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CHAPTER 2: ECOLOGICAL OVERVIEW-POSTFLOOD RECOVERY AND ECOSYSTEM MANAGEMENT 5 ABIOTIC CONTROLS The major abiotic controls in river-floodplain ecosys- tems are the hydrologic cycle. climate. and floodplain geo- morphology (Welcomme. 1979; Junk and others. 1989; Bayley. 1991), The hydrologic cycle (fig, 2-2) in the pre- dam era (late 1800's) was bimodal (spring and fall floods) for the UMR and unimodal (one extended flood) for the middle Mississippi and lIIinois Rivers, Hydrologic patterns differ between the upper and middle Mississippi Rivers because of the influence of the Missouri River. which joins the Mississippi near Sl. Louis. Missouri. The ecological sig- nificance of the difference in hydrologic patterns is unknown. but theories (Junk and others. 1989) predict that some level of resource partitioning and habitat development occurs with respect to river stage. Hydrologic cycles regulate floodplain habitat and nutrient availability, As ephemeral aquatic habitats appear and disappear with rising and falling river waters, resource availability is differentially proportioned between terrestrial and aquatic environments. In the evolutionary timescale the average hydrologic cycle created the dominant communities we see now (floodplain forests and wetlands). The annual hydrograph regulates community comJX>sition in any given season or location (fig, 2-3) (Junk and others. 1989), Climate plays an important role because biotic com- munities evolved in response to predictable patterns of tem- perature. rainfall (hydrology). and day length, Along the 800-mile length of the UMR, seasonal events at the northern edge of the basin can lag behind (in the spring) or precede (in the fall) those at the southern edge by 2-4 weeks (Lubin- ski. 1993). As a result. plant communities exhibit a grada- tion, having some subtropical species at the southern tip and north temperate species in the northern portion of the basin (KUchler. 1964: Curley and Urich, 1993; LTRMP. unpub, data). Mesothermal species of fish. such as northern pike and yellow perch. are found in higher abundance in the upper portions of the system. but most species occur lhroughoutthe system (Gutreuter. 1992). Usually. ecologi- cally functional equivalents fill similar niches at different extremes of the system. For example. both the redear and pumpkinseed sunfish eat snails, but the redear has a south- ern distribution and the pumpkinseed a northern distribution (pflieger. 1975), Local floodplain landform (geomorphology. topogra- phy) is an important determinant of floral and faunal com- munity composition at any particular location, The four reaches of the UMR (Lubinski. 1993) exhibit distinct differ- ences in floodplain geomorphology and thus habitat compo- sition, Each reach is likely to contain the broad habitat types shown in figures 3-1 and 3-2 of Chapter 3. Generalized biotic communities occur in these habitats based on physio- logical needs of the organisms. The aquatic habitat classifi- cations used most often, namely main channel, main chan- nel border and wingdams. islands, side channels, sloughs (side channels closed at the upper end). and floodplain lakes (isolated and contiguous), have been widely applied to the entire UMR (UMRBC. 1982). However. a more descriptive classification has been proposed by Wilcox (1993), I refer to seasonally flooded habitat as "floodplain" for discussion purposes but emphasize that both the river and tloodplain interact to make up the larger ecosystem. Junk and others (1989) call these dynamic habitats "aquatic terrestrial tran- sition zones," and Risser (1990) refers to them as ecotones. Ecological differences occur along both elevation and latitu- dinal gradients of the floodplain, Long-lived plant communities, such as forests, develop over time in relation to the average flood cycle. In wetland habilals, many plant species have adopted life history strate- gies that enable them to survive in a hydrologically dynamic environment. Some annual plants have tremendous growth rates on fertile alluvial soils exposed at low river stages. Others thrive equally well whether inundated or exposed. and many species may be present in the seed bank at a sin- gle location. The wetland plant community composition in any year is dictated by spring and summer hydrologic con- ditions. Animal communi lies are opportunistic, exploiting floodplain habitats as they occur and fulfill their life history needs (Bellrose, 1980; Bayley. 1991), HABITAT ASSOCIATlONS Pl.ASTS Plants respond primarily to abiotic factors in the river- floodplain environment. Because of distinct morphologic and hydrologic characteristics of river habitats, plant spe- cies/habitat associations can be identified (see figs. 3-1 and 3-2 in Chapter 3), Main channel habitats usually lack vas- cular plants because of deep water and high current veloci- ties; algal concentrations are low, presumably because of high concentrations of suspended sediments that block light penetration through the water, Channel border habitats are more likely to support submersed and emergent vascular aquatic plants because water depths are shallower and cur- rent velocities are low. Depositional areas also offer nutri- ent-rich alluvial soils, Algal production is likely to be higher in the low-flow, low-turbidity environments. The channel border can be a dynamic environment with rapid commu. nity shifts because of the patterns and frequency of water level fluctuations close to the main channel (Theiling and others. 1996). Side channel plant communities vary. depending on morphology and canopy vegetation of the side channel. Wide. slow-flowing side channels can develop similarly to main channel and channel borders, but narrow, swift- flowing channels under a forest canopy may not support aquatic plants, Sloughs and backwaters are very similar except that sloughs receive high flows during flooding.