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<br />Nutrient and Energy Transfer within a Large River Ecosystem
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<br />241
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<br />communities (Fontaine and Bartell, 1983). Plant and animal communi-
<br />ties adapted to seasonal flooding that created productive shallow habitats
<br />and promoted nutrient cycling, which is essential to the ecological in-
<br />tegrity and functioning of river-floodplain ecosystems (Copp, 1989; Junk
<br />et aI., 1989; Bayley, 1995; Johnson et al., 1995; Sparks, 1995; Welcomme,
<br />1995).
<br />The ecological integrity of aquatic ecosystems, especially flowing wa-
<br />ter systems, must be viewed from a landscape perspective (e.g., a water-
<br />shed) because rivers and streams are ultimately affected by all land-use
<br />practices in a watershed (Foreman and Godron, 1986; Ward and Stanford,
<br />1989; Sparks et aI., 1990; Schlosser, 1991; PeUs et aI., 1992; Wesche, 1993;
<br />Stanford et al., 1996; Poff et aI., 1997; Williams et al., 1997). Watersheds
<br />and riparian ecosystems serve as an ecotone or link between terrestrial and
<br />aquatic environments (Brinson et al., 1981; Naiman and Decamps, 1990)
<br />as well as the ecotone between surface water and groundwater (Gilbert
<br />et aI., 1990). Annual and periodic overbank flooding form various types of
<br />wetlands in floodplains of rivers. Some of these wetlands are maintained
<br />by connectivity with rivers for long periods of time, especially during high,
<br />prolonged runoff events. Wetlands serve various functions through their
<br />linkage to large river ecosystems (Table 9.2) by providing habitats and food
<br />resources that are required by communities of aquatic, avian, and terres-
<br />trial animals. Wetlands provide natural flood control by absorbing water
<br />during peak streamflows and returning it slowly as flows subside. Flood-
<br />plains also may abate water pollution by absorbing contaminants as water
<br />percolates through the floodplain sediments (Clark, 1978; Sparks, 1995;
<br />Finley, Chapter 7, this volume).
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<br />Nutrient and Energy Transfer within a Large River Ecosystem
<br />
<br />The importance of the land-water interface to the productivity of loticsys-
<br />tems has been recognized for over 25 years (Hynes, 1970, 1983; Karr and
<br />Schlosser, 1978; Allan,1995). However, interpretation of the complexity
<br />of biological responses (e.g., food webs and interactions of invertebrates
<br />and vertebrates) (Hildrew, 1992) and the importance of geomorphological
<br />or hydrological processes (Bevin and Carling, 1989; Sparks et aI., 1990)
<br />has occurred only recently. Rivers are characterized by a one-way flow
<br />of water that transports nutrients, sediments, pollutants, and organisms
<br />downstream, and all upstream activities affect all downstream reaches
<br />(National Research Council, 1992). The concepts of the river continuum
<br />(Vannote et al., 1980; Sedell et aI., 1989) and flood pulse (Junk et aI., 1989)
<br />apply to large river systems. Longitudinal transfer of nutrients and en-
<br />ergy occurs through the river continuum in small, headwater streams and
<br />high-gradient, restricted meander canyon reaches of larger streams. Lateral
<br />transfer of nutrients and energy occur through flood pulses in low-gradient,
<br />unrestricted reaches of floodplains in broad valley reaches. The major zone
<br />of productivity in a floodplain is the "moving littoral" (Le., a shallow zone
<br />that extends from the edge of the waterline to several meters in depth)
<br />because it covers the maximum area of a floodplain for a given flood as it
<br />traverses the floodplain during inundation and draining (Junk et aI., 1989;
<br />Mertes, Chapter 5, this volume).
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