High-resolution imaging of phosphorus mobilization and iron redox cycling in sediments from Honghu Lake, China

Abstract

Phosphorus (P) pollution is a significant environmental problem in lakes, especially in lakes highly impacted by human activities, e.g., Honghu Lake. Years of farming activities have led to a complex occurrence of P in the sediment which has not been studied in detail. Iron redox cycling is a key mechanism controlling P adsorption–desorption behavior, and its study is an effective means of assessing P dynamics, which could provide basic theoretical data for the control of internal phosphorus in such lakes. The high-resolution ZrO-Chelex DGT (ZrO-Chelex diffusive gradients in thin films) and HR-Peeper (high-resolution dialysis sampler) techniques were used to assess labile P, labile Fe, and soluble reactive P (SRP) in five sediment samples from four functional areas of Honghu Lake. The temperature, pH, and dissolved oxygen levels of water samples were measured in situ using a portable water quality analyzer, and the dissolved organic carbon (DOC) concentration was analyzed with a TOC analyzer after 0.45 μm membrane filtration. Total Fe concentrations were determined using the phenanthroline colorimetric approach. Sediment porosity was calculated based on weight loss after drying for 6 h at 105 °C. One-dimensional changes in labile P and Fe in different functional areas of the lake were quite heterogeneous, and there was a significant linear correlation between labile P and Fe near the sediment–water interface (SWI). Sediment samples HH-2 and HH-5 exhibited positive P diffusion flux (F) and tended to be a P source, while sites HH-1, HH-3, and HH-4 tended to take in P from overlying water. The ratio (R) of labile P to SRP was used as an indicator of potential soluble reactive P release from sediments into the surrounding water and initially increased at four sampling sites, then decreased deeper in the sediment except at site HH-4. The results indicated that the tendency of P released from sediments or uptake from overlying water was distinct in different functional areas and that iron redox cycling was a key mechanism controlling P dynamics in the sediments of Honghu Lake. Thus, the iron–phosphorus relationship can provide an accurate assessment of P release processes in a surface water environment.