Exploring riverine zooplankton in three habitats of the Illinois River ecosystem: Where do they come from?

We sampled three habitats (main channel, side channels, and backwater lakes) during 2 yr along 32 km of the Illinois River to compare zooplankton distribution and dynamics, as well as evaluate the possible effects of hydrology on taxonomic abundance and distribution. Zooplankton assemblages displayed both spatial and temporal variation. Whereas the riverine zooplankton assemblage was dominated by rotifers, the backwater lake assemblage was dominated by copepods. Zooplankton densities in the main channel peaked earlier in the season in both years than the backwater lake habitats. To determine if these patterns were caused by fluvial exchanges occurring between habitats during flooding, we sampled the connections between the backwater lake and main channel habitats and found that large numbers of zooplankton entered the main channel via these connections. Further, calculations of main channel population growth, birth, and death rates showed that population growth rates most commonly exceeded birth rates during the flooding period. Seasonal inoculums from off-channel habitats could play an important role in riverine zooplankton dynamics. However, for the main channel to achieve the measured zooplankton densities, ,400,000 backwater lakes would be required and zooplankton would need to travel an unrealistic number of days and distance based on estimated growth rates. Thus, other mechanisms (hatching of resting eggs or in situ reproduction) are likely responsible for zooplankton abundances. There is no clear understanding of how zooplankton assemblages in large rivers are structured (Davies and Walker 1986; Reynolds 1988) and their origin is often debated (Hynes 1970). Riverine zooplankton dynamics are thought to be controlled by factors affecting transport and growth (Hynes 1970). Main river channels produce harsh physical conditions (advection, turbulence, high turbidity) that may reduce zooplankton feeding (Hart 1988; Saiz et al. 1992), growth (McCabe and O’Brien 1983; Thorp et al. 1994), and reproduction (McCabe and O’Brien 1983; Thorp et al. 1994). Further, it is thought that these organisms are unable to swim against currents (Winner 1975). Despite harsh physical conditions, population growth does occur suggesting some taxa can successfully reproduce in the main channel of large rivers and densities often vary with shifts in the seasonal hydrologic regime. (Saunders and Lewis 1988a,b). River-floodplain ecosystems are comprised of both lotic (the main channel) and lentic, or off-channel, habitats. Although riverine zooplankton community composition may be controlled by advection and turbulence, physical changes during flooding likely play a critical role in structuring this community. Typical floods are associated with low water temperatures, decreased residence times, increased advection and decreased transparency in main 1 Present address: Great Lakes Fishery Commission, 2100 Commonwealth Blvd. Suite 100, Ann Arbor, Michigan 48105. 2 Present address: Department of Biology, University of South Dakota, 414 E. Clark St., Vermillion, South Dakota 57069