycle, leading to more extreme weather patterns. Some regions are experiencing more frequent droughts, which reduces water levels in rivers, lakes, and reservoirs, threatening ecosystems and human water supplies. In other areas, climate change is causing an increase in heavy rainfall events. These downpours can lead to floods that erode riverbanks and wash pollutants into waterways.
The world’s glaciers, which act as natural reservoirs of freshwater, are melting at an accelerated rate due to rising temperatures. In the short term, this glacial melt can increase water flow in rivers, a phenomenon referred to as “peak water.” As glaciers continue to shrink and eventually disappear, this source of meltwater will be lost. This poses a long-term threat to the water supply for people and ecosystems that rely on glacier-fed rivers.