Development of the DET technique for high-resolution determination of soluble reactive phosphate profiles in sediment pore waters

The DET (diffusive equilibrium in thin films) technique is developed to measure soluble reactive phosphate (SRP) profiles in sediment pore waters at a millimetre resolution. The analytical procedure includes equilibration of the gels in sediments, section of the gels after retrieval from the sediments, back elution of phosphorus in the gels, and analysis of SRP in the eluents. Recovery of phosphorus is improved from back elution with 0.25 M nitric acid relative to deionised water. SRP concentrations in pore waters of different sediments measured by DET probes agree well with those directly measured by the colorimetric method. Pore water profiles obtained simultaneously using gel probes and other techniques (including Rhizon and dialysis peeper) also show comparability at similar resolutions. The DET probes were used to investigate pore water SRP profiles in the sediments of two contrasting regions (algal-dominated and macrophyte-dominated) in Lake Taihu. An increasingly upward movement of SRP was observed in subsurface pore waters of the algal-dominated region coupled with an increase in water temperature from March to May. Peak-shape distribution of SRP and horizontal heterogeneity was observed in pore waters of the macrophyte-dominated region, which is most likely caused by the activity of submerged macrophyte roots.