Factors structuring zooplankton density and composition within a Louisiana river and floodplain tributaries with emphasis on hydrologic processes

Abstract

Zooplankters are important members of freshwater communities, facilitating the transfer of energy from primary production to higher trophic levels. Lentic floodplain systems are important in providing zooplankters with adequate habitat for growth and reproduction. Recently, The Nature Conservancy has been interested in cataloguing the aquatic taxa that inhabit the Ouachita River and floodplain in northern Louisiana, concurrent with an attempt to reestablish a river-floodplain connection with the river and eastern floodplain (the Mollicy Farms Unit, part of the Upper Ouachita National Wildlife Refuge). Sampling was conducted at seven sites along the Ouachita River and western floodplain monthly for one year to investigate zooplankton density and composition in relation to environmental variables and hydrology. Principal component analysis was used to illustrate relationships among zooplankton groups with environmental variables and sampling sites, and cladoceran taxa were found to be correlated to specific environmental variables by multivariate analysis of variance. Results indicated that highest densities were exhibited by rotifers, followed by copepods and cladocerans. Abundances of the zooplankton groups were correlated primarily with specific conductance, PO4, temperature, chlorophyll a, Secchi depth, and dissolved oxygen. Additionally, average densities of zooplankton groups were greater at floodplain sites than at sites near or within the river mainstem. During the low water phase, copepods and certain cladoceran taxa were found in greater densities than during the high water phase, indicating a potential concentration effect. These findings will contribute to our understanding of the interactions between environmental parameters and zooplankters within the Ouachita River and floodplain, as well as an overall understanding of zooplankton dynamics in river-floodplain systems.