Freshwater ecosystems support the lives of millions of species and, incidentally, provide us with drinking water. drinking waterThey nourish us and mitigate floods and droughts. In rivers, lakes, lagoons, ponds, and wetlands, a network of physical, chemical, and biological processes unfolds that, although sometimes unnoticed, are the foundation of human well-being and extraordinary biodiversity. In these bodies of water with low salinity, the variety of habitats and life forms is enormous, from the rapids of a torrent to the stillness of a deep lake, including marshes and swamps that filter water and store carbon. Collectively, they harbor a very significant portion of the planet's biodiversity; in fact, It is estimated that approximately 41% of fish species live in inland waters..
What is a freshwater ecosystem?
When we talk about freshwater ecosystems, we are referring to aquatic environments with low salinityThese include rivers, streams, springs, lakes, lagoons, ponds, and a wide variety of wetlands. They differ from marine environments in their salt concentration and the ecological dynamics of their communities, although in transition zones (estuaries, marshes) fresh and salt water can mix, giving rise to salinity gradients and very particular communities.
Their ecological and social importance is enormous: they provide water, food, and energy; they protect against extreme events and store large amounts of carbon, especially in peatlands and marshes. Furthermore, They are home to an essential part of the planet's flora and fauna, with fish, amphibians, macroinvertebrates and communities of algae and aquatic plants that interact with the physical (light, temperature, flow rate) and chemical (nutrients, pH, oxygen) factors of the environment.
The following basic terminology is often used in the study of these environments: sweet water for the medium, lotic ecosystem for moving water, lentic ecosystem for stagnant or slow-moving water, and limnology as the scientific discipline that studies them. This classification helps to understand processes and manage resources more effectively.
Main types: lotic, lentic and wetlands
The most widespread way of classifying freshwater ecosystems is by the water movementLotic systems include rivers, streams, and torrents, where flow is predominant; lentic systems encompass lakes, lagoons, reservoirs, and ponds, with still or very slow-moving water; and wetlands include areas that are flooded or saturated for part of the year, such as peat bogs, marshes, and some swamps. Although some wetlands may be brackish, many are strictly freshwater and perform critical ecological functions.
In more detail, there are common terms that are useful to have on hand. A swamp It is usually described as a wetland with woody vegetation and, in many cases, with some human intervention (for example, through water regulation in reservoirs or areas flooded by infrastructure). lagoon It is a body of fresh water smaller than a lake and, generally, shallower. pond It is a body of water with no appreciable flow, where the water remains in the same place And thermal stratification can be minimal or nonexistent. And the marshes They are wetlands (often brackish or salty water) in transition zones, closely linked to rivers and coasts.
Aquatic ecosystems can also be classified by the relationship of organisms to depth and water column. The community lives at the bottom. benthic (algae, invertebrates, detritivores), in the column move the nektonic (fish and other animals with active locomotion), already drifting they travel planktonic (phytoplankton and zooplankton) that are carried by the currents. On the surface are found the neustonicorganisms that float or move on the surface film of the water.
Rivers and torrents: the fluvial continuum
Rivers are probably the most visible face of freshwater. They transport water, nutrients, and sediments from their headwaters to their mouths, and they shape landscapes and cultures. Their ecological structure changes gradually from source to mouth, a phenomenon known as continuous riverThis concept explains why organisms and processes reorganize themselves as the channel increases in size and the flow becomes more stable.
In the headersNarrow, shaded streams are often heterotrophic: the community depends heavily on allochthonous organic matter carried from the watershed (leaf litter, branches). In the middle courseWith a less steep bed and more light, primary production increases and diversity usually reaches its peak. In the low courseThe waters are slower and turbid; less light penetration limits photosynthesis and changes the species composition towards communities better adapted to muddy bottoms and low oxygen levels.
This natural process is disrupted by dams, piping and extraction Water pollution disrupts connectivity, reduces ecological flows, and alters sediment pulses. This is compounded by diffuse and point-source pollution, as well as the extraction of sand and gravel from the riverbed. The result can be habitat loss, the blocking of migratory routes, and a substantial decline in river biodiversity.
Lakes and lagoons: zones and trophic state
In lentic systems, depth, water transparency, and nutrient availability are very important. Three main types are commonly distinguished. Zones: the littoral zone (next to the shore, with abundant vegetation and refuge for fry and invertebrates), the limnetic zone (open waters, where plankton and pelagic fish dominate) and the deep zone (with little or no light, dominance of decomposition processes and, often, limited oxygen in the hypolimnion).
Another classic way to characterize lakes and lagoons is by their trophic state. oligotrophic They have low nutrient concentrations, clear waters, and generally high water quality; The eutrophic They contain more nutrients, higher productivity, and, if the load increases significantly, a risk of algal blooms and oxygen deficiency. Eutrophication processes are driven by external inputs of nitrogen and phosphorus and by local pressures such as urban and agricultural runoff.
Algal communities, especially the diatomsThey are excellent indicators of water quality and changes in environmental conditions, as they grow rapidly and respond sensitively to light, nutrients, and pH. This accelerated response allows for the detection of environmental changes in short periods, which is key for adaptive management.
Wetlands, swamps and marshes
In everyday terminology, a swamp It may be linked to retained bodies of water (sometimes due to human activity) and with dominant vegetation; a lagoon is smaller and often shallow; and a pond It is a system with no appreciable current. marshes They are located in transition zones and usually have variable salinity; although strictly speaking they are not always freshwater, they are part of the river-coastal continuum and are crucial as buffers against storms and for the breeding of fish and aquatic birds.
