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<br />RESTORA nON OF REGULA TED RIVERS
<br />
<br />397
<br />
<br />
<br />appear to occur in zones along the river continuum; others must move long distances in search of resources
<br />needed for each life stage, sometimes involving migrations into the lakes (e.g, adfluvial bull charr, Sa/velinus
<br />confluenlus) or the ocean (e.g, anadromous salmon and trout: Onchorynchus spp,; Salmo salar and S, lrutla;
<br />Salvelinus spp,).
<br />Widely dispersed species often exist as metapopulations because local populations are linked by dispersal
<br />and gene flow into larger regional populations that may encompass the entire catchment (Hanski, 1991;
<br />Hanski and Gilpin, 1991), For example, metapopulation structure is thought to be particularly evident in
<br />many salmonid populations (Reisenbichler et ai" 1992; Rieman and McIntyre, 1993) and most likely influ-
<br />ences the probability of persistence for a species (Stacy and Taper, 1992). Metapopulation linkages allow for
<br />local extinction of populations, which can be re-established via colonization from adjacent populations (Lei-
<br />der, 1989; Milner and Bailey, 1989), The spatial arrangement oflarge- and small-scale habitat features within
<br />a catchment may serve as a template for metapopulation organization of fishes (Schlosser and Angermeier,
<br />1995). The mosaic of floodplain reaches and constrained segments (Figure 2) within the mainstem and tri-
<br />butaries influences size, spatial distribution and proximity of local spawning populations. Proximity of
<br />populations and favourability of connecting habitats can affect exchange of individuals between local popu-
<br />lations (Reiman and McIntyre, 1993; Li et ai" 1995; Schlosser and Angermeier, 1995) and thus influence
<br />potential for recolonization of habitats where local extinction has occurred.
<br />Since most river fauna are ectotherms, growth and reproduction is also vitally influenced by river tempera-
<br />ture, Most organisms adapted to the cold climes of the headwater reaches simply cannot survive in \'.iarmer
<br />reaches downstream. and vice versa, Indeed. species found in a particular thermal environment III one river
<br />generally will be found in very similar environments in other rivers within the geographical range of thai
<br />species. if all other resource needs are also met Because growth of ectotherms is strictly temperature depen-
<br />dent, temperature is a critical habitat attribute (Ward, 1985; Hall et at.. 1992). Stream insects and fish will be
<br />found in areas of the stream where their thermal needs are met and substratum, food and other resources are
<br />marginal. but rarely the inverse. at least for individuals that ultimately reproduce successfully ThIS IS
<br />because of the basic thermal energetics of growth and the fact that many life history stages, such as insect
<br />emergence (ecdysis) and flsh spawning are ini tiated by precise temperature cues (Brett 1971 ; Vannote and
<br />Sweeney. 1980; Ward and Stanford, 1982), In addition, because few riverine organisms have highly specia-
<br />lized food requirements, food limitation may be less prevalent than thermal limitation most of the time.
<br />For plants of the river food-web, availability of light and nutrients is crucial. In headwater streams shaded
<br />by riparian plants, decomposition of allochthonous (terrestrially derived) coarse particulate organic maller
<br />(leaves, grasses) usually drives instream bioproduction (Cummins et ai" 1984, 1989), Plant growth nutrients
<br />are released into transport by the decomposition of particulate organic matter entrained on the bottom. and
<br />are utilized by aquatic plants in better light environments downstream where the stream channel is wider and
<br />the riparian canopy opens. Of course, nutrients and other dissolved solids are also deri\'ed from dissolution
<br />of the bedrock and other geochemical reactions, Indeed, streams with high alkalinity from limestone disso-
<br />lution generally are more productive than streams draining more inert bedrocks, such as granite massifs
<br />(Kruger et at.. 1983; Waters et al., 1990), Dissolved solids that are required for growth by algae and macro-
<br />phytes spiral downstream, alternatively retained and released into transport by the river food-webs (New-
<br />bold et at., 1981, 1982), Conditions may shift back to heterotrophy in turbid, slow moving reaches near
<br />the river mouth as a consequence of planktonic microbial decomposition of organic matter transported
<br />from upstream reaches, reduced light reaching the bottom owing to deep and often turbid water and shifting
<br />substratum (Vannote and Sweeney, 1980; Minshall et ai" 1983; Naiman et ai" 1987)
<br />All of this underscores the complex linkages between the spatial dimensions of river ecosystems (Figure I).
<br />These interactive components and attributes are repeated throughout the river course. from headwaters to
<br />mouth, Floods maintain channel and floodplain habitats and pulse nutrient-enriched waters laterally into
<br />backwaters and on to floodplains, as well as downstream into the estuary, Because it is a continual habi-
<br />tat-forming process, river biota are adapted to frequency and duration of flood pulses (Copp, 1989; Junk
<br />et aI., 1989). Rivers that flood frequently (annually or more often) maintain different species and food-
<br />webs than systems that are more ecologically benign by rarely or never experiencing scouring floods (e,g.
<br />spnng-brooks and lake outlet streams). Food-webs are complex and change predictably along the stream
<br />
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