Streams

A stream is defined as a body of water with a current (a lotic environment), confined within a bed and stream banks. But a stream is much more than that. A stream is a dynamic, living entity that links ecological processes across space and time, creating habitat for a diverse assemblage of organisms. Streams are important as conduits in the water cycle, as instruments in groundwater recharge, and as corridors for fish and wildlife migration. Lotic waters are diverse in their form, ranging from springs only a few centimeters wide to major rivers kilometers in width.

Although streams and rivers have been critical to the development of civilization, they represent but a small fraction of the world's freshwater supply. Nevertheless, streams play an important role in connecting fragmented habitats and supporting biodiversity. Like many ecological systems, streams are highly imperiled by natural and anthropogenic stressors.

A stream is governed by its physical setting. Local geology determines its substrate type and sediment load. Rocks and sediments are transported by water in solution (dissolved load), as particles in suspension (suspended load), and/or as particles moving along the streambed (bed load). The size of particles determines how they are transported and where they are ultimately deposited. Stream channels vary in curvature and cross-sectional profile, alternating between pools, riffles, and runs. The degree of habitat variability is often used as a measure of stream health. Current is the defining physical variable of rivers and streams, shaping the streambed habitat and interacting with many other biotic and abiotic variables. Discharge is determined by climate and altitudinal gradient, and varies horizontally and vertically within the stream. Obstructions such as logs and boulders create variation in current that form different types of habitat. Even in smooth channels, water near the sides and bottom travels at lower velocities because of friction. Flow transports oxygen, carbon dioxide, and nutrients, and conveys food to waiting organisms.

To shelter from the current, organisms must either develop attachment devices or perform energetically costly work to hold position. Stream dwellers exhibit a wide variety of adaptations to the current, including a flattened profile, silk and other sticky secretions, hooks, claws, and suckers. In addition, many organisms adapt behaviorally, “holding station” in slow-moving water adjacent to faster-moving water, where they can dart out to capture passing prey. Others shelter near the streambed, where virtually no movement of water molecules occurs.

Other important physical factors affecting streams include sunlight, chemical constituents, and temperature. Water temperature is first affected by air temperature, then by the relative mix of surface water and groundwater contributing to the stream, the degree of shading, and the overall stream size. Small streams warm more by day and cool more by night because of solar heating of the streambed and their relatively small mass of water. Since the majority of stream organisms are cold-blooded, temperature strongly influences where organisms can live and their metabolic rates. While most streams range from 32 to 77 degrees F (0 to 25 degrees C), subtropical and tropical streams can reach 86 degrees F (30 degrees C), and some desert streams can reach 104 degrees F (40 degrees C).

Stream Vegetation

Energy flow through stream food webs is complex. In some streams, aquatic plants are the dominant energy sources; in others, terrestrial plant production dominates. Allochthonous sources of energy include leaves, fruits, and other plant materials that fall or are blown into the stream. This nonliving organic matter (detritus) is also called particulate organic matter (POM). Terrestrial energy sources also include detritus that decomposes on the forest floor, enters soil water, and eventually enters the stream as dissolved organic matter (DOM). Stream ecologists refer to energy produced within the stream channel as autochthonous production, and organic matter produced outside the stream as allochthonous production. Autochthonous energy sources include microscopic algae (though large colonies can be seen with the naked eye), cyanobacteria, and macrophytes (mosses, liverworts, and the true vascular plants, or angiosperms).

Studies suggest that in woodland streams, allochthonous inputs may be even more important than instream production. Terrestrial organic matter such as leaves, fruit, twigs, and logs produce coarse particulate organic matter (CPOM, greater than 1 millimeter in size) that is in many streams a major source of energy input. Through a combination of wetting, physical abrasion, microbial colonization, and shredding, CPOM is converted to fine particulate organic matter (FPOM). FPOM is readily available for bacterial colonization, serving as a food source for animals that collect it from the substrate or filter it from the water column. Found both within and outside the stream, dissolved organic matter (DOM) is the largest carbon pool in stream food webs. DOM comes dissolved in water and reaches the stream via canopy drip during rain, as surface flows, and via subsurface pathways.

