Spatial and temporal distribution of zooplankton in main channel and backwater habitats of pool 4, Upper Mississippi River

The Upper Mississippi River (UMR) is home to over 100 species of native fishes, most of which utilize zooplankton as a food item at sometime during their life history. Currently there is concern over the potential impact planktivorous Asian carps Hypophthalmichthys spp. may have on the zooplankton community. In an effort to provide baseline information for understanding ecosystem changes that may occur we used a stratified random sampling design to examine the spatial and temporal distribution of zooplankton in Pool 4 of the UMR. Analysis revealed significant differences in zooplankton density and community structure across habitats and seasons. The turbid backwater habitat of the upper reach of the pool had the highest total zooplankton density, comprised largely of rotifers that exhibited obvious seasonality. The lower reach of the pool is influenced by a natural riverine lake that retains sediment, improves water clarity and creates an environment favorable for large-bodied crustacean zooplankton that are exported to the main channel of the lower reach. The backwater habitat in this reach has abundant aquatic vegetation in which macrophyte-associated cladocerans are common.