Spatial and temporal relationships of zooplankton and planktivores in an invaded large river ecosystem

Introduction Large rivers exhibit spatiotemporal heterogeneity, where habitat structure affects spatial and temporal overlap between zooplankton and planktivores. In the central United States, the non-native, planktivorous silver carp ( Hypophthalmichthys molitrix ) dominates fish communities in complex riverscapes. Understanding how silver carp and native planktivores interact with zooplankton through space and time is essential for anticipating changes to trophic stability and energy pathways and can inform silver carp management. The objectives of this study were to assess spatiotemporal patterns in zooplankton and planktivorous fish densities, including investigating evidence of zooplankton diel vertical migration and whether planktivores display diel patterns in main channel and off channel habitat use. Methods The vertical distributions of three zooplankton taxa and the spatial distributions of silver carp and native planktivores were quantified over a 24-h period in main channel and off channel habitats of three locations in the Illinois River, USA during October 2018. Results Planktivorous fishes > 30 cm total length were dominated by silver carp and were consistently denser in off channel habitats during the day and night. At some sites, densities of cladocerans, copepods, and rotifers differed between the main channel and off channel habitats. Cladoceran, copepod, and rotifer taxa did not exhibit daily changes in vertical distribution. Cladoceran taxa were denser in off channel habitats at night than during the day, while copepod densities were consistently higher near the bottom of the off channel habitat. Discussion Declining densities of off channel cladocerans during the day may have been due to movement from nearshore areas or the sediment, planktivory by diurnally feeding fishes, or drift. Unchanging diel rotifer densities support other research showing that these small-bodied taxa with short life cycles persist in the face of high planktivory. Increases in cladocerans during the night in off channel habitats may have been due to a combination of sediment use, decreased nighttime planktivore foraging rates, drift from upstream sources, water flow, and movement from lateral areas. Zooplankton subsidies from the off channel areas of the riverscape support high densities of invasive silver carp and native planktivores.