Diatoms, algae, and cyanobacteria provide important direct sources of energy to stream heterotrophs (organisms that cannot produce their food) since most macrophytes are unpalatable (though they do contribute to instream production of DOM and FPOM). Unicellular diatoms create a brown, slippery coating comprised of millions of cells on stones or other instream objects exposed to sunlight. Diverse and abundant, they are the most important autochthonous energy source to stream food webs. Algae proliferate in streams with sufficient nutrients, a stable substrate, and adequate sunlight. They are common, occurring as single cells, colonies, or long filaments, and are generally palatable.

Cyanobacteria have the ability to “fix” atmospheric nitrogen that can be utilized by other autotrophs (organisms that can produce their food), but are probably less important as an energy source to stream food webs than diatoms and algae. Both autotrophs and heterotrophs benefit from their close association in biofilms, which are complex microecosystems and important regions of energy production. A self-contained ecosystem, biofilms—surface slime composed of bacteria, algae, and fungi bound within a polysaccharide (complex sugar) matrix—are in turn inhabited by protozoans and micrometazoans that consume the matrix. Within the matrix, dead algal cells and organic exudates nourish heterotrophic bacteria that, in turn, convert organic compounds into inorganic compounds used by algae for continued photosynthesis.

Mosses (commonly Fontinalis and Fisidens) are usually found growing on rocks and logs in and around the colder, well-shaded areas of streams. They can tolerate low light and low temperatures, and have a rapid rate of nutrient uptake and high resistance to being dislodged by high flow events. Poorly studied, mosses may rival algae for productivity, but because of their limited distribution, are probably of minor importance as an overall energy source. Macrophytes gain a toehold as streams increase in size and falling current velocities cause silt to settle out. Common genera include Potamogeton, Elodea, Ranunculus, and Nuphar, as well as watercress (Nasturtium) in smaller springs and brooks. Macrophytes play various roles in the ecology of streams: as an energy source both before and after death, as a substrate for attachment of other organisms, and as cover from predators. Their greatest role as an energy source is probably in the production of fine particulate organic matter (FPOM, less than 1 millimeter in size) produced after they die and their remains are broken down.

Stream Insects

Aquatic insects are by far the most abundant organisms in streams and have the greatest diversity of feeding methods. Stream ecologists classify these macroinvertebrates into functional feeding groups that encompass both their feeding roles and how their food is acquired. Functional feeding groups include shredders, grazers and scrapers, collectors (both gatherers and filterers), and predators. In addition, aquatic insects are excellent environmental indicators because they display a range of tolerances for environmental conditions.

Shredders are found in well-oxygenated waters with large accumulations of CPOM. Shredders obtain energy from the leaf and from the microbes, primarily fungi, which colonize it. Using their mouthparts, they shred and tear CPOM particles into smaller FPOM particles and also contribute FPOM via their fecal pellets. Common shredders include some stoneflies (e.g., the famous salmon fly Pteronarcys californica), many dipteran larvae (e.g., crane flies of the family Tipulidae), and most of the caddisflies that construct cases of organic material (Limnephilidae).

Grazers and scrapers are typically found where light reaches the stream bottom, promoting algal growth, their main food source. Grazers and scrapers have mouthparts adapted for scraping the film of algae growing on the surfaces of rocks and other large surfaces (periphyton) and produce copious amounts of FPOM through the production of fecal pellets and dislodgement of algal cells during feeding. Grazers (e.g., mayflies) browse the overstory of loosely attached algae, while scrapers close-crop the diatoms adhered to the substrate. One common grazer is Glossosoma, a small caddis that carries its small, tortoise shell-shaped case of coarse cemented sand grains around on its back while feeding. It is a general rule that caddis larvae with cases made from inorganic matter are scrapers, and those made from organic matter are shredders.

