High resolution spatiotemporal sampling as a tool for comprehensive assessment of zinc mobility and pollution in sediments of a eutrophic lake

•High-resolution sampling showed seasonal variations in dissolved and labile Zn.•Algal blooms reduced concentration of dissolved Zn by algal assimilation in summer.•Concentration of dissolved Zn decreased via Mn oxides adsorption of Zn in winter.•Mn oxides reduction and DOM complexation increased Zn mobilization in other seasons. To assess zinc (Zn) pollution risk from sediments, this study investigated the monthly changes of dissolved Zn and labile Zn in sediment-overlying water profiles in a eutrophic bay (Meiliang Bay) of Lake Taihu (China) using high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) at a 4 mm vertical resolution. In February and March, Mn oxides reduction caused high concentrations of DGT-labile Zn (14 ∼ 235 μg L−1), as evidenced by the significant correlation between DGT-labile Zn and DGT-labile Mn in sediments. In June and July, algal blooms reduced concentrations of dissolved Zn via algal assimilation. From August through October, concentrations of dissolved Zn in overlying water (338 ∼ 1023 μg L−1) exceeded the water quality limit for fisheries in China (100 μg L-1). This was attributed to reductive dissolution of Mn oxides in sediments caused by algal degradation followed by complexation of dissolved organic matter (DOM), which was identified in a simulated algal bloom experiment. In the winter, decreased Zn mobility was mainly attributed to adsorption by Mn oxides. It was concluded that enhanced Zn pollution risk from sediments is worthy of concern especially during algal degradation in eutrophic lakes.