The Effect of Turbidity on the Distribution and Life History of River Zooplankton

Differences in zooplankton community composition in rivers and their adjoining wetlands are likely due to differences in environmental factors, such as turbidity. The current study compared the species composition in the Illinois River and its associated wetlands, and used life table experiments to compare the fitness of two Daphnia species in response to turbidity and food levels. Over a three-day period in summer 1996, I sampled 12 sites in the Illinois River and associated backwaters and wetlands to determine community composition and environmental variables. Rotifers were the most widespread and abundant, copepods were most abundant in backwater lakes, and cladocerans were most abundant in the non-connected lake sites. The more turbid sampling sites (main channels, connected backwaters) had lower species diversity than the less turbid areas (non-connected backwaters). Canonical correspondence analysis indicated that Secchi depth, turbidity, conductivity, and depth were the most important environmental factors in determing species composition. Analysis of zooplankton samples from a long-term survey of the Illinois River indicated that the exotic Daphnia lumholtzi had higher abundances than the native Daphnia parvula. In the life table experiments I hypothesized that D. lumholtzi would have a higher tolerance for low food level and high turbidity than D. parvula. The life table experiments showed that D. lumholtzi had higher survivorship than D. parvula in most treatments, and had higher lifetime fertility than D. parvula in all treatments. D. lumholtzi achieved its highest intrinsic rate of growth (r = 0.40463) in the high turbidity/high food treatment. In contrast, D. parvula had its highest intrinsic rate of growth (r = 0.33584) in the low turbidity/high food treatment. The results of this study suggest that D. lumholtzi is better adapted to live in turbid river environments than the native D. parvula, which may contribute to the rapid dispersal of this exotic species