The largest and perhaps most interesting functional group, the collectors, can be further divided into filtering-collectors and gathering-collectors. Filtering-collectors filter FPOM from the water using nets (mainly caddisflies) or specially adapted body parts. Like fishermen, net-spinners construct a net across their nonportable case or fixed retreat and wait for current-borne FPOM to collect in the fine mesh. Then, they either feed on the trapped particles, or consume the net and its contents altogether. This ubiquitous group is highly variable with respect to stream order, temperature, and FPOM size, though body and net size generally decrease with distance downstream as CPOM inputs decrease and FPOM increases. Other filtering-collectors have specialized body parts to enable them to filter FPOM from the current. The caddis Brachycentrus sits inside its small, four-sided case with its hairy-fringed forelegs extended, ready to intercept FPOM from the current. Once the fringes are full, it combs the captured material from its forelegs with its mouthparts. Blackfly larvae anchor their abdomens to rocks and throw themselves into the current, filtering FPOM from the water with their fan-like mouthparts that are fitted with fine hairs and sticky mucous.

The mayfly Isonychia crouches in the current, holding its fringed front legs in front of its body like a basket. When the basket is full, it raises its front legs and consumes the contents with its mouthparts. Unlike filtering-collectors, gathering-collectors (such as mayflies of the genus Baetis) are typically generalists that scurry around the stream bottom, picking up particles wherever they find them or ingesting their way through the sediments like earthworms.

Predatory insects occur throughout the stream community and have many different adaptations to enable them to pursue and capture prey. Common predators include stoneflies (Plecoptera), dragonflies and damselflies (Odonata), hellgrammites (Megaloptera), and one family of free-roaming caddisfly (Rhyacophylidae) that does not construct cases. Other types of instream insects include miners that feed on detritus buried in fine sediments, piercers with sucking mouthparts that feed on plant fluids, and gougers that burrow into large woody debris while feeding on their associated fungal and bacterial colonies.

Animals in Streams

Fish also assume functional roles. Fish can be classified into feeding guilds—top-feeders, midwater-feeders, and bottom-feeders. Different species occur along a stream's length. Overall, species richness and variation in body size increases with distance from headwaters to river mouth. Species that prefer clean, swift gravel runs and feed on the aquatic insects found in such habitat are generally absent from low-gradient, soft-bottomed lower river reaches. As with macroinvertebrates, indices of stream health have been built on assessments of fish diversity and abundance.

Other instream organisms include mollusks, crustaceans, and meiofauna, which include protozoans, rotiferans, annelids, and the microcrustaceans (harpactacoid copepods and ostracods). Some minor groups include the macroscopic freshwater sponges (Porifera), freshwater hydroids and jellyfish (Coelenterata), flatworms (Turbellaria), horsehair worms (Nematomorpha), and mosslike animals (Bryozoa), and the microscopic gastrotrichs (Gastrotricha) and tardigrades (Tardigrada).

Reptiles, and especially amphibians, utilize streams where in certain cases, such as with the Pacific giant salamander (Dicamptodon ensatus) and tailed frog (Ascaphus truei), they assume a dominant role in terms of biomass. The tailed frog is the only New World amphibian that exhibits internal fertilization, presumably as an adaptation to rapidly flowing mountain streams where currents would wash eggs and sperm away.

Most mammals and birds visit streams at one time or another. Some, like beavers, river otters, and mink, and diving and wading birds, are intimately associated with them. The dipper or water ouzel (Cinclus mexicanus), a small songbird, is entirely dependent on stream ecosystems. A dipper actually walks underwater to pick aquatic invertebrates off submerged substrates, maintaining its position on the streambed by adjusting its wings against the force of the current. Other songbirds and bats consume emergent aquatic insects that are a large part of their food supply.

