The Lower Mississippi River, like most large rivers, has been heavily modified for flood control and navigation. These modifications have reduced the historic floodplain by 90% and restricted new habitat formation within the remaining floodplain. However, a large diversity of aquatic habitats remains within the system representing a range in main channel connectivity which in turn drives the suitability of a habitat for zooplankton assemblages both spatially and temporally. Specifically we asked: How do zooplankton assemblages respond to varying degrees of connectivity? Do zooplankton assemblage food sources change with changes in connectivity? Samples were collected over two years between spring and fall from 10 sites in the Lower Mississippi River floodplain representing a gradient in hydrologic connectivity with the river main channel. Additionally, we collected samples from 19 sites in 2016 for analysis of zooplankton food sources as indicated by their stable isotope composition.
Results/Conclusions
Main channel and highly connected sites were restricted to loricate rotifers, copepods and occasional small bodied cladocerans. Less connected sites had high variation in assemblage structure and overall zooplankton numbers throughout the sampling period. Least connected sites moved through a yearly successional pattern of assemblage earlier than intermediately connected sites. Analysis of stable isotope data showed that diets of assemblages in even moderately high connectivity habitats were almost exclusively lake (phytoplankton based) material. Only the diets of assemblages within the main channel and highly connected sites had a noteworthy contribution of riverine (bacteria and allochthonous based) material. However, that contribution was small compared to lake sources. This pattern may be explained by either selective feeding by zooplankton or that zooplankton may be washed into the main channel from areas of reduced flow and higher concentrations of phytoplankton. This research contributes to the lacking (albeit increasing) data on the ecological importance of variably connected habitats of the Lower Mississippi River at a time when new habitat formation is at a minimum.