Freshwater habitats and their surrounding wetlands are some of the most diverse ecosystems on the planet. Relationships between flora, fauna, and abiotic components are largely governed by the presence of water and oxygen. In streams and rivers, which are flowing systems with energetic and oxygen-rich waters, nutrient cycles help ensure that the needs of all animals – from microscopic creatures to massive apex predators – can be sustained.
Streams are rife with microhabitats that open up a wealth of niches for animals to occupy. Their bottom substrates tend to have different physical parameters depending on their location relative to the shoreline, their depth, and the presence of vegetation and detritus. The animals that occupy each area are usually equipped with a set of adaptations for making the most out of their surroundings.
For example, the middle of a river, which tends to have stronger currents due to the lack of obstructions, is more suited to animals with a means to propel their bodies against currents. In contrast, shorelines with high structural diversity can accommodate semi-aquatic visitors that prefer a calmer dip as they search for food or a mate. Below are some ubiquitous and rare (yet highly valued) creatures found in simple to complex streams.
1) Aquatic & semi-aquatic insects
This group of animals contains some of the most abundant living components of freshwater systems. Their multiple life stages play key roles in freshwater food webs. The eggs of aquatic insects, for example, can serve as a vital source of proteins and lipids for opportunistic fish. The larvae and nymphs are favored by insectivorous amphibians and mammals. Adult insects, which may either maintain a fully submerged lifestyle or may simply visit their natal streams to breed, are hunted by many stream dwellers that lie close to the surface.
Some of these insects are capable of diving into strong currents and latching onto underwater plants. They may rely on particles of organic material or on microbial mats for sustenance. More often than not, their juveniles act as decomposers of decaying organic matter. Some of them have gills that allow them to obtain oxygen from water, whereas others have hairlike structures for trapping bubbles. Below are some fantastic examples of these animals:
- Mayflies (order Ephemeroptera)
- Caddisflies (order Trichoptera)
- Diving beetles (Dytiscidae)
- Water striders (Gerridae)
- Dragonfly larvae (Anisoptera)
- Damselfly larvae (Zygoptera)
- Water boatmen (Corixidae family)
- Backswimmers (Notonectidae family)
2) Migratory fish
In streams and rivers with well-defined connections to estuarine systems, a fair portion of the freshwater fish population tends to have migratory habits and may have certain life phases that require saltwater exposure. These are referred to as anadromous or catadromous species. Anadromous fish spend the majority of their lives in marine environments, only entering rivers and streams to spawn. Inversely, catadromous fish primarily live in freshwater systems. Collectively, these two types of fish may be grouped under a single classification – diadromous fish.
Migratory fish have special adaptations for tolerating significant changes in salinity. They play a major role in the exchange of nutrients between freshwater and saltwater environments. Unfortunately, many of their wild populations are threatened by the buildup of toxic nutrients along coastlines. The construction of dams and the alteration of waterways can also force them to seek other routes as they attempt to search for adequate spawning or feeding environments. The species below are known for their migratory runs in North America:
- Atlantic salmon (Salmo salar)
- American shad (Alosa sapidissima)
- Sea lamprey (Petromyzon marinus)
- Blueback herring (Alosa aestivalis)
- Atlantic sturgeon (Acipenser oxyrhynchus)
- American eel (Anguilla rostrata)
- Striped bass (Morone saxatilis)
3) Obligate freshwater fish
Obligate freshwater fish are species that live out their entire life cycle in waters with very low or absent salinity levels. These fish are not equipped with adaptations for surviving in saltwater, which can cause them to become severely dehydrated and eventually die. Found in all levels of the water column, they aren’t generally known for having lengthy or long-distance runs from one body of water to another. Their populations may be perpetually confined to specific stretches of streams and rivers.
Occasionally, however, obligate freshwater fish are forced to migrate or drift due to drastic changes in water levels, anthropogenic disturbances, and unusually strong currents. This category of stream and river dwellers is important for allocating nutrients to different levels of the water column all throughout the year. They may serve as apex predators or as prey to larger aquatic, semi-aquatic, or even terrestrial carnivores. As indicated below, these include not only bony fishes but also cartilaginous ones.
- Catfish (order Siluriformes)
- Bluegill (Lepomis macrochirus)
- Common perch (Perca fluviatilis)
- Largemouth bass (Micropterus salmoides)
- Freshwater stingrays (Potamotrygonidae family)
- True freshwater sharks (Glyphis)
- Walleye (Sander vitreus)
- Shiners (Notropis, Notemigonus crysoleucas, Lythrurus spp., Luxilus spp., etc.)
- Minnows (Pimephalus, Phoxinus spp.)
- Carp (Cyprinus carpio, Ctenopharyngodon idella, Hypophthalmichthys nobilis, Carassius carassius)
4) Amphibians
Now considered one of the most endangered groups of animals on the planet, amphibians are set apart by their ability to thrive in a wide range of habitats. Despite their sheer diversity, however, all of them require full exposure to freshwater at some point in their life cycle. These four-legged vertebrates must undergo metamorphosis to grow into their sexually mature, adult stages. Their reliance on clean freshwater environments has made them extremely vulnerable to anthropogenic disturbances and climate change.
Amphibian larvae, which usually come in the form of gilled tadpoles, are found in practically all types of freshwater bodies (even seasonal pools). In streams and rivers, they tend to gather along productive shorelines with vertical structures. They initially feed on microbes, algae, and particulate organic matter. Over time, some of them may eventually develop an insectivorous or carnivorous diet. Once they have metamorphosed into adults, they may either remain close to water or venture into terrestrial environments.
