the surface area of the floodplain was 

 much larger. Sloughs were satrpled after 

 the water had ceased flowing off the 

 floodplain, and fish were concentrated by 

 falling water levels. Holder et al. 

 (1970) stated that "high water over the 

 floodplain provided space, food, and 

 increased habitat for the reproduction and 

 growth of fish. " 



Movement of fish on floodplains often 

 is keyed to temperature. Holder et al. 

 (1970) found ripe males and females of 

 several species trying to cross the sill 

 between the Okefenokee Swamp and the 

 Suwannee River coincident with high water 

 at the following time and water tempera- 

 tures: fliers, bowfin (February, March, 

 11°-13°C or 52°-56°F); yellow and brown 

 bullheads (March, 11°C or 52°F); warmouth 

 (March-April, 16°-19°C or 60°-67°F); chain 

 pickerel (March-April); lake chubsucker 

 (April, 21°-24°C or 70°-76°F). 



Floodplains are important spawning 

 areas for several species of herring 

 ( Clupeidae ). Hickory shad ( Alosa m.edio- 

 is) ^spawn in oxbow lakes, sloughs, and 



cr 



tributary streams of the Altamaha River 

 (GA) (between River Mile 20 and 137). 

 Blueback herring ( Alosa ae stivalis ) spawn 

 in the same areas of the bottomland hard- 

 woods; they have remarkably adhesive eggs 

 which adhere to twigs and objects on the 

 floodplain floor and resist being swept 

 away by sheet flow. Ripe bluebacks were 

 taken in an over 161-km (100-mi) long sec- 

 tion of the Altamaha in backwater lakes 

 and flooded low areas "that are accessible 

 to these fish only during spring flood 

 stages" (Adams and Street 1969). 



Studies of larval fish on the flood- 

 plain or in sloughs and waterways deep 

 within the floodplain suggest that the 

 immature stages of roughly one half of the 

 fishes of the lower Mississippi River used 

 the floodplain as a nursery (Gallagher 

 1979). Analysis of the temporal, spatial, 

 and size distribution of larval fishes 

 supported this contention; spawning of 7 

 out of 10 of the most common taxa took 

 place in backwater habitats (Atchafalaya 

 Basin, LA) (Hall 1979). Temporal and 

 spatial delineation of niches of larval 

 fish on the floodplain have been summar- 

 ized further by Larson et al. (1981) and 

 Wharton et al. (1981). 



TROPHIC RELATIONSHIPS 



Energy flow in riverine systems 

 involves both detritus and grazing path- 

 ways. Although rivers appear to shift 

 from autotrophy (predominantly grazing 

 pathway) in mid-sections to heterotrophy 

 (predominantly detritus pathway) in lower 

 sections (Vannote et al. 1980), many lower 

 river "detrital" food chains may still 

 involve zooplankton "crazing" on phyto- 

 plankton. For example, Wallace et al. 

 (1977) found over 300,000 diatoms/liter in 

 the lower Altamaha River (GA). The graz- 

 ing pathway is important even in Coastal 

 Plain blackwater streams; Patrick (1972) 

 characterized these streams as being domi- 

 nated by the diatom genera Eu notia and 

 Actinel la . For clarity, trophic pathways 

 on the floodplain have been divided into 

 two systems (dry system pathways and wet 

 system pathways). These two "systems" are 

 not always clear cut. For example, mal- 

 lards prefer to feed on acorns when they 

 float during inundation. The dry system 

 (Figure 51) is functional during drydown 

 when the floodplain is not inundated. 

 While the system is largely detrital, the 

 grazing pathway of the terrestrial faunal 

 assemblage is also pronounced, with appre- 

 ciable consumption of the products (nuts, 

 berries, leaves, bark) of the bottomland 

 hardwoods and other primary producers. 



Trophic pathways in the litter layer 

 of the dry system are similar to those in 

 the uplands. Gist and Crossley (1975) in 

 trophic studies of upland forest found 

 that millipedes consume up to 120 g/m^/yr 

 of deciduous litter detritus. Fungal 

 hyphae were the principal food of snails 

 and collembolans. The predatory mites 

 consumed primarily collembolans. 



The second trophic system, (Figure 

 52) is a wet system functioning in pools 

 and during inundation. Primarily detri- 

 tal, it involves the bulk export of detri- 

 tus into sloughs, oxbows and rivers, thus 

 feeding a largely aquatic fauna. Aerial 

 swarms of midge flies, mosquitoes, and 

 mayflies emerge periodically, however, to 

 regale legions of swifts, tree frogs, 

 bats, and dragonflies. 



Since much of the energy of the wet 

 system is exported, the process needs to 

 be summarized in more detail. Detritus 



95 



