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

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.

Successful control of internal phosphorus loading after sediment dredging for 6years: A field assessment using high-resolution sampling techniques.

The effectiveness of sediment dredging for the control of internal phosphorus (P) loading, was investigated seasonally in the eutrophic Lake Taihu. The high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were used to measure the concentrations of soluble Fe(II) and soluble reactive P (SRP) as well as DGT-labile Fe/P in the non-dredging and post-dredging sediments. The P resupply kinetics from sediment solids were interpreted using DGT Induced Fluxes in Sediments (DIFS) modeling. The results showed no obvious improvement in water and sediment quality after dredging for 6years, due to their geographical proximity (a line distance of approximately 9km). However, dredging significantly decreased the concentrations of soluble Fe(II)/SRP and DGT-labile Fe/P in sediments, with effects varying at different depths below the sediment-water interface; More pronounced effects appeared in January and April. The diffusive flux of pore water SRP from sediments decreased from 0.746, 4.08 and 0.353mg/m2/d to 0.174, 1.58 and 0.048mg/m2/d in April, July and January, respectively. DIFS modeling indicated that the P retention capability of sediment solids was improved in April in post-dredging site. Positive correlations between pore water soluble Fe(II) and SRP as well as between DGT-labile Fe and P, reflect the key role of Fe redox cycling in regulating dredging effectiveness. This effect is especially important in winter and spring, while in summer and autumn, the decomposition of algae promoted the release of P from sediments and suppressed dredging effectiveness. Overall, the high-resolution HR-Peeper and DGT measurements indicated a successful control of internal P loading by dredging, and the post-dredging effectiveness was suppressed by algal bloom.