- North American bullfrog (Lithobates catesbeianus)
- Fire salamander (Salamandra salamandra)
- Common frog (Rana temporaria)
- Mediterranean tree frog (Hyla meridionalis)
- Common toad (Bufo bufo)
- Common midwife toad (Alytes obstetricans)
- Eastern newt (Notophthalmus viridescens)
5) Semi-aquatic reptiles
Freshwater reptiles in streams and rivers often look as though they’ve been around for millennia. Many of these are considered living fossils because their general adaptations and morphology have been conserved for thousands of years. Studies into their DNA actually show that they are quite well-evolved (i.e. they have dynamic evolutionary histories). Often feared, these are perhaps some of the most robust and hardy animals in their freshwater environments.
Freshwater reptiles may remain in a single body of freshwater or they may be able to migrate over considerable stretches of terrestrial environments as they search for mates, new territories, or food. They are often considered “semi-aquatic” animals as they do not have gills for breathing in water. Instead, they must constantly rise to the water’s surface for a breath of air. Masters of stealth, many of these river dwellers are apex predators.
- American alligator (Alligator mississippiensis)
- Alligator snapping turtle (Macrochelys temminckii)
- Painted turtle (Chrysemys picta)
- Common watersnake (Nerodia sipedon)
- Nile crocodile (Crocodylus niloticus)
- Flattened musk turtle (Sternotherus depressus)
- Red-eared slider (Trachemys scripta elegans)
- Diamondback watersnake (Nerodia rhombifer)
- Banded watersnake (Nerodia fasciata)
6) Waterfowl
Waterfowl refers to a large and remarkably diverse group of birds that have adaptations for swimming and feeding in water. These birds spend the majority of their waking hours in freshwater systems. With webbed feet and specialized feathers, they are able to perform shallow, graceful dives. Most of these birds belong to the Anatidae family, which includes many modern types of ducks, swans, and geese. As they are naturals in water (and air – in the case of some migratory species), they may appear to move quite awkwardly on land.
Waterfowl are important components of rivers and streams. They may feed on a diversity of insects, amphibians, and fish. In water systems with controlled populations of these birds, their waste is a source of nutrients for microbes, decomposers, and plants. Unfortunately, in the absence of predators, they can become numerous enough to significantly damage water quality. Apex predators, such as crocodiles and large, carnivorous fish, are crucial for maintaining manageable waterfowl populations.
- Mallard duck (Anas platyrhynchos)
- Canada goose (Branta canadensis)
- Common merganser (Mergus merganser)
- Bufflehead (Bucephala albeola)
- Wood duck (Aix sponsa)
- Canvasback (Aythya valisineria)
- Greater scaup (Aythya marila)
- Trumpeter swan (Olor buccinator)
7) Aquatic & semi-aquatic mammals
Some mammals spend the majority of their time in freshwater systems. These may live partly or almost entirely in aquatic environments. Of course, those that have evolved to lose their limbs are unable to exit into outer wetlands or terrestrial habitats. Others, such as those belonging to the Castoridae or Mustelidae families, rely on their highly evolved limbs to build their watery nests and search for prey. These may migrate, via fragments of terrestrial habitats, to other freshwater systems.
Though aquatic mammals are experts in the water and are often able to swim as efficiently and expertly as fish, they must always return to the surface to breathe. These animals are equipped with adaptations for withstanding changes in water pressure as they dive. They may or may not have a generous coat of water-repellent fur. Some of them may venture into estuarine or even coastal areas in search of food.
- African manatee (Trichechus senegalensis)
- Amazon river dolphin (Inia geoffrensis)
- Giant otter (Pteronura brasiliensis)
- North American river otter (Lontra canadensis)
- Baikal seal (Pusa sibirica)
- Eurasian beaver (Castor fiber)
- American water shrew (Sorex palustris)
- Muskrat (Ondatra zibethicus)
- Hippopotamus (Hippopotamus amphibius)
8) Aquatic mollusks
Freshwater mollusks are found in stream systems all over the world. Known for their slow rate of movement, they occupy a wide range of microhabitats on streambeds and along the shoreline. Some of them must be perpetually latched onto a surface to move, whereas others have developed fascinating ways of propelling through the water. Moreover, some remain stationary whereas others are able to travel over surprisingly great distances!
- Freshwater limpets (Ancylidae)
- Freshwater leeches (Glossiphoniidae)
- Prosobranchiate snails (Neritidae)
- River snails (Viviparidae)
- Asiatic clam (Corbicula fluminea)
- Zebra mussel (Dreissena polymorpha)
9) Microscopic animals
A single drop of freshwater from an outdoor river or stream contains a surprising amount of minuscule animals. These microscopic creatures are often overlooked as they are invisible to the naked eye. However, they tend to be extremely important for the ecological workings of their microhabitats. Absolutely crucial to the health of natural freshwater systems, their absence could spell complete disaster. As they feed and reproduce, these tiny animals reallocate nutrients and convert them into tissues that benefit the development of their predators.
Microscopic animals can also serve as bioindicators of relatively clean aquatic systems. Many of them are extremely sensitive to ambient conditions. Spikes in water pH, temperature, or nutrient levels can decimate their populations. Conversely, these may also trigger infestations of pestilent species. Composed of just a few to more than a thousand cells, microscopic animals are unrivaled in their diversity and omnipresence.
- Rotifers
- Tardigrades
- Gastrotrichs
- Copepods
- Bryozoans
- Ostracods
- Daphnia
