What Lives in Freshwater? From Microbes to Mammals

Freshwater lakes, rivers, streams, and wetlands cover a small fraction of Earth’s surface but support a staggering variety of life, from viruses smaller than a wavelength of light to half-ton fish and semi-aquatic mammals. The diversity is easy to underestimate because so much of it is invisible or hidden beneath murky surfaces. What ties these organisms together is a shared challenge: surviving in water that contains very little dissolved salt, which creates specific physiological demands that every freshwater resident, from a single-celled bacterium to a river dolphin, has to solve in its own way.

The Invisible Majority

Most of the living biomass in any freshwater body is made up of organisms you will never see without a microscope. Bacteria cycle nutrients, break down dead organic matter, and form the base of microbial food webs. Some freshwater bacteria are closely related to marine groups that scientists long assumed were confined to the ocean. Lake Baikal in Siberia, the deepest and largest freshwater lake by volume, hosts microbial communities that include relatives of the marine bacterium Pelagibacter and members of the Chloroflexi group, organisms typically associated with saltwater environments. Its deep waters also support communities of microbes that derive energy from chemicals like ammonia and methane rather than sunlight.¹

Viruses are even more abundant than bacteria in most freshwater systems. Bacteriophages, the viruses that infect bacteria, are not simply killers. They also transfer genetic material between bacterial hosts, manipulate host metabolism, and redirect the flow of carbon and nutrients through microbial communities. In aquatic ecosystems, this “viral shunt” diverts organic carbon away from larger grazers and back into the dissolved pool, where bacteria can use it again.² The balance between viral killing and grazing by tiny predators like flagellates shifts with the seasons and can reshape the entire microbial food web over the course of weeks.

Fungi round out the microbial picture, though they tend to get overlooked in conversations about freshwater life. In streams, aquatic fungi called hyphomycetes are the dominant decomposers of fallen leaves and wood. They colonize submerged leaf litter, soften it, and enrich it with nutrients, making it palatable to larger invertebrates like shredding insects. Without fungi conditioning that leaf material, a huge portion of the energy entering headwater streams from surrounding forests would go largely unused by other organisms.³ This process is especially well studied in temperate and tropical low-order streams, where leaf inputs from overhanging trees are the primary energy source.⁴

Zooplankton and Microcrustaceans

Step up one rung from microbes and you find the tiny animals that graze on bacteria and algae: the zooplankton. Among the most studied freshwater zooplankton are water fleas in the genus Daphnia, translucent crustaceans typically a few millimeters long. Daphnia are sometimes called keystone species because of how effectively they filter algae from the water column and how central they are to the diets of small fish.

One of the more fascinating behaviors in freshwater zooplankton is diel vertical migration. Each day, Daphnia and other zooplankton move between warm surface waters and cooler, darker depths, timing their trips to avoid visual predators like fish. They rise toward the surface at night to feed on algae and descend before dawn. The behavior is flexible, shaped by trade-offs between growth, reproduction, and predator avoidance, and influenced by factors like light intensity, chemical cues from predators, and temperature.⁵ Anthropogenic stressors like artificial light at night and warming water temperatures can disrupt this migration, with cascading effects on algal blooms and water clarity.

Insects and Bottom-Dwelling Invertebrates

Freshwater insects are enormously diverse, and most people encounter them only when an adult mayfly or caddisfly lands on a porch light. But the larval stages of these insects spend months or years underwater, clinging to rocks, burrowing in sediment, or building protective cases from tiny pebbles and silk. The major groups include mayflies (Ephemeroptera), stoneflies (Plecoptera), and caddisflies (Trichoptera), often referred to collectively by ecologists as EPT taxa.

These insects serve as reliable indicators of water quality. Mayflies and stoneflies are generally sensitive to pollution, so their presence in high numbers signals clean water. Conversely, larvae like Chironomus midges and rat-tailed maggots (Eristalis) tolerate polluted, low-oxygen conditions, so finding them in abundance can indicate degraded water. Biomonitoring studies use indices based on the composition of these insect communities to assess the health of rivers and lakes. In a study of a Himalayan river, upstream sites with high proportions of EPT taxa scored well on multiple water-quality indices, while downstream sites showed declining scores as pollution increased.⁶ Similarly, high densities of pollution-tolerant species like Chironomus in an Indian lake confirmed the water body’s degraded condition.⁷

Freshwater Mussels as Living Filters

Freshwater mussels are among the most underappreciated animals in rivers and lakes. Buried partway in sediment with their shells slightly agape, they pull water through their bodies and filter out suspended particles, algae, and bacteria. This filtration removes excess nutrients and helps prevent the runaway algal growth known as eutrophication. Beyond water quality, mussel beds create physical habitat structure on the river bottom, offering shelter and food resources for smaller invertebrates and fish.⁸

Freshwater mussels are also among the most endangered groups of animals on Earth. Many species have complex life cycles that depend on specific host fish: mussel larvae called glochidia attach to fish gills, grow for weeks, then drop off to begin their sedentary life. When dam construction, pollution, or invasive species eliminate the right host fish, mussel populations collapse. The irony is that the animals providing the most direct water-cleaning service are often the first to disappear when a waterway degrades.

