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

George Q. Schaffer¹Kristopher A. Maxson²Andrya L. Whitten²Ashley L. Johnson²Patrick J. Kennedy¹Jason A. DeBoer²James E. Garvey¹Cameron Davis¹David P. Coulter^3*

• ¹Southern Illinois University- Carbondale, Carbondale, United States
• ²University of Illinois Urbana-Champaign Illinois Natural History Survey, Champaign, United States
• ³South Dakota State University, Brookings, United States

Large rivers exhibit much spatiotemporal heterogeneity, where habitat structure can affect spatial and temporal overlap between zooplankton and planktivores. These dynamics have become increasingly important in rivers of the central United States, where the non-native planktivore silver carp (Hypophthalmichthys molitrix) dominates fish communities. As this invasion progresses, 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 diurnal patterns in main channel and off channel habitat use. The vertical distributions of three zooplankton taxa and the spatial distributions of silver carp and native planktivores were quantified over a 24-hour period in main channel and off channel habitats of three locations in the Illinois River, USA during October 2018. Planktivorous fishes > 30 cm 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 off channel habitat. 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. Rotifer densities did not change between day and night, supporting 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.