Zooplankton Assemblage Structure of the Lower Mississippi River and Floodplain in Relation to Site Hydrological Connection

ABSTRACT The pattern of flow or hydrological connection between a river's main channel and its floodplain can vary seasonally and spatially in frequency, duration, direction, and magnitude. Such variation is expected to promote habitat and biotic diversity. We examined this proposition in relation to zooplankton assemblage structure and environmental parameters in the Lower Mississippi River System (LMRS). Over an 18‐month period, we sampled channel and off‐channel floodplain lake habitats representing four categories of mean annual connection duration to the main channel (MC), or “potamal states,” ranging from frequently to infrequently connected. We compared zooplankton assemblages in terms of density, richness, dominance, and turnover, and identified habitat conditions associated with variation across potamal states. Principal findings were as follows: (1) we identified 70 distinct zooplankton taxa, most not previously recorded in the LMRS; (2) assemblage structure aligned strongly with habitat conditions influenced by the MC; (3) the most pronounced differences among potamal states occurred between channel habitats and floodplain lakes collectively; (4) mean zooplankton density and richness were lowest in channel habitats, regardless of season or river stage, and highest in the least frequently connected lakes; (5) indicator taxa distinguished assemblages among potamal states; and (6) densities of large crustaceans were markedly lower than those recorded in the only previous study of LMRS zooplankton assemblages, conducted 35 years earlier. Our results highlight the critical role of variable hydrological connection in shaping habitat conditions and promoting biological diversity in large river–floodplain ecosystems.