Inundation of different river bank heights influences organic matter concentrations and zooplankton abundance

Regulation and water extraction change flow regimes in lowland rivers, affecting ecosystem functions and wetting patterns of riverbanks. River connectivity to lateral environments is crucial for organic matter cycling and the life cycles of diapausing microinvertebrates. While extreme hydraulic periods (floods and cease‐flow) are well‐studied, the impact of small to medium flows on riverine carbon flux is less understood. We conducted a mesocosm study to examine litter, nutrient, and zooplankton contributions from different bank heights in the Mehi River, Australia. Sediment from three bank heights (lower, lower + middle, and lower + middle + upper) was added to 1000 L mesocosms. Upper bank heights had more organic matter, leaf litter, and live plant coverage. Sediment from upper and middle banks increased organic carbon and phosphorus concentrations. Zooplankton abundance was higher in treatments with upper bank sediment compared to lower bank sediment. Zooplankton communities varied, with rotifer taxa including Keratella valga and Filinia passa in upper bank treatments. We estimated zooplankton biomass contributions under current regulated hydrology and compared them to a predevelopment scenario without water extraction. Regulation has reduced zooplankton input from banks by about 8.8%. Inundating higher banks increases carbon and microinvertebrate availability for food webs compared to only inundating lower sections. These findings inform effective flow management strategies and highlight how targeted environmental water use can enhance lowland river ecosystem productivity.