River Continuum Concept

The River Continuum Concept (RCC) describes how a hypothetical stream might change physically and ecologically along its length as a result of variable energy inputs and their resultant changes to the community of organisms. This model uses stream order, which describes the different links in a stream network, and can give an indication of their discharge. For example, a first-order stream lies at the very headwaters of a stream network where the stream begins to flow year-round. Further downstream, first-order streams merge into larger, second-order streams, and so forth. The RCC generates many useful predictions about patterns that can be seen in any geographical region or biome. For example, a stream's headwaters (orders 1–3) might flow through a heavily shaded forest reach, then through a midreach of more open country (orders 4–7), and finally through a lowland reach of relatively flat, open country as a large, deep, heavily silted river (orders 8 and above).

Upper reaches are generally narrow and well-shaded by streamside trees. The stream substrate may be rocky or sandy, depending upon the geology of the region. Here, mosses are the dominant aquatic primary producers because of insufficient light for instream algal growth, and the current is usually too swift, nutrients too low, and the substrate too unstable to support the growth of macrophytes. Considerable CPOM enters the stream from leaves, twigs, and branches. This matter is important in meeting the energy demands of instream organisms, shredders who break down the coarse organic matter into FPOM, and collectors who gather this suspended material from the water column. Fishes in upper reaches include minnows, trout, sculpins, and other species tolerant of high-current velocities and seasonal and daily cold temperature regimes. Upper reaches export a considerable amount of FPOM to middle reaches where the stream widens, warms, and is well-lit by direct sunlight. These conditions lead to a proliferation of algae, filamentous greens, and/or diatoms. The diatom layer makes middle-reach rocks slippery. Rooted macrophytes find purchase in protected places where the current slows and sediments accumulate. Biomass of collectors is generally similar to upper reaches (though the individual species may differ), but shredders are much reduced. However, grazer biomass is much increased to take advantage of the proliferation of algae on the stream bottom.

Unlike other reaches, organisms in middle reaches produce more food than they consume, exporting large amounts of FPOM to lower reaches. The majority of the world's trout streams are located in these highly productive middle reaches. Lower reaches of streams generally fall into the river category. The RCC is well-supported in most streams around the world. Exceptions occur in streams with different geographical or geological conditions, and/or where changes occur because of human alterations to the pristine pattern. Even in relatively undisturbed streams, reset mechanisms interrupt the predictable patterns of the river continuum. These can occur through natural means—such as when tributaries join larger streams, resetting the conditions in the receiving stream to those higher in the continuum—or through human activities, such as damming or deforestation.

Examples of streams around the world include cold-water trout streams that support a wide variety of organisms and provide world-class recreational opportunities for anglers; midwestern United States warm-water streams that are, in their pristine state, the prototype of the RCC; southeastern United States blackwater streams that, while entirely heterotrophic, provide habitat for a wide variety of threatened and endemic species; mineral-laden thermal springs and streams and their resulting unique biota; ephemeral hot-desert streams driven almost entirely by seasonal precipitation; cold-desert spring streams that store excess primary productivity until released by high-flow flushing events; alpine streams that, despite a short growing season, support flourishing communities of algae, macroinvertebrates, and fishes; cave streams in which energy flows are based almost entirely on detritus or chemosynthesis; and tropical streams that differ from temperate streams in their seasonality regimes, high interannual variation, frequent storm events, and differences in species composition. For example, many tropical streams are dominated by decapod crustaceans (mainly atyid shrimp), rather than insects.

Human activities have a profound influence on streams. Dams alter flow, temperature, and sediment regimes. Dams also fragment river systems, block fish passage, and isolate populations. Nonpoint source pollution is the dominant contributor to nutrient loading in streams and rivers. Acid rain has lowered pH in many streams in western Europe, Scandinavia, the northeastern United States, and some regions of the Rocky Mountains, killing much of the life in these systems. Mining also produces acid and toxic metal runoff that has poisoned many streams in Appalachia, the Rockies, and elsewhere, virtually extinguishing life. Introduced species displace native species. And climate change is almost certain to alter thermal and hydrologic regimes in streams, with associated changes to the ecology of organisms in these systems.