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50 OVERVIEW OF RIVER-FLOODPLAIN ECOLOGY IN THE UPPER MISSISSIPPI RIVER BASIN preventing rapid throughput of materials (Minshall and oth- ers, 1985), The river continuum and resource spiraling concepts were developed largely from small temperate streams, and their usefulness as generalized paradigms for large rivers has been questioned (Statzner and Higler, 1985; Welcomme, ]985; Davies and Walker, ]986; Cuffney, ]988; Junk and others, 1989; Sedell and others, 1989), A more relevant framework has now emerged in which to test and clarify concepts about the structure and function of large flood- plain-river ecosystems (Dodge, ] 989). This complementary perspective is termed the flood pulse concept (Junk and oth- ers, 1989), Junk and others postulate that the bulk of aquatic biomass in many unaltered large floodplain rivers is derived directly or indirectly from production within the floodplain and not from downstream transport or organic matter pro- duced elsewhere in the basin, Whereas longitudinal linkages in small to moderate-sized streams are the basis for the con- tinuum aspect within the RCC. lateral exchange between the floodplain and river channel and nutrient recycling within the floodplain have a more direct impact on the biota and biological activity in large rivers. Whereas downstream losses of organic matter in small streams are reduced pri- marily by instream structure (e.g., pools, debris dams), geo- morphic features within the lateral floodplain (e.g" sloughs, side channels, backwaters) are largely responsible for reten- tion of organic matter and nutrients in large low-gradient rivers, The foundation of the flood pulse concept is that sea- sonal pulsing of flood flows onto the floodplain is the driv- ing force controlling the river-floodplain complex (Junk and others, 1989; Welcomme and others, ]989; Sparks and oth- ers, 1990; Bayley, ]991; Schlosser, 1991). While contributions of organic matter from floodplains may be quantitatively smaller than from upstream sources, they may be nutritionally of higher quality. Fremling and others (1989) postulate that organic matter from tributary sources consists largely of dissolved humic acids or refrac- tory particles by the time it is delivered to the main-stem Mississippi River. The more nutritious fractions have been utilized or retained by upstream communities. They con- clude that local sources of primary production, largely from within the floodplain, are responsible for the high fish pro- duction observed in large floodplain rivers. Floodplain wetlands are regarded as among the most productive ecosystems in the world (Lieth and Whittaker, 1975; Brinson and others, 1981). In situ primary production is high, and effective retention mechanisms contribute to efficient internal recycling of most carbon and nutrients (Junk and others, 1989). Although nutrient and organic mat- ter losses from the floodplain complex to the river channel may be small in relation to internal inputs within the flood- plain, leakage from the floodplain to the river during the annual flood pulse is the principal source of these materials to the main channel in unaltered rivers (Mulholland, 1981; Level of floodplain ----.. Floodplain surface _ D~ ~a~n_ Inundated Terrestrial Floodplain depressions Connected to river Isolated ephemeral and permanent aquatic habitats Confined to channel River Connected to floodplain 1 2 3 4 8- 9- 10 5 6 7- 11- 12- 13 Figure 5-1. Idealized changes in water level over an annual cycle for a riverine floodplain. Numbered horizontal bars indicate characteristic patterns of annual periodicity for some major inter- actions as follows: I, nutrients released as floodplain surface is flooded; 2. nutrient subsidy from river; 3. rapid growth of aquatic plants and invertebrates on floodplain; 4. major period of detrital processing on floodplain; 5. dissolved organic matter and fine par. ticulate organic matter exported to river; 6. maximum plankton production in floodplain depressions; 7. drift of plankton, benthos, and macrophytes to river; 8. fishes that enter floodplain from river and fishes that survived dry season in floodplain depressions move to floodplain surface; 9, major period of fish spawning on flood- plain; 10, period of maximum fish growth; II, fishes move from floodplain to river; 12. heavy fish predation losses at mouth of drainage channels: 13, high mortality of fishes stranded in flood- plain depressions (source: Ward. 1989). Cuffney, 1988; Grubaugh and Anderson, 1989; Junk and others, 1989; Ward, 1989; Sparks and others, 1990). Fishes capitalize on this highly productive floodplain environment for feeding, spawning, nurseries. and as refuge from adverse river conditions (fig. 5-]). Indeed, floodplain wetlands are considered the essential component responsi- ble for the high fish production recorded in large, low- gradient rivers (Welcomme, 1985; Ward, 1989). Risotto and Turner (] 985) showed that variation in commercial fish har- vest in the Mississippi River basin was positively associated with acreage of bottomland hardwoods in the basin flood- plain. This benefit of the floodplain and the flood pulse to aquatic productivity of large rivers has been termed the flood pulse advantage by Bayley (1991). He defines the flood pulse advantage as the hypothesized increase in multi- species fish yield over that which would result from the same water-surface area with no flood pulse (i.e., from a system with constant water level). He further argues that particularly strong year-classes of fish tend to result from