Freshwater ecosystems are a type of Earth's aquatic ecosystems that include living things in bodies of water such as lakes, ponds, rivers, streams, springs, bogs, and wetlands. These ecosystems differ from marine ecosystems, which have much saltier water. Freshwater habitats can be grouped based on factors like temperature, how much light reaches the water, the amount of nutrients, and the presence of plants.

There are three main types of freshwater ecosystems: lentic (slow-moving water, such as pools, ponds, and lakes), lotic (faster-moving water, like streams and rivers), and wetlands (areas where soil is wet or covered with water for part of the year). Freshwater ecosystems are home to 41% of the world's known fish species.

Over time, freshwater ecosystems have changed, which has affected their features. Early efforts to study and monitor these ecosystems began because of dangers to human health, such as cholera outbreaks caused by polluted water. At first, monitoring focused on chemicals, then bacteria, and later on algae, fungi, and protozoa. A newer method involves counting different groups of organisms, such as macroinvertebrates, plants, and fish, and measuring the conditions of the water where they live.

Threats to freshwater biodiversity include overuse of resources, water pollution, changes in water flow, damage to habitats, and the spread of non-native species. Climate change is adding more stress to these ecosystems because water temperatures have risen by about 1°C, and ice coverage has decreased significantly, causing further challenges for ecosystems.

Types

There are three main types of freshwater ecosystems: lentic (slow-moving water, such as pools, ponds, and lakes), lotic (faster-moving water, like streams and rivers), and wetlands (areas where the soil is soaked or covered with water for part of the year). The study of freshwater ecosystems is called limnology, and it includes the study of freshwater biology.

River ecosystems are flowing waters that move across the land. They include interactions between living things, such as plants, animals, and tiny organisms, and nonliving factors like physical and chemical conditions. Rivers are part of larger networks called watersheds or catchments, where small streams flow into larger rivers. The major areas within rivers are shaped by the slope of the riverbed or the speed of the water. Faster-moving water usually has more dissolved oxygen, which supports more life than slower-moving water. These differences help classify rivers as upland (steep, fast-moving) or lowland (gentler, slower-moving).

In streams within riparian forests (areas near rivers with trees), the main food source comes from the trees. However, wider streams or those without tree cover rely more on algae for food. Fish that travel from the ocean to rivers to spawn also add nutrients. Threats to rivers include losing water, building dams, chemical pollution, and introducing non-native species. Dams can harm ecosystems downstream by reducing spring floods, which harms wetlands, and by trapping sediment, which can destroy wetlands near river deltas.

Rivers are examples of lotic ecosystems. The term "lotic" comes from the Latin word lotus, meaning "washed." Lotic systems range from tiny springs to large rivers. This information applies to all lotic systems, including streams and springs. Lotic ecosystems differ from lentic ecosystems, which involve still water like lakes, ponds, and wetlands. The word "lentic" comes from the Latin lenis, meaning "calm." Together, lotic and lentic systems form the broader study of freshwater or aquatic ecology. These two types of ecosystems do not have a clear boundary between them.

A wetland is a special type of ecosystem where the ground is covered with water, either all the time or seasonally. Flooding in wetlands creates oxygen-poor conditions in the soil. Wetlands are areas between water and dry land, and their plants have roots that grow in waterlogged, oxygen-poor soil. Wetlands are among the most diverse ecosystems, supporting many plants and animals. Plants in wetlands help improve water quality by removing extra nutrients like nitrates and phosphorus.

Wetlands are found on every continent except Antarctica. Their water can be freshwater, brackish (a mix of salt and fresh water), or saltwater. Wetlands are grouped based on the plants they support and their water source. For example, marshes have plants like reeds and cattails that grow above the water. Swamps have woody plants like trees and shrubs, though some swamps in Europe are dominated by reeds. Mangrove forests have trees and plants that can survive in salty water.

Examples of wetlands based on their water source include tidal wetlands (water from ocean tides), estuaries (a mix of river and tidal water), floodplains (water from overflowing rivers or lakes), and bogs or vernal ponds (water from rain or melting snow, sometimes from underground springs). Some of the world's largest wetlands include the Amazon River basin, the West Siberian Plain, the Pantanal in South America, and the Sundarbans in the Ganges-Brahmaputra delta.

Threats

Freshwater biodiversity is threatened by five major factors: overuse of resources, water pollution, changes to natural water flow, destruction or damage to habitats, and the arrival of non-native species. Recent declines in species populations are often linked to soil and sand buildup, broken river systems, harmful chemicals, dams, and invasive species. Common chemical problems in freshwater environments include acid levels rising, too many nutrients causing algal growth, and contamination from copper and pesticides.

Freshwater ecosystems are facing serious challenges. The World Wide Fund for Nature reports that the number of freshwater vertebrates dropped by 83% between 1970 and 2014. These declines are happening faster than similar drops in ocean or land environments. Key causes of these declines include:

• Rapid changes in the climate
• Trade of wildlife online and introduction of invasive species
• Spread of diseases
• Growth of harmful algae
• Dams built for hydropower, which split rivers
• New types of pollutants, such as hormones
• Tiny materials made in labs
• Tiny plastic particles in water
• Too much light or noise
• Salt levels increasing in coastal freshwater due to rising sea levels
• Calcium levels dropping below what some species need
• The combined effects of these problems

Invasive plants and animals harm freshwater ecosystems by competing with native species and changing water conditions. These species are especially harmful to areas with endangered animals. For example, Asian carp compete with paddlefish in the Mississippi River. Common ways invasive species enter freshwater systems include being released from aquariums, introduced for fishing, or used as food.

Over 123 freshwater animal species in North America have gone extinct since 1900. In North America, about 48.5% of mussels, 22.8% of snails, 32.7% of crayfish, 25.9% of amphibians, and 21.2% of fish are endangered or at risk. Extinction rates may rise sharply in the future due to invasive species, loss of key species, and species that can no longer reproduce. Even with low estimates, freshwater fish in North America are going extinct at a rate 877 times faster than the usual background rate. Projected extinction rates for freshwater animals are about five times higher than for land animals and similar to those in rainforests. Because of this crisis, scientists and experts worldwide created an Emergency Action plan to help restore freshwater biodiversity.

Current methods to monitor freshwater health focus on the types of organisms present. Some programs also measure how much oxygen is used by organisms, how much oxygen is in the water, and how much oxygen is needed by sediments. Macroinvertebrates (small animals without backbones) are often studied because they are easy to collect, sensitive to many stressors, and important to the ecosystem. Algae, especially diatoms, are also studied because they are diverse, easy to collect, and sensitive to changes. Algae grow quickly, so their presence can show fast changes in water conditions.

In addition to studying species, scientists test how stressors affect organisms by observing changes in behavior, growth, reproduction, or survival. Results from tests on single species in labs may not match real-world conditions in complex ecosystems.

Reference sites are used to compare the health of freshwater ecosystems. These sites are chosen based on their distance from human activity or by studying preserved materials like diatom remains, pollen, insect parts, and fish scales to understand past conditions. These historical comparisons are easier in still water than in flowing water because sediments in still water preserve biological materials better.

Climate change makes other stressors worse for many fish, invertebrates, plants, and other organisms. It is raising water temperatures and worsening problems like changes in riverbed material, oxygen levels, and other changes that affect the entire ecosystem. Water temperatures have already risen by about 1°C, and less ice cover has caused stress in ecosystems.