Although mixing in lakes has significant environmental and ecological implications, knowledge of mixing dynamics for shallow floodplain lakes has received little attention. In this study, hydrological, thermal and isotopic investigations were undertaken to provide evidence for the mixing in the large, shallow floodplain Poyang Lake (China). Depth profiles of water velocity, water temperature and stable hydrogen and oxygen isotope compositions were measured throughout the lake, with results showing that the water velocity differences in depth profiles are generally less than ∼0.2 m/s, indicating weak stratification. Although water temperature differences of up to ∼2 °C are observed occasionally, Poyang Lake appears to have isothermal mixed layers from the epilimnion to the hypolimnion, attributed to the presence of mostly small temperature differences (<1 °C). Additionally, isotope compositions reveal that the lake's water columns are almost homogeneous during various water-level periods. Relative to many lakes exhibiting either no mixing or partial mixing, Poyang Lake appears to be fully mixing on a seasonal basis, depending on hydrological forcings within the lake rather than meteorological conditions. The current study will help to improve our knowledge of water flow patterns and pollutant transport in Poyang Lake and other similar floodplain lakes.

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