Fish That Stay and Fish That Travel

Fish are the most visible residents of fresh water, and they range from tiny killifish in seasonal puddles to massive sturgeons and catfish in deep rivers. A key challenge for any freshwater fish is osmoregulation: because their body fluids are saltier than the surrounding water, water constantly flows in through their gills and skin, while salts leak out. Freshwater fish compensate by producing large volumes of dilute urine and actively absorbing ions like sodium and chloride through specialized cells in their gills.⁹ This basic machinery is shared, in modified forms, by crustaceans, aquatic insects, and mollusks living in the same water.

Some fish refuse to pick a side. Diadromous species move between fresh and salt water during their life cycles. Salmon famously hatch in freshwater streams, migrate to the ocean to grow, and return to fresh water to spawn. Eels do the reverse, spawning at sea and spending their growth phase in rivers and lakes. But the old textbook categories can be misleading. Research on tropical eels in the genus Anguilla has found that their migration into fresh water is not obligatory at all. Many individuals spend their entire growth phase in brackish or even marine water, and the choice seems to depend on local conditions and competition rather than a rigid biological program.¹⁰ The transition between salt and fresh water involves hormonally driven changes in gill cells, increased drinking to compensate for water loss in salt water, and adjustments to kidney output.¹¹

Amphibians and Reptiles

Amphibians have the most intimate relationship with fresh water of any vertebrate group. Frogs, salamanders, and newts typically depend on ponds or streams for breeding, and many spend their larval stages entirely underwater, breathing through gills before metamorphosing into air-breathing adults. Even as adults, amphibians rely heavily on their thin, permeable skin for gas exchange, a strategy called cutaneous respiration. That same permeability makes them extremely vulnerable to drying out. Amphibians face some of the highest water-loss costs of any terrestrial vertebrate precisely because the skin that lets them breathe also lets moisture escape.¹² This is why you rarely find frogs far from moisture and why amphibian populations are so sensitive to habitat drying and climate shifts.

Freshwater reptiles include turtles, water snakes, and, in tropical regions, crocodylians. Crocodiles and alligators are often described as apex predators and keystone species in freshwater systems, and popular accounts frequently credit them with engineering their habitat and transporting nutrients between land and water. The actual evidence, though, is thinner than you might expect. A large-scale review found that most claims about crocodylians’ ecological importance as apex predators, ecosystem engineers, or nutrient transporters are based on anecdotal observations rather than rigorous study.¹³ That does not mean crocodylians are unimportant, only that the science has not yet caught up to the intuition.

Waterbirds and Wetland Chemistry

Herons, ducks, geese, cormorants, and other waterbirds are not just visitors to freshwater habitats. They actively reshape wetland chemistry. Birds deposit nitrogen- and phosphorus-rich guano into the water, and in wetlands with large bird populations, this nutrient input can significantly alter algal communities and nutrient cycling.¹⁴ The effect is not always welcome: excessive bird-derived nutrients can push a wetland toward algal blooms and oxygen depletion, the same problems caused by agricultural runoff.

Waterbirds also function as biological connectors between habitats. As they fly between feeding and roosting sites, they carry plant seeds, invertebrate eggs, pathogens, and even contaminants on their feathers, feet, and in their guts. Tracking studies have shown that generalist species like gulls create functional connectivity networks linking terrestrial sites such as agricultural fields and landfills with wetlands and other aquatic habitats.¹⁵ This means that whatever is at one site, from weed seeds to antibiotic-resistant bacteria, can end up at another.

Mammals in Fresh Water

When people think of aquatic mammals, whales and dolphins come to mind first, but several mammal lineages have adapted to freshwater life. River dolphins survive in the Amazon, Ganges, and Yangtze (the baiji is now functionally extinct). Manatees graze on aquatic vegetation in warm rivers and springs. Hippos spend their days submerged in African rivers and lakes, emerging at night to graze on land. Beavers reshape entire landscapes by building dams that create new ponds and wetlands.

Semi-aquatic mammals face a particular thermal challenge. Water conducts heat away from the body far more efficiently than air, so animals that spend long hours submerged need ways to stay warm. Otters, for instance, have exceptionally dense fur that traps an insulating layer of air against the skin. Research on the Eurasian otter suggests that diet plays a role in thermoregulation as well. Otters eat a diet heavy in fish, which is extremely high in protein. Protein digestion generates more metabolic heat than fat or carbohydrate digestion, and the extra warmth from this process may help compensate for the constant heat loss of being in cold water. The otter’s estimated intake runs around 61% protein and 39% lipid by energy, a ratio higher in protein than typical hypercarnivores, and the protein share tends to increase as water temperatures drop.¹⁶ In other words, eating a lot of fish may be part of what makes being an otter physically possible.

