River and stream systems constantly transport material downstream from the mountains to the ocean. These waterways transport not only precipitation as it falls on the mountains, but sediment (including boulders, cobble, gravel, sand, silt and clay) as the mountains are slowly eroded. With these sediments are nutrients that are dissolved in the water, attached to sediment particles, and enclosed in the organic material of plants and animals.
The water chemistry of a stream or river is shaped the quality of precipitation falling from the atmosphere, the geology of the watershed, land use and vegetation in that watershed, natural stability of the stream or river, and biological activity. In running waters, dissolved nutrients are transported some distance until uptake by aquatic organisms. Because first-order streams constitute such a large portion of the total length of stream within any watershed, maintaining the health of water quality in small headwaters and even intermittent streams determines water quality in larger streams and rivers. This water quality in turn has a large impact on the type of aquatic life that lives in the stream environment.
An intermediate-sized stream where on-site or "autochthonous" production by periphyton and off-site "allochthonous" production from adjacent woodlands are both important sources of energy to the stream ecosystem. Credit: Jeff Reardon
Stream ecosystems ultimately run on energy derived directly or indirectly from solar energy and photosynthesis. The products of photosynthesis fuel the biological activity in the stream so that energy (carbon) travels up the food chain. For example, a tiny net-forming caddisfly will trap particles of leaves in its net, while a dragonfly, a trout, or another predator will eat the caddis. Because the energy that fuels stream ecosystems is in the form of organic matter, whether living (a caddisfly), or dead (the particles of organic matter trapped in the caddisfly's net), stream ecologists often track energy by accounting for the flow of organic matter, or the flow of the element carbon, which makes up most of the organic matter in a stream.
There are two main sources of carbon or energy in stream systems: primary production (photosynthesis) that occurs on land near the stream, and primary production that occurs within the stream itself. The relative balance between the two varies depending on the size of the stream, the placement of the stream within the stream network, and watershed conditions.
Typical Freshwater Aquatic Food web. Credit: Federal Interagency Stream Restoration Working Group (1998)
In much of the world, healthy small headwater streams are cold and typically shaded for much of the growing season by forests and riparian vegetation. These systems are typically allochthonous, where carbon is derived from dead plant material or detritus that enter the stream. Large amounts of organic material are produced in adjacent forests and fall or are washed into the stream each year in the form of leaves, wood, shed bud scales, flowers and seeds. Because the stream is shaded, little photosynthesis can occur. Stream life therefore is geared toward processing this dead plant material and detritus directly, or by trapping the tiny particles of detritus released into the water column. The functional feeding groups of the aquatic insect life are primarily "shredders" that breaks down larger carbon sources like leaves into smaller particles and "collectors" that harvest these smaller particles or other particles released as bacteria decompose the detritus. See Macroinvertebrates. Respiration, the breakdown of organic material by living organisms, greatly exceeds photosynthesis.
As the stream network grows, streams widen and more light penetrates the surrounding forest canopy to reach the stream bottom, photosynthesis occurs at higher rates and increases in relative importance. Algae and diatoms living and growing on the stream bottom, called periphyton, produce carbon through photosynthesis. Simultaneously, as the stream and the floodplain become wider, the relative amounts of allochthonous material entering the streams from the uplands declines. The macroinvertebrate assemblage is a mix of "shredders," "collectors" and "grazers." See Macroinvertebrates.
As streams become rivers that are wide and open to the sunlight most of the year, carbon is primarily derived from photosynthesis. This local carbon production is called "autochthonous" by stream ecologists. In larger river systems, photosynthesis by algae, diatoms, cyanobacteria, dinoflagellates and aquatic plants dominate inputs from the surrounding lands. Photosynthesis greatly exceeds respiration. Larger river ecosystems often support rich plankton communities that live in the water column that are preyed upon by sight predators like small fish as well as by filter feeders like freshwater mussels. Functional feeding groups in larger rivers are therefore mostly "collector-gatherers" as well as "filterers." Only during floods, when mobile aquatic organisms can access the floodplain, and immediately following floods, when organic material has washed into the river from the floodplain, will allochthonous production rival onsite production. In all types of stream systems, predators exist that feed on the other types of respective feeding groups.
Allen, J. David. 2001. Stream Ecology. Norwell, Massachusetts: Kluwer Academic Publications.
Caduto, Michael J. Pond and Brook: A Guide to Nature in Freshwater Environments. 1990. University Press of New England.
Vannote, R.L., G.W. Minshall, K.W. Cummins., J.R. Sedell, and C.E. Cushing. 1980. The River Continuum Concept. Canadian Journal of Fisheries and Aquatic Science 37: 130-137.