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Water-level fluctuations (WLFs) are a key influence on aquatic biodiversity in seasonally inundatedfreshwater ecosystems. However, how unregulated WLFs affect macroinvertebrates in lake-floodplain systemsexperiencing considerable annual fluctuations remains unclear. We explored spatial and temporal variability intaxonomic α and β diversity in the largest fluctuating lake in China, Poyang Lake, during two hydrological seasons.We hypothesized that taxa richness (α diversity) is greater in the floodplain than in the lake channel due to greateravailability of trophic resources, and that variability in assemblage composition (β diversity) in the channel isgreater during the high-water season (HWS) than the low-water season (LWS) due to increased habitat heterogeneity.Benthic macroinvertebrate assemblages were sampled, water physicochemical and hydrological variables weremeasured, and geographical coordinates were determined at 34 sites during the HWS (October 2017) and LWS(January and April 2018). A total of 74 taxa were recorded. Macroinvertebrate α diversity was comparable in thefloodplain and the lake channel. Beta diversity in the channel was greater during HWS than LWS. Hydrologicalvariables influenced β diversity during LWS and geographical distance between sites increased β diversity duringHWS, whereas physicochemical variables did not influence β diversity in either hydrological season. Our resultssuggest that extensive WLFs altered macroinvertebrate biodiversity among hydrological seasons by extending waterinto floodplains during HWS and reducing substrate heterogeneity in the lake channel during LWS. We thushighlight the importance of WLFs that maintain such environmental seasonality in supporting the biodiversity ofbenthic macroinvertebrates in naturally dynamic freshwater ecosystems.

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