The anatomical adaptations across aquatic and semi-aquatic mammals are remarkably varied, spanning modifications to skin, limbs, eyes, ears, and digestive systems. Fossil and living species alike show convergent solutions to the demands of underwater life, with animals as different as manatees, seals, and pygmy hippos arriving at similar body plans through independent evolutionary paths.¹⁷

Aquatic Plants as the Foundation

No survey of freshwater life is complete without plants. Submerged, floating, and emergent aquatic plants are primary producers that convert sunlight into the chemical energy that feeds herbivores and, through them, the rest of the food web. Their root systems stabilize sediments and reduce turbidity, while their leaves and stems provide shelter for fish, invertebrates, and amphibians. Aquatic plants also take up nutrients like nitrogen and phosphorus from the water, acting as natural buffers against nutrient overload, and they sequester carbon in their tissues and in the sediments they stabilize.

The loss of aquatic vegetation from a lake or river tends to trigger a cascade of problems: cloudier water, more algal blooms, less habitat for fish, and accelerated bank erosion. Restoration projects often focus on replanting native macrophytes for exactly this reason.

Parasites That Depend on Fresh Water

Fresh water is not just a habitat for free-living organisms. It is also a critical stage in the life cycles of many parasites that infect wildlife and people. Trematodes, or flukes, are a prominent example. Their life cycles typically involve freshwater snails as intermediate hosts. In a xenomonitoring study in Africa, researchers found that about one in five Biomphalaria pfeifferi snails were infected with schistosome parasites based on visual inspection, but molecular methods revealed the true infection rate was closer to 45%. Individual infected snails could release thousands of parasitic larvae (cercariae) per day into the water.¹⁸

Wildlife is heavily affected too. In wild ruminants like deer and antelopes, common freshwater-transmitted trematode infections include fascioliasis and schistosomiasis, while wild carnivores can pick up lung flukes from contaminated prey. These parasitic diseases tend to peak after monsoon seasons, when standing water and snail populations expand.¹⁹ Freshwater habitats, in other words, are not just homes for animals but transmission hubs for the parasites that exploit them.

Freshwater Megafauna Under Pressure

The largest animals in fresh water are in serious trouble. Freshwater megafauna, defined as freshwater animals weighing 30 kilograms or more, include giant catfish, sturgeons, river dolphins, crocodylians, and large turtles. Thirty-four of these species are listed as critically endangered by the IUCN, and assessments remain incomplete for about a quarter of all freshwater megafauna species.²⁰ These animals tend to be slow to mature, produce few offspring, and need large, connected stretches of river, making them especially vulnerable to dams, overfishing, and habitat fragmentation. The Mekong giant catfish, the Chinese paddlefish (now likely extinct), and the Ganges river shark are emblematic of a broader crisis affecting large freshwater animals on every continent.

Detecting What You Cannot See

Given how many freshwater organisms are small, cryptic, or nocturnal, counting them by traditional methods like netting or visual surveys inevitably misses species. Environmental DNA, or eDNA, offers a different approach. Every organism sheds DNA into its environment through skin cells, mucus, waste, and decomposition. By filtering water samples and sequencing the DNA fragments they contain, researchers can identify which species are present without ever seeing or catching them.

In a study of two Sicilian lakes, eDNA analysis identified four classes of vertebrates across 14 orders and 16 families, providing a broad snapshot of biodiversity from a few liters of water.²¹ In a tropical rainforest in Madagascar, eDNA sampling from streams detected significantly more species of birds, mammals, amphibians, reptiles, and fish than conventional surveys of the same sites.²² The technique is not perfect. It cannot tell you how many individuals are present, and DNA degrades quickly in warm or acidic water, so it works best as a complement to traditional methods rather than a replacement.²³ Still, for a habitat where so much life is hidden, having a tool that detects organisms from traces they leave in the water is transformative.

When New Species Move In

Freshwater ecosystems are particularly vulnerable to invasive species because rivers and lakes are isolated from one another, meaning native communities evolved without exposure to many potential competitors and predators. When a non-native species arrives, whether through accidental introduction, deliberate stocking, or canal connections, the effects can be severe and long-lasting.

An analysis of invasive species impacts in Canada found that freshwater systems had roughly as many recorded ecological impacts as terrestrial systems, around 1,200 documented cases, despite covering a far smaller area. Among the most damaging freshwater invaders were zebra mussels and rainbow trout, both recognized globally as highly harmful invasive species.²⁴ Zebra mussels, native to Eastern Europe, encrust water intake pipes, smother native mussel beds, and filter so aggressively that they strip the plankton from large stretches of water, fundamentally altering food webs. Rainbow trout, prized by anglers, outcompete and hybridize with native trout and char species in streams where they have been introduced. The fact that freshwater habitats are fragmented by nature, split into discrete lakes, river basins, and wetlands, means that once an invader establishes itself in one water body, native species have few places to retreat.