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, similar to 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.