Biogeochemical and hydrological controls on phosphorus transport in a floodplain fen
Surridge, Benjamin William James
PublisherUniversity of Sheffield
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This study investigates the biogeochemical controls on the release of phosphorus (P) from riparian wetland sediments to solution, and the subsequent delivery of P to receiving waters. The research was conducted at Strumpshaw Fen, a riparian wetland in the floodplain of the River Yare in the UK. Novel mesocosm work identifies the reductive-dissolution of iron-bound P as the dominant control on P release to solution. The timing of P release to pore water and surface water is also influenced by the concentration of nitrate-nitrogen in floodwater, because redox potential is poised above the level necessary for the reduction 0 f ferric iron whilst nitrate remains in solution. Field-based hydrochemical monitoring confirms the findings from laboratory mesocosm work on the controls on P release to solution, and indicates that P concentrations indicative of hyper eutrophic conditions may be established in the pore water and surface water of this riparian wetland. Sequential filtration establishes that the vast majority of the MRP fraction in 0.45 J.1m filtrates may be composed of free orthophosphate. An investigation of hydrological connectivity between the shallow peat and (i) the underlying mineral aquifer and (ii) the adjacent open-water network is used to indicate whether P in solution might be exported from the peat in large quantities. Reliable characterisation of the hydraulic conductivity (K) of the peat deposits at Strumpshaw Fen is achieved after a thorough evaluation of the piezometer technique, and the application of a novel laboratory-based method. K is found to be relatively high, indicating a potential for substantial flows of water between the near-surface peat and the ditch network at the site, given suitable hydraulic gradients. Field-based hydrological monitoring suggests that lateral exchanges between the peat and adjacent open-water bodies are more important than vertical exchanges between the peat and the underlying mineral aquifer. It is shown that the combination of P availability and hydrological connectivity between the peat and open water, will result in bioavailable P reaching the ditch network at Strumpshaw. In the light of these findings, riparian fen wetlands should not be assumed a priori to be 'protectors' of the chemical and ecological quality of receiving waters, but may in fact be sources for significant quantities of P that have the potential to be exported from the fen system.