The problem is that wetlands have been massively drained and transformed for agricultural and urban uses. The data is compelling: historical disappearance is around 87% globally, and More than half has been lost since 1900Hydrological restoration (for example, by returning the water table to peatlands) is one of the most effective tools for restoring ecological functions and, incidentally, preventing stored carbon from ending up in the atmosphere.
Threats to freshwater biodiversity
Five major pressures explain the accelerated decline of continental species: overexploitation (fishing and extraction of resources beyond their renewal capacity), contamination (from wastewater and plastics to industrial and agricultural runoff), flow modification (dams, diversions, flows without natural regime), habitat destruction or degradation (channeling, occupation of riverbanks) and invasion of exotic species that compete with or prey on the local biota. These are compounded by intensive water extraction for irrigation, energy, and industry, and the exploitation of aggregates (sand and gravel) that alters the riverbed.
Among the most common chemical pressures are the acidification of bodies of water, the eutrophication due to excess nutrients and pollution copper and pesticidesClimate change introduces synergies that are difficult to predict: heat wavesDroughts, more intense storms, and changes in flow regimes amplify the effects of other stressors and put freshwater and marine species alike at risk.
The figures for losses are alarming in some regions. In North America, for example, there have been extinction of more than 123 species of freshwater fauna Since 1900, an estimated 48,5% of mussels, 22,8% of gastropods, 32,7% of crayfish, 25,9% of amphibians, and 21,2% of fish are threatened or endangered. Extinction rates for freshwater fish are up to 877 times higher than the background rate (1 per 3.000.000 years), and projections for freshwater animals are about five times higher than those for terrestrial fauna, comparable to those of tropical rainforest communities.
This situation has prompted the scientific community and managers to propose a emergency action plan focused on restoring freshwater biodiversity: reducing pressures, reconnecting rivers, protecting and restoring wetlands, and ensuring minimum environmental flows that maintain ecological processes.
How to assess the health of freshwater ecosystems
Biomonitoring typically focuses on the community structureespecially in sensitive and diverse groups that adapt to changing environmental conditions over time. Benthic macroinvertebrates (caddisflies, mayflies, stoneflies, among others) are a classic example due to their taxonomic diversity, ease of sampling, and response to multiple stressors. Algal communities are also monitored, with particular attention to diatoms, whose rapid life cycle reflects fast-moving environmental changes.
In addition to biological structure, functional indicators of ecosystem metabolism are measured, such as biochemical oxygen demand (BOD), oxygen demand in sediments (DOS) and the dissolved oxygenThese parameters allow us to infer organic loads, nutrient inputs and hypoxia risks, which are fundamental for diagnosing eutrophication and guiding sanitation measures.
In ecotoxicology, experimental trials help estimate the effect of specific stressors by measuring changes in behavior, growth rates, reproduction, or mortality. It is important to remember that the results with a single species under controlled conditions They do not always reflect what happens in multi-species natural communities, so they are complemented by fieldwork and integrative approaches.
To contextualize the data, we use Reference sites with minimal human disturbance or, when that is not possible, to temporal references reconstructed using preserved bioindicators (diatom valves, macrophyte pollen, insect chitin, fish scales) that allow us to infer conditions prior to major alterations. This reconstruction is generally more viable in lentic systems than in lotic ones, since the stable sediments Lakes and lagoons better preserve the biological record.
Protection and restoration measures
Protecting and restoring freshwater ecosystems requires addressing the problem at its source. Proper treatment of sewage water Pre-discharge measures and the reduction of diffuse pollutants from the field (nutrients, pesticides) alleviate eutrophication and improve oxygen levels. Controlling fishing and mining in rivers and deltas reduces pressure on sensitive habitats and key species, and helps to slow degradation.
In rivers, the removal of obsolete dams or their redesign with wildlife crossings Environmental flow regimes restore connectivity and ecological functions. Water extraction management ensures minimum flows compatible with aquatic life and ecosystem services. In wetlands and peatlands, restoring natural water levels allows them to recover their capacity to sequester carbon and buffer floods, with climate and biodiversity benefits.
These actions work best when supported by a basin planningwith clear objectives, continuous biomonitoring, and social participation. Coordination between administrations, irrigators, companies, and citizens, combined with quality dataIt makes it easier to prioritize actions where they are most needed and to evaluate their effectiveness in the medium and long term.
Characteristic fauna and examples
Freshwater ecosystems are home to a wide variety of wildlife. Among the fish Among the well-known and popular freshwater fish in the aquarium hobby are guppies, platies, mollies, bettas, gouramis, ramirezi, discus, and the so-called angelfish (there are freshwater species of the genus Pterophyllum). amphibiansFrogs, toads, and salamanders are common in riverbanks, ponds, and floodplains, where they need water for reproduction and the development of their larvae.
In the invertebratesDragonflies and their nymphs are excellent indicators in rivers and lakes; crayfish are part of benthic communities and, in many places, suffer competition from exotic species. Oysters, which are mostly marine or estuarine (more often brackish), and earthworms, which live in adjacent moist soils, are also mentioned; this type of citation highlights the close relationship between freshwater and aquatic life. transitional habitatswhere borders are not strict and communities share environmental gradients.
In summary of everyday terms used in environmental education and educational games: a swamp It could be a wetland with some human intervention, a lagoon It's smaller than a lake, a pond It is a still water system and the marshes They are transitional ecosystems with saline influence. Although the ocean It is another world (marine and the largest on the planet), the comparison serves to place the mosaic of aquatic environments on a gradient from freshwater to saltwater.
Looking at the whole picture, the combination of good sanitation, hydrological restoration, pressure control, and scientific monitoring allows for the recovery of the functionality of rivers, lakes, and wetlands. There are no magic formulas, but there is a growing consensus and proven tools: Reconnect, decontaminate, and protect Freshwater ecosystems are an investment with extraordinary ecological and social returns.
