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In riverine systems, sediment flux and organic matter inputs are important components of habitat structure and dynamics. Natural sediment regimes (Figure 7) are those that accompany natural flow variation. Natural organic matter regimes include seasonal inputs from terrestrial environments. Terrestrial organic matter inputs, especially in smaller rivers and streams, are particularly important sources of energy and nutrition, while large, coarse, woody material provides substrate and habitat for organisms. The invertebrates, algae, bryophytes, vascular plants, and bacteria that populate the bottoms of freshwater systems are responsible for much of the water purification, decomposition, and nutrient cycling that occurs. They are highly adapted to the specific sediment and organic matter conditions of their environment, as are many fish species, and do not persist if changes in the type, size, or frequency of sediment inputs occur.

Humans have severely altered the natural rates of sediment and organic matter supply to aquatic systems in ways that both increase and decrease inputs. Poor agricultural, logging, or housing development practices promote high rates of soil erosion. Sediment capture behind dams truncates normal sediment supply to downstream reaches, erodes streambeds, degrades habitat, and prohibits flood events from rejuvenating wetland and riparian areas. Siltation from agricultural, urban construction, and unspecified nonpoint sources is the cause of impairment. Channel straightening, overgrazing of riparian areas, and clearing of streamside vegetation reduce organic matter inputs, but also often increase erosion.

Dissolved Oxygen

Establishing an appropriate flow regime in a stream or river corridor may do little to ensure a healthy ecosystem if the physical and chemical characteristics of the water are damaging to that ecosystem. For example, streams or rivers with high concentrations of toxic materials, high temperatures, low dissolved oxygen (DO) concentrations, or other harmful physical/chemical characteristics cannot support healthy stream corridor ecosystems.

Figure 8 illustrates some of the key water quality processes affecting the oxygen content and hence the biology of surface waters.

Figure 7 The bank-full discharge typically has the largest total sediment load. Modified from Loucks DP and van Beek E (2005) Water Resources Systems Planning and Management. Paris: UNESCO.
Basic Chemistry Oxygen
Figure 8 Some of the basic chemical and biological processes affecting dissolved oxygen in waters. Modified from Loucks DP and van Beek E (2005) Water Resources Systems Planning and Management. Paris: UNESCO.

DO is a basic requirement for a healthy aquatic ecosystem. Most fish and aquatic insects 'breathe' the oxygen dissolved in the water body. Some fish and aquatic organisms, such as carp and sludge worms, are adapted to low oxygen conditions, but most sport-fish species, such as trout and salmon, suffer when DO concentrations fall below a concentration of 3-4 mgl-1.

Larvae and juvenile fish are more sensitive and require even higher concentrations of DO. Many fish and other aquatic organisms can recover from short periods of low oxygen concentrations in the water. However, prolonged episodes of depressed DO concentrations of 2 mgP1 or less can result in 'dead' (anaerobic) water bodies. Prolonged exposure to low DO conditions can suffocate adult fish or reduce their reproductive survival rate by suffocating sensitive eggs and larvae, or starve fish by killing aquatic insect larvae and other sources of food. Low DO concentrations also favor anaerobic bacteria that produce the noxious gases often associated with polluted water bodies.

Water absorbs oxygen directly from the atmosphere, and from plants as a result of photosynthesis. The amount of oxygen that can be dissolved in water is influenced by its temperature and salinity. Water loses oxygen primarily by respiration of aquatic plants and animals, and by the mineralization of organic matter by microorganisms. Discharges of oxygen-demanding wastes or excessive plant growth (eutrophication) induced by nutrient loading followed by death and decomposition of vegetative material can also deplete oxygen.

In addition to oxygen and water, aquatic plants require a variety of other elements to support their bodily structures and metabolism. Just as with terrestrial plants, nitrogen and phosphorus are important among these elements. Additional nutrients, such as potassium, iron, selenium, and silica, are also needed by many species but are generally not limiting factors to plant growth. When any of these elements are limited, plant growth may be limited. This is an important consideration in ecosystem management.

Dissolved organic O Dissolved organic nitrogen Export to downstream [> nitrogen

Figure 9 Dynamics and transformations of nitrogen in a stream ecosystem. Modified from Loucks DP and van Beek E (2005) Water Resources Systems Planning and Management. Paris: UNESCO.

Nutrients cycle from one form to another depending on nutrient inputs, as well as temperature and available oxygen. The nitrogen cycle is illustrated in Figure 9 as an example. Table 1 lists some common sources of nitrogen and phosphorus nutrient inputs and their typical concentration ranges.

Management activities can interact in a variety of complex ways with water quality. This in turn can affect ecosystem species, as shown in Figure 10.

Water Temperature

Light and heat properties are influenced by climate and topography, and by a waterbody's chemical composition, suspended sediments, and primary productivity. Water temperature directly regulates oxygen concentrations, organism metabolism, and associated life processes. The thermal regime greatly influences organismal fitness and, by extension, the distribution of species in both space (e.g., along latitudinal and altitu-dinal gradients) and time (e.g., seasonal variation at one location).

Water temperature can drop and water clarity can increase downstream of dams. Species richness will likely decline and species composition will likely change. Water temperature dropped in the Colorado River after closure of the Glen Canyon Dam in 1963 and, along with a dramatic increase in water clarity, this allowed for development of a non-native trout population and an unusual food web more commonly found in Nearctic regions. Water clarity is now routinely 0.7 m, whereas prior to dam closure the water column was opaque with suspended sediments.

Table 1 Sources and concentrations of pollutants from common point and non-point sources. These data show little or no impact on nutrient removals from basic wastewater treatment facilities

Total nitrogen

Total phosphorus




Urban runoff



Livestock operations



Atmosphere (wet




90% forest



50% forest



90% agriculture







Treated wastewater



Modified from Loucks DP and van Beek E (2005) Water Resources Systems Planning and Management. Paris: UNESCO.

Modified from Loucks DP and van Beek E (2005) Water Resources Systems Planning and Management. Paris: UNESCO.

Chemical and Nutrient Characteristics

Natural nutrient and chemical conditions are those that reflect local climate, bedrock, soil, vegetation type, and topography. Natural waters can range from clear, nutrient-poor rivers and lakes on crystalline bedrock, to much more productive and chemically enriched freshwaters in catchments with productive soils or limestone bedrock. Cultural eutrophication occurs when additional nutrients from human activities substantially increase productivity beyond the original or natural state.

has the potential to push functionally intact freshwater ecosystems beyond the bounds of resilience or sustainabil-ity, threatening their ability to provide important goods and services on both short and long timescales. Further, introduction of non-native species that can thrive under the existing or altered range of environmental variation can severely modify food-web structure and processes such as nutrient cycling. Exotic species are often successful in modified systems, where they can be difficult to eradicate.

Ecological Connectivity

Healthy ecosystems also depend on conductivity and width. Connectivity is a measure of how spatially continuous a corridor or a matrix is. A stream corridor with connections among its natural communities promotes transport of materials and energy and movement of flora and fauna.

Width is the distance across the stream and its zone of adjacent vegetation cover. Factors affecting width are edges, community composition, environmental gradients, and disturbance effects of adjacent ecosystems, including those due to human activity.

Width and connectivity interact throughout the length of a stream corridor. Corridor width varies along a stream and may have gaps. Gaps across the corridor can interrupt and reduce connectivity. Ensuring connectivity and adequate width can provide some of the most useful ways to mitigate disturbances.

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