High-resolution profiles of dissolved reactive phosphorus in overlying water and porewater of Lake Taihu, China

The diffusive gradients in thin films (DGT) technique was applied to obtain high spatial resolution of dissolved reactive phosphorus (DRP) concentrations in overlying water and sediment porewater in Lake Taihu. A strong positive correlation between total phosphorus (TP) in sediments and DRP concentrations using DGT was found in both of the sampled lake regions. For stations ZSW and DPG, which have high TP background values in sediments, DRP concentrations in overlying water and porewater were much higher than those at stations MLW, DPG, and HX. Also, a high potential for P release at the sediment–water interface was revealed at stations ZSW and DPG due to a significant concentration gradient between overlying water and porewater. In sediment cores from stations ZSW and DPG, DRP porewater concentration profiles using DGT with three diffusive gel thicknesses displayed a similar trend, increasing down to −2 cm or −4 cm, then decreasing down to −8 cm, and then becoming relatively stable in deeper layers. High-resolution profiles of DRP were related to dissolved oxygen concentrations and organic matter concentrations in different sediment layers at stations ZSW and DPG. For most sediment with low TP concentrations at stations MLW, DPG, and HX, DRP concentrations could not be determined at and near the sediment–water interface, then increased gradually down to −8 cm or −10 cm, and remained stable. At stations MLW, DPG, and HX, most DRP_DGT (DRP concentrations measured by DGT) values were 10–30 % of the DRP concentrations in porewater, indicating a partial resupply of the sediments to porewater.